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Keywords:

  • adverse pregnancy outcome;
  • ALSPAC;
  • factor V Leiden;
  • fetal growth restriction;
  • genetic polymorphisms;
  • pre-eclampsia;
  • prothrombin;
  • single nucleotide polymorphism;
  • SNP

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Addendum
  9. Acknowledgements
  10. Disclosure of Conflict of Interests
  11. References

Summary. Background: Adverse pregnancy outcomes have been related to environmental and/or genetic factors. Of interest are genes associated with the clotting system as any perturbation in the balance of thrombotic and thrombolytic cascades could affect the placental circulation and hence the viability of the developing fetus. Several previous reports using relatively small numbers of cases and controls have suggested that there is a relationship between poor pregnancy outcomes and two polymorphisms, one in the factor V gene, the 1691G to A change (rs6025) located on chromosome 1q23 (factor V Leiden, FVL), and the other in the prothrombin gene, 20210G to A change (rs1799963) on chromosome 11p11-q12 (PT). These results, however, are conflicting. Methods: We genotyped 6755 mother/infant pairs from the Avon Longitudinal Study of Parents and Children (ALSPAC) to determine whether maternal or fetal FVL or PT, either alone or in combination, are associated with fetal growth restriction (FGR) or pre-eclampsia (PE). We also added the present results to previous cohort studies using meta-analysis. Results: Smoking, primiparity and lower body mass index (BMI) were all associated with FGR, but neither maternal nor fetal FVL or PT, singly or in combination, were associated with FGR in the ALSPAC cohort. Meta-analysis confirmed the lack of association between maternal FVL and FGR with a pooled odds ratio (OR) of 1.15 [95% confidence interval (CI) 0.95–1.39]. High BMI, primiparity, diabetes and chronic hypertension were all associated with pre-eclampsia. Combining ALSPAC results with previous studies in a meta-analysis indicated that maternal FVL is significantly associated with pre-eclampsia, with a pooled OR of 1.49 (95% CI 1.13–1.96). Conclusion: Neither maternal nor fetal FVL or PT, singly or in combination, are associated with FGR; this contradicts previous case–control studies and meta-analyses based on these studies. In a meta-analysis of all published cohort studies to date, maternal FVL appears to increase the risk of pre-eclampsia by almost 50%. This result is robust, homogeneous and does not appear to be affected by publication bias.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Addendum
  9. Acknowledgements
  10. Disclosure of Conflict of Interests
  11. References

Adverse pregnancy outcomes including fetal growth restriction (FGR) and pre-eclampsia (PE) remain a major cause of maternal and infant morbidity and mortality. A successful pregnancy is highly dependent on a complex interplay between a variety of maternal, fetal and placental factors. Given the importance of establishing and maintaining an adequate placental circulation, hereditary thrombophilias have been postulated as a possible cause of placental insufficiency. Despite early reports supporting an association between hereditary thrombophilias and adverse pregnancy outcomes [2–6] a number of other studies have yielded conflicting results [7–10]. To address this discrepancy, three large meta-analyses [11–13] were published to help clarify the contribution of inherited thrombophilias to the risk of FGR. Although two studies [11,12] reported an association between FGR and factor V Leiden (FVL), their results were dominated by small, case–control studies.

Several large meta-analyses summarizing the association between thrombophilia and pre-eclampsia report that FVL is associated with an increased risk of hypertensive disease of pregnancy [13–15], and especially of severe PE [11,14]. Robertson et al. [13] also reported an association between the prothrombin 20210G-A polymorphism and PE [odds ratio (OR) 2.54 95% confidence interval (CI) 1.52–4.23], but these meta-analyses reported frequent heterogeneity between studies and may be subject to publication and time-lag bias. Restricting the literature to population-based studies yields conflicting results, which may be as a result of differences in study design, study populations or power [1,9,16,18].

As placental circulation consists of both a fetal and maternal component, it is possible that hypercoagulability not only within the maternal circulation but also within the fetal placental circulation may be associated with an increased risk of adverse pregnancy outcome. A number of small case–control studies and parent triad studies evaluating an association between fetal FVL and adverse pregnancy outcomes have also produced conflicting results [7,17,19–22]. To overcome the shortfalls observed in the large number of small and possibility underpowered studies, we conducted a large cohort study evaluating the association between both the maternal and fetal FVL and prothrombin (PT) genotype and risk of FGR and PE. To increase the power of detecting an association, data from other published cohort studies were combined in the meta-analyses.

Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Addendum
  9. Acknowledgements
  10. Disclosure of Conflict of Interests
  11. References

Study population and design for the nested case-control study

The Avon Longitudinal Study of Parents and Children (ALSPAC) [23] recruited 14663 pregnant mothers with an expected date of delivery between 1st April 1991 and 31st December 1992 in the defined county of Avon in the UK. One of the main aims of the ALSPAC cohort was to promote the study of how genotype differences influence the reactions of individuals to the environment. Details of the ALSPAC study area, eligibility criteria, enrolment, data collection and questionnaire validation are reported elsewhere [23], or can be found at http://www.alspac.bris.ac.uk. Briefly, data were collected by (i) self-completion questionnaires during pregnancy and post-delivery completed by the mother and her partner, (ii) review of medical records and (iii) biological samples from the mother, her partner and child.

Outcomes

Outcomes were as follows:

  • 1
    FGR, as defined by 10th centile on gender-specific UK population charts.
  • 2
    PE, defined as a systolic blood pressure of 140 mmHg or a diastolic of 90 mmHg in addition to 3 g/24 h of proteinuria. This was assessed independent of a previous history of hypertension, although previous history was included as a confounder in the analyses for this outcome.

The outcome of stillbirth was not analyzed as a result of insufficient power.

DNA collection

Blood collection, DNA extraction and processing have been described previously [24]. Maternal DNA samples were extracted from blood collected in EDTA or heparin from mothers during their antenatal care and children’s samples from umbilical cord blood collected in heparin or from venous samples taken later in life collected in EDTA. DNA is available for approximately 70% of the cohort children. Ethical approval for the study was obtained from the ALSPAC Law and Ethics committee and the local ethics committees, as well as the Newcastle University Human Research Ethics Committee (HREC). All participants had given consent to participate in genetic association studies.

Of the initial ALSPAC sample of 14 663 pregnancies, 13 432 pregnancies were single pregnancies and had non-Down Syndrome children. DNA was available for 10 232 children, and 9117 mothers; because these were not completely overlapping, this represents 6755 mother–child pairs with DNA.

Genotyping

Genotyping on stored DNA was performed retrospectively by KBiosciences (Hoddesdon, UK) using KASPar, an allele-specific PCR system developed by the company (http://www.kbioscience.co.uk).

The variants that were genotyped were the factor V Leiden 1691G-A polymorphism (rs 6025), in F5 (coagulation factor V) gene on chromosome 1q23 and a common prothrombin variant 20210G-A (rs1799963) in the F2 [coagulation factor II (thrombin)] gene on chromosome 11p11-q12.

The genotyping failure rates for each of the groups tested were:

  • 1
    mothers: FVL 3.01% and prothrombin 3.26% and
  • 2
    children: FVL 9.58% and prothrombin 9.11%.

This difference in failure rates may be as a result of differences in sample processing. The blood samples for children were taken at follow-up visits, when they were older, and the samples taken in EDTA rather than heparin as for the mothers. In addition, many of the children’s samples were frozen unseparated at −20 °C for up to 3 weeks before DNA extraction.

A series of duplicate assays were also performed to determine the efficiency of the genotyping assays. There were 805 duplicate samples of mothers and 226 children’s samples tested and no differences in the genotyping were observed. None of the genotypes in any of the groups tested deviated from the Hardy–Weinberg equilibrium (P < 0.05).

Statistical analysis

Stata version 8.0 was used for logistic regression analysis to examine the relationship between a number of predictors and outcomes. Predictors included: (i) maternal factor V Leiden; (ii) maternal prothrombin G20210A polymorphism; (iii) fetal factor V Leiden; (iv) fetal prothrombin G20210A polymorphism; and (v) combined maternal and foetal genotype. Rare homozygous results for FVL or PT were analyzed in combination with heterozygous results. Exclusion of homozygous women or infants from the analysis did not change the results. There were no individuals homozygous for both FVL and PT. The results are presented as ORs with 95% CIs. Univariate regression analysis was used to determine possible predictors of the outcome prior to multivariate analysis. Age (<35, 35–39, ≥40 years), body mass index (BMI) (≥30, 25–29, <25), smoking (≥20, 10–19, 1–9, no cigarettes) and parity (primiparous, multiparous) were included in the FGR univariate analysis; and age, BMI, smoking, parity, diabetes (yes, no) and chronic hypertension (yes, no) were included in the PE univariate analysis.

