Description of the intervention
Folate is a generic term for both the endogenous form of the vitamin occurring naturally in food and the synthetic form found in supplements and fortified foods (Bailey 1995). It should be noted, however, that folate is a naturally occurring vitamin while folic aid is the synthetic replacement of folate used in most supplements and in fortified foods. Humans are fully dependent on dietary sources or dietary supplements and microorganisms in their intestinal tract for their folate supply. Folate derivatives are essential for the synthesis of nucleic acid, amino acids, cell division, tissue growth, and DNA methylation (Krishnaswamy 2001; Morrison 1998; Scholl 2000).
Inadequate folate intake leads to a decrease in serum folate concentration, resulting in a decrease in erythrocyte (red blood cell) folate concentration, a rise in homocysteine (Hcy) concentration, and megaloblastic changes in the bone marrow and other tissues with rapidly dividing cells (Dietary Ref 1998; Willoughby 1968). During pregnancy, fetal growth causes an increase in the total number of rapidly dividing cells, which leads to increased requirements for folate (Bailey 1995). With inadequate folic acid intake, concentrations of folate in maternal serum, plasma, and red blood cells decrease from the fifth month of pregnancy onwards (Açkurt 1995; Bates 1986). If inadequate folate intake is sustained during pregnancy, megaloblastic anaemia (a blood disorder characterised by anaemia, with red blood cells that are larger than normal and cell contents that are not completely developed) occurs (Willoughby 1968). Folate concentrations continue to decrease for several weeks after pregnancy (Bruinse 1995; Smith 1983), and by the second to third month postpartum, a third of all mothers can have subnormal concentrations of folate in serum and red blood cells (Açkurt 1995). Possible causes for the decline in blood folate during pregnancy include increased folate demand for growth of the fetus due to an increase in the number of rapidly dividing cells (Bailey 1995) and growth of uteroplacental organs, decreased folate absorption, low folate intake, hormonal influence on folate metabolism as a physiologic response to pregnancy (Chanarin 1969), and dilution of folate due to blood volume expansion (Bruinse 1995). Folate demands may be further increased in women with sickle cell disease and women living in areas where malaria is endemic (Lawson 1988); in these areas, anaemia in pregnancy is a major health problem. Increased folate catabolism and urinary folate excretion (Fleming 1972; Landon 1971) may also contribute to increased folate needs in pregnancy (Caudill 1998; Gregory 2001b; Higgins 2000; McPartlin 1993), but the findings are controversial. As a consequence of folate deficiency, Hcy accumulates in the serum and is found to be associated with an increased risk in cardiovascular disease (Refsum 2008), late pregnancy complications such as pre-eclampsia (Makedos 2007; Patrick 2004; Tamura 2006), and neural tube defects around the time of conception (De Benoist 2008).
The recommended folate intake for pregnant women is 400 µg/day (Food and Nutrition Board 1970). It was revised in 1999 after evaluating its bioavailability from food and synthetic folate, and the recommendation was increased to 450 µg (600 DFEs/day (dietary folate equivalent)) (Institute of Medicine 2000). It should be noted that as per NICE guidelines, this amount of folic acid when supplemented to pregnant women (and those intending to become pregnant), before conception and throughout the first 12 weeks, reduces the risk of having a baby with a neural tube defect (NICE 2008). However, the Food and Nutrition Board of the Institute of Medicine have suggested that an increased folate intake might delay the diagnosis of vitamin B-12 deficiency by correcting the anaemia, or even exacerbate its neurologic and neuropsychiatric effects (Food and Nutrition Board 1998; Herbert 1997; Rush 1994). Further research is still needed in this area.