We also analyzed a possible interaction between the maternal and fetal genotype. To reduce multiple subgroup analyses, a decision was made to combine FVL and PT genotypes together as a thrombophilic tendency. The description ‘either heterozygous’ includes mothers or infants who were heterozygous for one or both FVL or PT polymorphisms. ‘Wild type’ refers to mothers and infants who were homozygous for the wild-type FVL and PT alleles.

Meta-analysis of cohort studies

We also performed a MEDLINE and EMBASE search (up to February 2007) using the headings: (i) factor V leiden (textword) and pregnancy, OR pre-eclampsia OR fetal growth restriction (MeSH headings) and (ii) prothrombin (text word) and pregnancy, OR pre-eclampsia OR fetal growth restriction (MeSH headings). The search was limited to human studies published in English. Inclusion criteria were:

  • 1
    cohort design;
  • 2
    outcomes clearly defined as fetal growth restriction or pre-eclampsia;
  • 3
    entire cohort, or at least both nested cases and controls, tested for FVL or prothrombin G20210A mutation; and
  • 4
    sufficient data to enable the calculation of an odds ratio.

Data were extracted independently by two of the authors (Attia and Dudding) on (i) the general characteristics of the study, e.g. age ranges of mothers, ensuring that definitions of predictors and outcomes were comparable, etc; (ii) research questions and methodology and (iii) outcome data. Testing for heterogeneity was performed using the Breslow–Day method with the P-value threshold for heterogeneity set at P < 0.1, and using I2. If there was no heterogeneity, a fixed effects model was used to pool data; otherwise a random effects model was used. Study size (publication) bias was assessed using Egger’s test. Pooling was performed using StatsDirect (version 2.5.7, StatsDirect Ltd, Cheshire, UK, http://www.statsdirect.com).

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Addendum
  9. Acknowledgements
  10. Disclosure of Conflict of Interests
  11. References

Cohort study

Fetal growth restriction Among 13 601 pregnancies, 10 976 pregnancies where data for fetal growth and at least one of FVL and PT gene for the mother/child were available. These subjects were included in the analysis. Characteristics between included and non-included subjects were similar except that included mothers were slightly older and smoked less than non-included mothers (Table 1); all analyses were adjusted for these factors. Less than 5% of the cohort was of non-Caucasian ethnicity. Table 2 describes the maternal and fetal gene frequencies of FVL and PT for the pregnancies where the infant weighed < 10th centile for gestational age (based on population growth charts). As can be seen from Table 2, the gene frequencies in all the subgroups are consistent with expected population frequencies of 5% for FVL and 2% for PT. A number of predictors of FGR were explored by univariate analysis including maternal age, BMI, smoking and parity. From this analysis, smoking, parity and BMI emerged as statistically significant predictors of FGR.

Table 1.   Characteristics of mothers included and not included in the associations between genotype and fetal growth restriction (FGR) or pre-eclampsia (PE)
Maternal characteristicsFGRPE
IncludedNot includedP-valueIncludedNot includedP-value
  1. Inclusion was based solely on availability of DNA or availability of data from manual record review. BMI, body mass index.

Parity
 Primiparous4586 (44.4)1024 (45.4)0.3802199 (51.9)3481 (40.9)<0.001
 Multiparous5753 (55.7)1233 (54.6)2037 (48.1)5029 (59.1)
Smoking/day
 ≥20298 (2.8)93 (3.9)<0.001136 (77.0)262 (2.9)0.313
 10–19905 (8.5)275 (11.6)369 (11.5)831 (9.4)
 1–91149 (10.8)353 (14.8)500 (8.4)1020 (11.6)
 None8317 (77.9)1660 (69.7)3366 (3.1)6721 (76.1)
BMI
 ≥30589 (6.3)121 (6.5)0.664271 (7.0)444 (5.9)0.001
 25–291731 (18.4)327 (17.6)759 (19.7)1319 (17.6)
 <257065 (75.4)1410 (75.92828 (73.3)5735 (76.5)
Age group
 <359833 (89.6)2419 (92.2)<0.0014028 (90.0)8318 (90.1)0.695
 35–391002 (9.1)181 (6.9)389 (8.7)807 (8.7)
 ≥40141 (1.3)25 (0.9)60 (1.3)108 (1.2)
Table 2.   Genotype frequencies of (a) maternal factor V Leiden (FVL) (1691G-A) and fetal FVL (1691G-A) and (b) maternal prothrombin (PT) (20210G-A) and fetal PT (20210G-A) in relation to the presence of birth weight <10th centile
(a) Maternal FVL(b) Maternal PT
  1. Percentages are based on row totals.

Birth weight < 10th centileA/A or A/GG/GA/A or A/GG/G
33 (5.6%)554 (94.4%)16 (2.7%)575 (97.3%)
Birth weight > 10th centile 368 (5%)6914 (95%)162 (2.2%)7089 (97.8%)
Fetal FVLFoetal PT
Birth weight < 10th centileA/A or A/GG/GA/A or A/GG/G
37 (6.8%)508 (93.2%)15 (2.7%)542 (97.3%)
Birth weight > 10th centile396 (5.1%)7408 (94.9%)211 (2.7%)7655 (97.3%)

These factors were included in multivariate logistic models in which maternal and fetal genotypes were added separately. As seen in Table 3, neither maternal nor fetal prothrombotic genotypes were associated with FGR. The power of these tests to detect the effect sizes seen for FVL and PT was only 45.6% and 44.7%. Smoking, parity and BMI continued to be associated, indicating independent effects. Given the maternal and fetal contribution to placental circulation, we went on to explore a possible interaction between maternal and fetal genotypes.

Table 3.   Odds ratios for maternal factor V Leiden (FVL) and prothrombin G20210A and (FGR) and odds ratio for fetal FVL and prothrombin G20210A and fetal growth restriction (FGR): multivariate analyses allowing for all variables in the table
 MaternalFetal
  1. BMI, body mass index; CI, confidence interval; OR, odds ratio, PT, prothrombin.

CharacteristicsOR (95% CI)P-valueOR (95% CI)P-value
FVL
 A/A and A/G1.09 (0.71, 1.69)0.6931.25 (0.83, 1.89)0.286
 G/G1 1 
PT
 A/A and A/G1.37 (0.79, 2.39)0.2660.97 (0.55, 1.74)0.930
 G/G1 1 
Smoking/day
 ≥204.45 (2.86, 6.92)<0.0013.45 (2.06, 5.80)<0.001
 10–193.56 (2.70, 4.70)<0.0012.68 (1.98, 3.64)<0.001
 1–92.20 (1.67, 2.88)<0.0012.16 (1.65, 2.83)<0.001
None1 1 
Parity
 Primiparous2.18 (1.78, 2.67)<0.0012.57 (2.09, 3.16)<0.001
 Multiparous1 1 
BMI
 ≥300.66 (0.42, 1.04)0.0730.56 (0.33, 0.94)0.027
 25–290.69 (0.53, 0.91)0.0090.79 (0.60, 1.03)0.084
 <251 1 

Considering the multiple possible combinations of maternal and fetal FVL and PT (16 possible combinations) and the ensuing loss of power with this many degrees of freedom, we combined FVL and PT into one variable reflecting thrombophilia, which was positive if the individual was heterozygous for FVL or PT or both. This was supported not only on the basis of power consideration, but also on the lack of any clear difference in effect size, or indeed effect, between the two genes and FGR. The term ‘wild type’ was used to denote the homozygous normal genotype for both FVL and PT. Table 4 describes the interaction between maternal and fetal thrombophilia defined in this way in a multivariate analysis on FGR. Compared with the dyads where both had the wild type, when the mother and the fetus had a thrombophilia allele, the OR was 0.94 (0.5–1.78). When the mother had a thrombophilia allele and the infant was homozygous wild type, the odds ratio was 1.92 (95% CI 1.13–3.27, P = 0.016). When only the infant had a thrombophilia allele, there appeared to be no effect on FGR (OR = 1.09, 95% CI 0.61–1.96). Results were similar when the definition of birth weight <10th centile was based on customized growth charts, which incorporates maternal weight, height, parity and ethnic group (data not shown).