How the intervention might work
The relationship between pregnancy outcome and maternal blood folate concentrations, folate intake and hyperhomocysteinaemia cannot be ignored (Smits 2001). Plasma total homocysteine (tHcy) is regulated by folate status (Selhub 1993), and hyperhomocysteinaemia is linked to vaso-occlusive disease (Green 1995). Impaired placental perfusion due to hyperhomocysteinaemia is implicated in having a negative effect on pregnancy outcome, as are inadequate folate intake and low serum folate concentrations (Scholl 2000). Folate has long been used as a supplement in combination with iron during pregnancy, largely on the basis of haematological benefits (Fleming 1968), although deficiency has also been associated with pregnancy complications and congenital malformations (Scholl 2000). Periconceptional supplementation with folic acid, three months before and early in pregnancy is recommended (Czeizel 1992; MRC 1991), and has been shown to reduce the risk of neural tube defects by almost three-quarters (De-Regil 2010). Although still unproven, folic acid supplementation has also been suggested to help prevent other fetal malformations such as congenital heart defects (Botto 1996; Czeizel 1993; Czeizel 1996; Shaw 1995), urinary tract anomalies (Li 1995), limb defects (Czeizel 1993), oro-facial clefts (Czeizel 1993; Li 1995; Shaw 1995), and pyloric stenosis (Shaw 1995).
Why it is important to do this review
The role of folate deficiency in increasing the risk of spontaneous abortion and birth outcomes such as low birthweight, preterm birth, and perinatal mortality is unclear (Bukowski 2009; Scholl 2000). Hence, the aim of this review is to assess the effect of folic acid supplementation alone in pregnant women on haematological and biochemical parameters, adverse events during pregnancy, and on pregnancy outcomes. We did not assess periconceptional folic acid supplementation, or supplementation of folic acid along with iron during pregnancy and with other micronutrients, as these have been addressed by other reviews (Haider 2006; De-Regil 2010; Pena-Rosas 2006).
To assess the effectiveness of oral folic acid supplementation alone or with other micronutrients versus no folic acid (placebo or same micronutrients but no folic acid) during pregnancy on haematological and biochemical parameters during pregnancy and on pregnancy outcomes.
Criteria for considering studies for this review
Types of studies
We included randomised or quasi-randomised controlled trials of folic acid supplementation alone or with other micronutrients versus no folic acid (placebo or same micronutrients but no folic acid).
Types of participants
We included pregnant women of any age and parity.
Types of interventions
- Folic acid alone versus no treatment/placebo (no folic acid)
- Folic acid+ iron versus iron (no folic acid)
- Folic acid + other vitamins and minerals versus other vitamins and minerals (but no folic acid)
We excluded studies that supplemented folic acid in the form of fortification or home fortification alone or in combination with other micronutrients. We also excluded studies in which women were supplemented during periconception.
Types of outcome measures
- Pre-delivery anaemia (less than 10 g/dL haemoglobin or haematocrit below 30%
- Mean pre-delivery haemoglobin level
- Low pre-delivery serum folate (less than 3 mg/L or 7 nmol/L or 3 ng/mL)
- Mean pre-delivery serum folate level
- Low pre-delivery red cell folate (less than 100 mg/L or 300 nmol/L or 140 ng/mL)
- Mean pre-delivery red cell folate
- Preterm birth (delivery before 37 weeks of gestation)
- Low birthweight (birthweight less than 2500 g)
- Miscarriage (loss of pregnancy before 22 weeks of gestation)
- Perinatal mortality - includes stillbirth (deaths after 22 weeks of gestation) and mortality in the first seven days of life
- Pre-eclampsia- defined as blood pressure of > 140 mmHg systolic or > 90 mmHg diastolic after 20 weeks of gestation, and proteinuria of more than 0.3 g in 24 hours
- Respiratory disease in child
- Allergic disease in child
- Megaloblastic anaemia
- Hyperhomocysteinaemia (more than 16 micromol/L)
Search methods for identification of studies
We contacted the Trials Search Co-ordinator to search the Cochrane Pregnancy and Childbirth Group’s Trials Register (31 December 2012)
The Cochrane Pregnancy and Childbirth Group’s Trials Register is maintained by the Trials Search Co-ordinator and contains trials identified from:
- monthly searches of the Cochrane Central Register of Controlled Trials (CENTRAL);
- weekly searches of MEDLINE;
- weekly searches of EMBASE;
- handsearches of 30 journals and the proceedings of major conferences;
- weekly current awareness alerts for a further 44 journals plus monthly BioMed Central email alerts.
Details of the search strategies for CENTRAL, MEDLINE and EMBASE, the list of handsearched journals and conference proceedings, and the list of journals reviewed via the current awareness service can be found in the ‘Specialized Register’ section within the editorial information about the Cochrane Pregnancy and Childbirth Group.