Table 4.   Maternal and fetal gene effects on fetal growth restriction (FGR): multivariate analysis
CharacteristicsOR95% CI of ORP value
  1. The description ‘either heterozygous’ includes mothers or infants who were either factor V Leiden (FVL) heterozygous or prothrombin (PT) heterozygous or both FVL and PT heterozygous. ‘Wild type’ refers to mothers and infants who were homozygous wild type at both FVL and PT alleles.

Maternal FVL-PTFetal FVL-PT   
Either heterozygousEither heterozygous0.940.50, 1.780.858
Either heterozygouswild type1.921.13, 3.270.016
wild typeEither heterozygous1.090.61, 1.960.774
wild typewild type1  
Smoking/day
 ≥20 3.711.20, 6.89<0.001
 10–19 3.232.25, 4.65<0.001
 1–9 2.051.46, 2.88<0.001
 None 1  
Parity
 Primiparous 2.531.96, 3.26<0.001
 Multiparous 1  
BMI
 ≥30 0.650.37, 1.140.131
 25–29 0.700.50, 0.980.039
 <25 1  

Pre-eclampsia As previous meta-analyses have suggested an association between thrombophilia and PE, we explored this possible association. Only 4477/14 273 pregnancies had at least one of FVL and PT genotypes and sufficient data for assessing pre-eclampsia; the latter had to be extracted manually by medical record review and this process is not yet complete for the entire ALSPAC cohort. Characteristics between those pregnancies that did/did not have data were similar except that included mothers were more likely to be primiparous and have slightly higher BMI (Table 1); all analyses were adjusted for these factors. Table 5 describes the maternal gene frequencies for FVL and PT in those who were diagnosed with PE compared with controls. As a result of small numbers and some missing data, this analysis was not run on fetal FVL and PT.

Table 5.   Genotype frequencies of maternal factor V Leiden (FVL) (1691G-A) and prothrombin (PT) (20210G-A) and pre-eclampsia (PE)
Maternal FVL 1691G-AMaternal prothrombin variant 20210G-A
  1. Percentages are based on row totals.

Pre-eclampsiaA/A or A/GG/GA/A or A/GG/G
17 (7%)226 (93%)5 (2.1%)234 (97.9%)
No pre-eclampsia204 (4.8%)4002 (95.2%)85 (2%)4091 (98%)

On univariate analysis, parity, BMI, diabetes and chronic hypertension were independent predictors of pre-eclampsia. Including these variables in a multivariate logistic model along with maternal genotype showed that heterozygosity in the mother for FVL or PT was not associated with a risk of PE with OR of 1.19 (95% CI 0.64–2.23) and 1.39 (95% CI 0.54–3.58) respectively (Table 5). The power of these tests to detect the effect sizes seen for FVL and PT was 33.1% and 3.4% respectively. Parity, BMI, diabetes and chronic hypertension continued to be associated, indicating an independent effect.

Meta-analysis of cohort studies

Fetal growth restriction In order to investigate whether or not the lack of association noted above was as a result of insufficient power (type 2 error), we pooled our results by meta-analysis with data from other published cohort studies. The study characteristics and OR for the cohort studies evaluating the risk of FGR in unselected groups of FVL positive mothers were available from six studies including ours. Nurk et al. [16] provided data from the first pregnancy of the 5874 women in the previously published Hordaland study. The combined OR for studies where FGR was defined as a birth weight <10th centile for gestational age were homogeneous (Breslow–Day P = 0.75, I2 = 0%) with a combined OR of 1.15 (95% CI 0.95–1.39) (Fig. 1). With the sample size in this meta-analysis, and assuming a power of 80%, we would have been able to detect an OR of 1.09. There was no evidence of study size (publication) bias (Egger test P = 0.83).

image

Figure 1.  Pooled odds ratio for the association between fetal growth restriction (FGR) (<10th centile) and maternal factor V Leiden (FVL).