Trials identified through the searching activities described above are each assigned to a review topic (or topics). The Trials Search Co-ordinator searches the register for each review using the topic list rather than keywords.
Searching other resources
For identification of ongoing or unpublished studies, we contacted major organisations working in micronutrient supplementation, including UNICEF Nutrition Section, World Health Organization (WHO) Maternal and Reproductive Health, WHO Nutrition Division, and National Center on Birth defects and Developmnetal Disabilities, US Centers for Disease Control and Prevention (CDC).
We did not apply any language restrictions.
Data collection and analysis
Selection of studies
Two review authors, Zohra Lassi (ZSL) and Rehana Salam (RAS), independently assessed for inclusion all the potential studies we identified as a result of the search strategy. We resolved any disagreement through discussion and, if required, we consulted the third review author, Zulfiqar Bhutta (ZAB)
Data extraction and management
We designed a form to extract data. For eligible studies, two review authors (RAS and ZL) extracted the data using the agreed form. We resolved discrepancies through discussion and, if required, we consulted the third review author. Data were entered into ReviewManager software (RevMan 2011) and checked for accuracy.
When information regarding any of the above was unclear, we attempted to contact authors of the original reports to provide further details.
Assessment of risk of bias in included studies
Two review authors (ZSL and RAS) independently assessed risk of bias for each study using the criteria outlined in the Cochrane Handbook for Systematic Reviews of Interventions (Higgins 2011). Any disagreement was resolved by discussion or by involving a third assessor (ZAB).
(1) Random sequence generation (checking for possible selection bias)
We described for each included study the method used to generate the allocation sequence in sufficient detail to allow an assessment of whether it produced comparable groups. We assessed the method as:
- low risk of bias (any truly random process, e.g. random number table; computer random number generator);
- high risk of bias (any non-random process, e.g. odd or even date of birth; hospital or clinic record number);
- unclear risk of bias.
(2) Allocation concealment (checking for possible selection bias)
We described for each included study the method used to conceal the allocation sequence in sufficient detail and determine whether intervention allocation could have been foreseen in advance of, or during recruitment, or changed after assignment.
We assessed the methods as:
- low risk of bias (e.g. telephone or central randomisation; consecutively numbered sealed opaque envelopes);
- high risk of bias (open random allocation; unsealed or non-opaque envelopes, alternation; date of birth);
- unclear risk of bias.
(3.1) Blinding (checking for possible performance bias)
We described for each included study the methods used, if any, to blind study participants and personnel from knowledge of which intervention a participant received. We considered that studies were at low risk of bias if they were blinded, or if we judged that the lack of blinding would be unlikely to affect results. We assessed blinding separately for different outcomes or classes of outcomes.
We assessed the methods as:
- low, high or unclear risk of bias for participants;
- low, high or unclear risk of bias for personnel.
(4) Incomplete outcome data (checking for possible attrition bias through withdrawals, dropouts, protocol deviations)
We described for each included study, and for each outcome or class of outcomes, the completeness of data including attrition and exclusions from the analysis. We stated whether attrition and exclusions were reported, the numbers included in the analysis at each stage (compared with the total randomised participants), reasons for attrition or exclusion where reported, and whether missing data were balanced across groups or were related to outcomes. Where sufficient information was reported, or could be supplied by the trial authors, we re-included missing data in the analyses which we undertook. We assessed methods as:
- low risk of bias (e.g. no missing outcome data; missing outcome data balanced across groups);
- high risk of bias (e.g. numbers or reasons for missing data imbalanced across groups; ‘as treated’ analysis done with substantial departure of intervention received from that assigned at randomisation);
- unclear risk of bias.
(5) Selective reporting bias
We described for each included study how we investigated the possibility of selective outcome reporting bias and what we found.
We assessed the methods as:
- low risk of bias (where it was clear that all of the study’s pre-specified outcomes and all expected outcomes of interest to the review have been reported);
- high risk of bias (where not all the study’s pre-specified outcomes have been reported; one or more reported primary outcomes were not pre-specified; outcomes of interest were reported incompletely and so could not be used; study failed to include results of a key outcome that would have been expected to have been reported);
- unclear risk of bias.
(6) Other bias (checking for bias due to problems not covered by (1) to (5) above)
We described for each included study any important concerns we had about other possible sources of bias.
We assessed whether each study was free of other problems that could put it at risk of bias:
- low risk of other bias;
- high risk of other bias;
- unclear whether there is risk of other bias.
(7) Overall risk of bias
We made explicit judgements about whether studies were at high risk of bias, according to the criteria given in the Handbook (Higgins 2011). With reference to (1) to (6) above, we assessed the likely magnitude and direction of the bias and whether we considered it was likely to impact on the findings.
Measures of treatment effect
For dichotomous data, we presented results as summary risk ratio with 95% confidence intervals.
For continuous data, we used the mean difference if outcomes were measured in the same way between trials. We used the standardised mean difference to combine trials that measured the same outcome, but used different methods.
Unit of analysis issues
We included cluster-randomised/quasi-randomised trials in the analyses along with individually-randomised trials. We incorporated the data of cluster-randomised/quasi-randomised trials using generic inverse variance method in which logarithms of risk ratio estimates were used along with the standard error of the logarithms of risk ratio estimates.
We also looked for any cross-over trials on this topic, and such trials were deemed eligible for inclusion, However, we did not find any eligible cross-over trials.
Dealing with missing data
We noted levels of attrition for included studies. We also planned to explore the impact of including studies with high levels of missing data in the overall assessment of treatment effect by using sensitivity analysis. For all outcomes, we carried out analyses, as far as possible, on an intention-to-treat basis, i.e. we attempted to include all participants randomised to each group in the analyses, and all participants were analysed in the group to which they were allocated, regardless of whether or not they received the allocated intervention. The denominator for each outcome in each trial was the number randomised minus any participants whose outcomes were known to be missing.
Assessment of heterogeneity
We assessed statistical heterogeneity in each meta-analysis using the T², I² and Chi² statistics. We regarded heterogeneity as substantial if the I² was greater than 30% and either T² was greater than zero, or there was a low P value (less than 0.10) in the Chi² test for heterogeneity.
Assessment of reporting biases
If there were 10 or more studies in the meta-analysis, we investigated reporting biases (such as publication bias) using funnel plots. We assessed funnel plot asymmetry visually, If asymmetry was suggested by a visual assessment, we performed exploratory analyses to investigate it.
Mostly studies were old and we suspected reporting bias, therefore, we attempted to contact study authors, where possible, asking them to provide missing outcome data.
We carried out statistical analysis using the Review Manager software (RevMan 2011). We used fixed-effect Mantel-Hanzel meta-analysis for combining data where it was reasonable to assume that studies were estimating the same underlying treatment effect: i.e. trials were examining the same intervention, and the trials’ populations and methods were judged to be sufficiently similar. If there was clinical heterogeneity sufficient to expect that the underlying treatment effects differed between trials, or if substantial statistical heterogeneity was detected, we used random-effects meta-analysis to produce an overall summary if an average treatment effect across trials was considered clinically meaningful. The random-effects summary was treated as the average range of possible treatment effects and we discussed the clinical implications of treatment effects differing between trials. If the average treatment effect was not clinically meaningful, we did not combine trials.
If we used random-effects analyses, the results were presented as the average treatment effect with 95% confidence intervals, and the estimates of T² and I².
Subgroup analysis and investigation of heterogeneity
We planned to carry out subgroup analyses based on following factors.
• Different doses of folate used (< 400 μg and > 400 μg)
• Different durations of folate supplementation
• Haemoglobin level of participants
Not all included studies mentioned the baseline haemoglobin levels of participants and since duration and start of folic acid supplementation in women during pregnancy varied, we, therefore, did not carry out these subgroup analyses. However, subgroup analyses were carried out on studies in which iron was additionally provided with folic acid. We also performed subgroup analyses on the dosage of folic acid.
We also reported the outcomes based on how the outcome was defined in the individual study.
We assessed subgroup differences by the interaction tests available within RevMan (RevMan 2011). We reported the results of subgroup analyses quoting the χ² statistic and the P value, and the interaction test I² value.
We did not perform sensitivity analyses as studies were old and of mediocre quality.