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Only two cohort studies evaluated the effect of fetal FVL on the risk of FGR. These studies were homogeneous with a pooled OR of 1.2 (95% CI 0.88–1.63). There were insufficient studies to perform a meta-analysis of PT positive mothers or infants and FGR.

Pre-eclampsia The same six cohort studies evaluated the effect of maternal FVL on the risk of PE. These studies were homogeneous (Breslow–Day P = 0.93, I2 = 0%) and indicated a statistically significant increased risk of pre-eclampsia with a pooled OR of 1.49 (95%CI 1.13–1.96, P = 0.003) (Fig. 2). There was no evidence of study size (publication) bias (Egger test P = 0.20).

image

Figure 2.  Pooled odds ratio for the association between pre-eclampsia (PE) and maternal factor V Leiden (FVL).

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Two studies evaluated the effect of maternal PT on the risk of PE. The studies were homogeneous with a pooled OR of 0.96 (95% CI 0.48–1.88). There were insufficient studies to perform a meta-analysis of fetal FVL or PT and PE (Table 6).

Table 6.   Odds ratio for maternal factor V Leiden (FVL) and prothrombin (PT) and pre-eclampsia (PE): multivariate analysis total pooled odds ratio (OR) (heterogeneity = 0.7)
CharacteristicsOR95% CI of ORP-value
  1. BMI, body mass index; HT, hypertension.

FVL
 A/A and A/G1.190.64, 2.230.586
 G/G1  
PT
 A/A and A/G1.390.54, 3.580.500
 G/G1  
BMI
 ≥301.921.23, 3.000.004
 25–291.120.77, 1.620.550
 <251  
Parity
 Primiparous2.321.66, 3.24<0.001
 Multiparous1  
Diabetes
 Yes4.182.00, 8.73<0.001
 No1  
Chronic HT
 Yes5.383.78, 7.66<0.001
 No1  

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Addendum
  9. Acknowledgements
  10. Disclosure of Conflict of Interests
  11. References

Fetal growth restriction

Overall the results of our ALSPAC cohort study show no statistically significant association between maternal FVL, maternal PT, fetal FVL or fetal PT and birth weight <10th centile. The one statistically significant OR of 1.92 (1.13–3.27) (Table 4) when the mother has a thrombophilia (heterozygous for fVL and/or PT) and the infant is wild type at both loci may represent a true association; however, the result is inconsistent with the other results and is more likely to represent a chance statistically significant result owing to the multiple number of analyses. Furthermore, our fetal growth restriction (FGR) cohort meta-analysis does not support this lone result and finds no evidence of an effect of maternal FVL on FGR. Given the size of the pooled sample, we had 80% power to detect an OR of 1.09, indicating that if there is an effect of FVL on FGR that has been missed by this meta-analysis then it must be quite small clinically. It is also possible that there is an association between thrombophilia and more severe FGR, e.g. <3rd centile, but the size of cohort required to test this hypothesis would need to be extremely large as a result of the small number of infants with this outcome.

The result of this cohort meta-analysis contradicts the conclusion of previous large meta-analyses [11,12], which noted an association between FVL and FGR. However, these previous meta-analyses reported statistically significant heterogeneity between studies, possibly related in part to study designs, i.e. case–control versus population cohorts. There are a number of possible explanations for this discrepancy between the cohort studies and case–control studies, which include biased recruitment of cases, poor selection of controls and interaction effects. It is possible that although cases with FGR or PE are enriched for the FVL or PT mutations compared with controls, this effect is manifest because of other unmeasured genotypes or clinical factors [6,22]; this comes about because cases are selected based on having had an adverse event. In other words, FVL or PT are only part of the high-risk ‘profile’ but because the rest of the factors are not measured, the entire risk is assigned to the FVL or PT mutation. Those with FVL or PT but no other unmeasured risk factors do not have events and are unrepresented in the case–control studies. Conclusions about PT gene effects are limited because of a lack of studies and there is room for further investigation in this area. There is also a dearth of studies addressing the effect of fetal thrombophilia on pregnancy outcomes.