Description of studies
Results of the search
A total of 94 trial reports were considered for inclusion into this review, finally 31 studies involving 17,771 women were included in this review (Figure 1).
|Figure 1. Study flow diagram.|
Thirty-one studies have been included in this review. The majority of these studies were quite old and were conducted during the 1960s (Castren 1968; Chanarin 1965; Chanarin 1968; Chisholm 1966; Dawson 1962; Edelstein 1968; Fleming 1968; Hibbard 1969a; Menon 1962; Metz 1965; Willoughby 1967); the 1970s (Balmelli 1974; Batu 1976; Baumslag 1970; Fletcher 1971; Giles 1971; Iyengar 1975; Rae 1970; Rolschau 1979; Trigg 1976; Weil 1977), and the 1980s (Blot 1981; Harrison 1985; Lira 1989; Roth 1980; Srisupandit 1983; Tchernia 1982; Pack 1980). Three studies were published in 2005 (Charles 2005; Christian 2003; Decsi 2005), however, Charles 2005 re-analysed data that were collected in 1966. Seven studies (Chanarin 1965; Christian 2003; Dawson 1962; Decsi 2005; Hibbard 1969a; Metz 1965; Pack 1980) were were not included in the meta-analyses because they either did not mention their standard deviations/standard errors; or they reported the rise or fall in the haematological and biochemical levels.
Most of the outcomes were defined in the same way across different trials except for preterm birth, pre-delivery anaemia, and low birthweight which were defined differently, however, we still included them and they were presented in subgroup according to their defined cut-offs (Refer to Table 1). The majority of the studies were conducted in Europe (Balmelli 1974; Blot 1981; Castren 1968; Chanarin 1965; Chanarin 1968; Charles 2005; Chisholm 1966; Dawson 1962; Decsi 2005; Fletcher 1971; Hibbard 1969a; Rae 1970; Rolschau 1979; Tchernia 1982; Trigg 1976; Weil 1977; Willoughby 1967), Africa (Baumslag 1970; Edelstein 1968; Fleming 1968; Harrison 1985; Metz 1965) and Asia (Batu 1976; Christian 2003; Iyengar 1975; Menon 1962; Srisupandit 1983). One study was conducted in South America (Lira 1989), one in Australia (Giles 1971) and one in New Zealand (Pack 1980). One study (Roth 1980) did not mention the setting. The time for initiation of supplementation varied from 8th week of pregnancy till three days postpartum. Most of the studies supplemented women with folic acid in combination with iron (Balmelli 1974; Batu 1976; Baumslag 1970; Blot 1981; Castren 1968; Chanarin 1965; Chanarin 1968; Chisholm 1966; Christian 2003; Edelstein 1968; Fletcher 1971; Giles 1971; Harrison 1985; Iyengar 1975; Lira 1989; Menon 1962; Metz 1965; Rae 1970; Rolschau 1979; Roth 1980; Srisupandit 1983; Tchernia 1982; Trigg 1976; Weil 1977; Willoughby 1967) however, only a few compared folic acid alone with placebo (Charles 2005; Chisholm 1966; Decsi 2005; Fleming 1968; Pack 1980).
Please refer to the Characteristics of included studies table for more details.
A total of 25 studies were excluded from the review as they did not satisfy the inclusion criteria. Hamilton 1973 was not a randomised controlled trial. There were four studies in which folic acid was given in combination with other micronutrients compared with a no supplement group (Bjerre 1967; Ma 2008; Wang 2012; Zeng 2008). Similarly, Giles 1960 compared the intervention group with historical controls; Gregory 2001 compared pregnant women with non pregnant women; Khanna 1977 evaluated the therapeutic use of folic acid in women with anaemia; and there were a few studies in which the association of folic acid supplementation was observed, with breast cancer, fetal apoptosis (Klinger 2006), congenital anomalies (Ulrich 1999) and with malaria when given with sulphadoxine pyrimethamine (Ouma 2006). We excluded studies in which therapy of iron and folic acid was compared with no therapy at all (Taylor 1979; Taylor 1981). We also excluded studies in which folic acid was given in a fortification form (Colman 1974; Colman 1975). We excluded studies that compared the duration of folic acid supplements (Ellison 2004; Polatti 1992), and different dosage of folic acid supplements (Hekmatdoost 2011; Hibbard 1969; Manizheh 2009). Trials were also excluded that were in the form of published abstracts only and had insufficient information to extract (Hague 1998; Kristoffersen 1979; Melli 2008; Thomson 1982). Also, one study in which results from three trials were re analysed was excluded (Tchernia 1982a).
Please refer to Characteristics of excluded studies table for more details.
Risk of bias in included studies
Most of the studies were conducted over 30 to 45 years ago, and we found poor subjective and objective compliance with random allocation, adequate concealment and blinding. Bias and confounding thus seem to us the likely explanation for our findings.
|Figure 2. Methodological quality graph: review authors' judgements about each methodological quality item presented as percentages across all included studies.|
|Figure 3. Methodological quality summary: review authors' judgements about each methodological quality item for each included study.|
Sequence generation and adequate allocation concealment was a problem in almost all the studies and control of selection bias at entry was often difficult to assess as many authors stated that women were 'randomly allocated' without actually describing the technique, still there were studies that managed to report the methods of allocation concealment adequately (Blot 1981; Edelstein 1968; Fleming 1968; Giles 1971; Rolschau 1979).
Incomplete outcome data
Mostly studies provided insufficient information regarding attrition rates, which meant we were unable to make any judgment. There were only a few studies that discussed their exclusion and attrition rates and reported their reasons. (Balmelli 1974; Batu 1976; Blot 1981 Castren 1968; Fleming 1968; Giles 1971 Harrison 1985; Iyengar 1975; Srisupandit 1983; Tchernia 1982).
Other potential sources of bias
No other bias was identified but we had insufficient information available to fully assess this 'Risk of bias' domain. Consequently, we assessed all included studies as being at 'unclear' risk of other bias.
Effects of interventions
a. Clinical measures of untoward events during pregnancy and of pregnancy outcome
None of the included studies reported preterm birth in accordance with our definition of the outcome. We found two studies, of which one defined it as birth of a baby between 36 to 38 weeks, and another defined it as birth before 38 weeks of pregnancy. We pooled them both to look for an association with folic acid supplementation in pregnancy. Our analysis showed that administration of folic acid supplementation during pregnancy has no impact on reducing preterm birth (risk ratio (RR) 1.01, 95% confidence interval (CI) 0.73 to 1.38; three studies, 2959 participants ( Analysis 1.1)).
None of the included studies reported perinatal mortality. However, three studies reported stillbirth and neonatal mortality as a composite outcome, hence we pooled them to obtain data for perinatal mortality. Folic acid supplementation during pregnancy did not show any impact on reducing stillbirths/neonatal deaths (RR 1.33, 95% CI 0.96 to 1.85; three studies, 3110 participants ( Analysis 1.2)).
Folic acid supplementation during pregnancy did not show any impact on reducing low birthweight (less than 2500 g) (RR 0.83, 95% CI 0.66 to 1.04; four studies, 3113 participants ( Analysis 1.3)).
We also attempted to look at the impact of folic acid supplementation during pregnancy on mean birthweight (g) of newborns and found no association (mean difference (MD) 104.96 g, 95% CI -0.25.50 g to 235.41 g; five studies, 774 participants; random-effects, T² = 21694.29, I² = 90% ( Analysis 1.4)). All the studies pooled for this outcome compared folic acid + iron versus iron alone.
The standard errors for Trigg 1976 were very small as compared to the other trials for being plausible, therefore, we conducted a sensitivity analysis after removing this study. Heterogeneity was reduced from 90% to 50% (MD 135.76, 95% CI 47.85 to 223.68; four studies, 625 participants; random-effects, T² = 4841.10, I² = 50% ( Analysis 1.5)
Outcomes not reported in the included studies
The included studies did not report on the impact of folic acid supplementation on miscarriage, pre-eclampsia, respiratory disease or allergic disease in children.
b. Haematological and biochemical parameters
The included studies used different definitions of anaemia. Eight studies reported pre-delivery anaemia as an outcome, but only two studies used our definition of anaemia. We included all studies reporting anaemia but pooled them separately according to the definition of anaemia used. Folic acid supplementation did not show any impact on reducing pre-delivery anaemia (any cut-off point) (average RR 0.62, 95% CI 0.35 to 1.10; eight studies, 4149 participants; random-effects, T² = 0.51, I² = 90% ( Analysis 1.6)). When studies were separately pooled according to the definition described in the earlier section of this review, we found that supplementation had no impact on reducing anaemia (haemoglobin less than 10 g/dL) (average RR 0.35, 95% CI 0.05 to 2.42; two studies, 2448 participants; random-effects, T² = 1.86, I² = 97% ( Analysis 1.6)).