The results of our ALSPAC cohort study confirm previous reports that smoking, parity and BMI are independent predictors of FGR [25,26].

Pre-eclampsia

Overall, the results of previous meta-analyses, based largely on case–control studies, show an association between PE and FVL, particularly in relation to severe PE [11,13–15]; however, a previous meta-analysis limited to cohort studies, reflecting an unselected group of women, showed no association between FVL and PE with an OR of 1.1 (95% CI 0.4–2) [11]. Although the results of our ALSPAC cohort study show no statistically significant association between maternal FVL, maternal PT and PE, increasing the power by combining our study with other cohort studies by meta-analysis confirmed a positive association between maternal FVL and pre-eclampsia with an OR of 1.49 (95% CI 1.13–1.96, P = 0.003). There is insufficient data to make any firm conclusions about maternal PT or fetal thrombophilias and PE.

The results of our ALSPAC cohort study also confirm previous reports that BMI, parity, diabetes and chronic hypertension are independent predictors of PE [27].

Clinical implications

The results of this study and the meta-analysis of cohort studies are important in a number of contexts:

1) Women with FVL or PT mutations identified through cascade testing. These results support the evidence that maternal FVL is an independent risk factor for PE. The magnitude of this risk may vary depending upon the presence of other environmental or genetic risk factors, which means that a pregnancy management plan should be formulated on an individual basis combining history of other known risk factors and evaluation for multiple thrombophilia. Whether or not FVL carrier women should just be monitored more closely or treated during pregnancy remains to be determined. There appears to be no increased risk of FGR for FVL carriers. There is still insufficient data specifically relating to the risk of PT carriers.

2) Screening women who have experienced an adverse pregnancy outcome. There are no studies evaluating the subsequent risk of PE in a second pregnancy for women carrying an inherited thrombophilia compared with the subsequent risk of PE in a second pregnancy of women without a thrombophilia. However, these results support the screening of women with a previous history of PE for FVL. Whether those who test positive should simply be monitored or treated during their subsequent pregnancy remains to be determined. There is insufficient data to recommend any course of action regarding PT genotyping.

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Addendum
  9. Acknowledgements
  10. Disclosure of Conflict of Interests
  11. References

We find that previous estimates of FVL increasing the risk of FGR were driven largely by small case–control studies and are not supported by our cohort study or our meta-analysis of cohort studies. Conversely, the results from our study combined with other cohort studies by meta-analysis does support an association between maternal FVL and PE, and this is consistent between case–control and cohort studies. The effects of fetal thrombophilia on these outcomes are yet to be clarified.

Addendum

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Addendum
  9. Acknowledgements
  10. Disclosure of Conflict of Interests
  11. References

While this manuscript was being submitted and reviewed, another large cohort study addressing the association between FVL and poor pregnancy outcomes was published online [Clark P et al. The GOAL study: a prospective examination of the impact of factor V Leiden and ABO(H) blood groups on haemorrhagic and thrombotic pregnancy outcomes. Br J Haematol (published online 19 November 2007)]. Consistent with our findings, the authors found no association between FVL and fetal growth restriction (defined as <5th centile) with an OR = 0.91 (95% CI 0.32–2.58) but did find a higher point estimate for association with PE, although this did not reach significance (OR = 1.83, 95% CI 0.51–6.13).

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Addendum
  9. Acknowledgements
  10. Disclosure of Conflict of Interests
  11. References

We are extremely grateful to all the families who took part in this study, the midwives for their help in recruiting them, and the whole ALSPAC team, which includes interviewers, computer and laboratory technicians, clerical workers, research scientists, volunteers, managers, receptionists and nurses. The UK Medical Research Council, the Wellcome Trust and the University of Bristol provide core support for ALSPAC. This publication is the work of the authors and J. Heron and A. Thakkinstian will serve as guarantors for the contents of this paper. This research was specifically funded by the National Health and Medical Research Council of Australia, grant number 858908.

Disclosure of Conflict of Interests

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Addendum
  9. Acknowledgements
  10. Disclosure of Conflict of Interests
  11. References

The authors state that they have no conflict of interest.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Addendum
  9. Acknowledgements
  10. Disclosure of Conflict of Interests
  11. References
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