We also looked at the impact of folic acid supplementation in pregnancy on mean pre-delivery haemoglobin level, and found no difference in the mean haemoglobin concentration among those in the intervention arm compared with those in the placebo arm (MD -0.03, 95% CI -0.25 to 0.19; 12 studies, 1806 participants; random-effects, T² = 0.12, I² = 95% ( Analysis 1.7)). All the studies pooled for this outcome compared folic acid + iron versus iron alone.
With regard to subgroup analysis based on dosage of folic acid supplementation, we found no differences on improving haemoglobin concentrations and the interaction test was insignificant (Chi² = 1.18, df = 1 (P = 0.28), I² = 15.1%). Analysis 1.8
We also ran a funnel plot to assess the publication bias and we found studies were equally distributed on each side except for two outliers Figure 4.
|Figure 4. Funnel plot of comparison: 1 Folic acid versus no folic acid, outcome: 1.7 Mean pre-delivery haemoglobin level.|
Pre-delivery serum folate
Folic acid supplementation in pregnancy showed a reduction in the incidence of low pre-delivery serum folate by 62% (RR 0.38, 95% CI 0.25 to 0.59; two studies, 696 participants ( Analysis 1.11)).
We found non-significantly higher mean pre-delivery serum folate levels among those in the folic acid supplementation arm compared with those in the placebo arm (standardised mean difference (SMD) 2.03, 95% CI 0.80 to 3.27; eight studies, 1250 participants; random-effects, T² = 2.96, I² = 98% ( Analysis 1.9)). All the studies pooled for this outcome compared folic acid + iron versus iron alone.
For subgroup analysis based on dosage of folic acid supplementation, we found significant improvements in mean serum folate concentration when the dose was less than 400 μg (SMD 3.70, 95% CI: 0.28 to 7.11, four studies n = 253, random effects, I² = 99%), however, no impact was seen of folic acid > 400 μg (SMD 0.68, 95% CI: -0.75 to 2.10, four studies n = 997, random effects, I² = 98%) Analysis 1.10. The interaction test for the overall estimate was not significant (Chi² P value = 0.11, I² = 61%) suggesting no difference between groups.
Pre-delivery red cell folate
None of the included studies reported data for pre-delivery red cell folate deficiency status. However, mean red cell folate levels were reported in four studies. Folic acid supplementation during pregnancy did not show any impact on reducing mean pre-delivery red cell folate levels (SMD 1.59, 95% CI -0.07 to 3.26; four studies, 427 participants; random-effects, T² = 2.79, I² = 97% ( Analysis 1.12)). All the studies pooled for this outcome compared folic acid + iron versus iron alone.
Folic acid supplementation during pregnancy significantly reduced the incidence of megaloblastic anaemia by 79% (RR 0.21, 95% CI 0.11 to 0.38; four studies, 3839 women ( Analysis 1.13)).
Outcomes not reported in the included studies
The included studies did not report on the impact of folic acid supplementation on hyperhomocysteinaemia, respiratory disease and allergic disease in the child.
Summary of main results
From our meta-analysis of randomised controlled trials on folic acid supplementation, we found no evidence of an effect of supplements on preterm birth, stillbirth/neonatal death, mean birthweight/low birthweight, low pre-delivery haemoglobin and serum red cell folate. However, we found a risk reduction on low pre-delivery serum folate and megaloblastic anaemia.
Quality of the evidence
First, all the included studies were conducted over 30 to 45 years ago, and we found poor subjective and objective compliance with random allocation, adequate concealment and blinding. Bias and confounding thus seem to be the likely explanation for our findings.
Second, for combining studies, it is important that the outcome measures are comparable. Of note, trials included in this analysis reported outcomes quite differently from each other. This could have resulted in higher risk of bias due to selective reporting in these trials. However, we pooled them separately, wherever possible, to minimise this bias.
Potential biases in the review process
We undertook a systematic, thorough search of the literature to identify all studies meeting the inclusion criteria and we are confident that the included trials met the set criteria. Study selection and data extraction were carried out in duplicate and independently and we reached consensus by discussing any discrepancies. A protocol was published for this review. All the analyses were specified a priori, with the exception of a post hoc analysis of the different cut-off values for biochemistry markers.
Agreements and disagreements with other studies or reviews
Previous observational studies have suggested that higher folate status in pregnancy is associated with higher birthweight, higher placental weight, and prolonged gestation (Goldenberg 1992; Neggers 1997; Tamura 1992). Preconception folic acid supplementation has also shown effects on decreasing preterm births (Bukowski 2009). However, the findings from this review are inconclusive.
A review on folic acid supplementation during pregnancy by Charles et al (Charles 2005b) that included results from large randomised controlled trials found no conclusive evidence of benefit for folic acid supplementation in pregnant women. An earlier version of this Cochrane review also reached the same conclusion (Mahomed 1997).
Implications for practice
Our meta-analysis of folic acid supplementation in pregnancy included 31 studies and provided non-conclusive evidence of folic acid supplementation for pregnant women on pregnancy outcomes except for improvement in mean birthweight. A reduction in the risk of megaloblastic anaemia and improvement in folate levels, however, has been noted with folic acid supplementation against supplementation with placebo but the limitation to this finding is the few number of studies reporting the outcome.
Implications for research
More well-designed, large scale randomised controlled trials are needed to establish the benefit of folic acid supplementation during pregnancy. Researchers of future trials should also make efforts to describe the participants in more detail before enrolment and should undertake long-term follow-up of the participants and their children in order to study the long-term effects of folic acid supplementation. Bias should also be reduced by adequate randomisation and allocation concealment of the assignment of intervention by achieving blinding of the participants, providers and the outcome assessors and by minimising loss to follow-up of the participants, in order to produce trials of adequate methodological quality.
We thank Kate Barton and Rebecca Gainey as translators of Lira 1989; Elena Intra as translator of Polatti 1992; Alison Ledward as translator of Weil 1977, Austin Anderson Leirvik as translator of Tchernia 1982 and Caroline Summers as translator of Balmelli 1974 and Roth 1980.
As part of the pre-publication editorial process, this review has been commented on by three peers (an editor and two referees who are external to the editorial team), a member of the Pregnancy and Childbirth Group's international panel of consumers and the Group's Statistical Adviser.
Data and analyses
- Top of page
- Authors' conclusions
- Data and analyses
- Contributions of authors
- Declarations of interest
- Sources of support
- Differences between protocol and review
- Index terms
Protocol first published: Issue 1, 2008
Review first published: Issue 3, 2013
Contributions of authors
Zohra Lassi entered the data, created the comparisons, conducted the analyses and wrote the text of the review under the guidance of Dr Zulfiqar Bhutta. The draft protocol was written by Dr Batool Haider (BAH) who also designed the eligibility and data extraction forms. Rehana Salam (RAS) took part in assisting with data analysis.
Declarations of interest
Sources of support
- The Aga Khan University, Pakistan.
- No sources of support supplied
Differences between protocol and review
Outcome measures have been separated into 'Primary' and 'Secondary' outcomes.
We have added two additional outcomes: respiratory disease in the child; allergic disease in the child.
This review has been developed to update the previously published review, 'Folate supplementation in pregnancy' , which was withdrawn from publication in Issue 3, 2006, of The Cochrane Library because it was out of date. See Other published versions of this review.
Medical Subject Headings (MeSH)
*Maternal Welfare; *Pregnancy Outcome; Anemia [blood; prevention & control]; Birth Weight; Folic Acid [*administration & dosage]; Hemoglobin A [analysis]; Micronutrients [*administration & dosage]; Pregnancy Complications, Hematologic [blood; prevention & control]; Premature Birth [prevention & control]; Randomized Controlled Trials as Topic; Stillbirth; Vitamin B Complex [*administration & dosage]
MeSH check words
Female; Humans; Pregnancy
* Indicates the major publication for the study