To study whether a routine with a routine ultrasound examination (routine scan) at 41 gestational weeks as compared with ultrasound on clinical indication (indicated scan), lowered the risk of severe adverse fetal outcome in post-term period.
A retrospective cohort study.
Karolinska University Hospital, Stockholm, Sweden.
Eight years of deliveries, 2002–2009.
One of the two delivery units at Karolinska University Hospital used a routine scan at 41 week of gestation and the other unit used an indicated scan. Severe adverse fetal outcome were defined: severe asphyxia, death or cerebral damage. The study was analysed using logistic regression with adjustment for potential confounders.
Main outcome measures
Differences in post-term severe adverse fetal outcome.
No increased risk of post-term severe adverse fetal outcome was seen at the unit using a routine scan; conversely, a 48% significantly increased risk was seen at the unit using an indicated scan (OR 0.89, 95% confidence interval, CI, 0.5–1.5 and OR 1.48, 95% CI 1.06–2.1, respectively). Comparing post-term periods, there was no significantly increased risk at the unit using indicated scans (OR 1.6, 95% CI 0.9–3.0). There was a 60% increased prevalence of small-for-gestational age (SGA) newborns in the post-term period at the unit using indicated scans (OR 1.6, 95% CI 1.1–2.4), but no differences in operative delivery.
A policy to use routine scans at 41 weeks of gestation seems to normalise an increased post-term risk of severe adverse fetal outcome, possible due to increased awareness of SGA and/or oligohydramniosis.
Intrauterine growth restriction (IUGR) is a complication of pregnancy that occurs during the second half of 3–5% of all pregnancies. Fetuses that are IUGR are more prone to serious complications such as severe fetal distress, cerebral damage, long-term neurological sequelae, and death.[2-5] A large proportion of small-for-gestational age (SGA) fetuses are IUGR, and the majority of IUGR are also SGA. It has been shown that antepartum identification of SGA, as compared with non-identification and the employment of a surveillance strategy, was related to a four-fold lower risk of severe adverse fetal outcomes. The number needed to treat (NNT) was 11, that is for every 11 cases of SGA identified prenatally, one instant of severe adverse fetal outcome could be avoided. Many obstetric units use SGA as a proxy for IUGR and have a programme for identifying SGA pregnancies before delivery in order to lower perinatal morbidity and mortality.
Postdate pregnancies have been reported to be at an increased risk of fetal death, a risk that was specifically attributed to fetuses found to be SGA. Therefore, beginning in 1998, one of the two delivery sites at Karolinska University Hospital (Karolinska Huddinge) introduced screening by ultrasound examination with fetometry and measurement of amniotic fluid index (AFI) at 41 weeks of gestation weeks, whereas the other (Karolinska Solna) used ultrasound fetometry on clinical indication. The main object was to lower the increased risk of severe adverse fetal outcomes in post-term pregnancies. Depending on the results of ultrasound examination, the degree of surveillance, timing of delivery and mode of delivery were determined.[5, 7-9]
The aim of the present study was to elucidate whether a policy with a routine scan in gestational week 41, as compared with an indicated scan, lowered the risk of post-term severe adverse fetal outcomes
To compare the effect of routine versus indicated ultrasound protocols between the two delivery units of Karolinska University Hospital, all pregnancies delivered above 32 weeks of gestation without chromosome aberrations or major malformations during an 8-year period (January 2002–December 2009) were evaluated in this study. Only those who were AGA or SGA were included in the study. The main outcome of this study was the difference in the composite variable ‘severe adverse fetal outcomes’ (see below) in post-term pregnancies (>293 days of gestation). The study was approved by the Ethics Committee of Karolinska University Hospital (No. 2010/998).
All pregnant women were invited to an ultrasound examination without charge at 18–20 weeks of gestation for a delivery dating scan. About 97% of these women participated in this screening. Ultrasound fetometry in the third trimester was carried out on clinical indication in both units (e.g., suspected small fetus) but pregnant women at Huddinge were also scheduled for a late routine scan including fetometry and measurement of AFI at 41 gestational weeks free of charge. On that occasion, the fetus was measured for biparietal diameter (BPD), abdominal diameter (AD), and femur length. Estimated and expected fetal weight was assessed using Swedish reference algorithms. Fetal weight deviation was calculated and expressed as a percentage from expected weight-for-gestational age (predictive weight less expected weight-for-gestational age)/(expected fetal weight-for-gestational age). AFI was defined as the sum of the deepest pocket of amniotic fluid in all quadrants. Oligohydramnios (AFI < 50 mm) in the postdate period was usually considered an indication for induction. Newborn weight deviation was categorised as: between <+22% and >−16.5% as the AGA group; between ≤−16.5% and ≥−22% as the 10th percentile group and those <−22% as SGA group (approximately 4% of the population). Intrauterine-estimated SGA (eSGA) were those fetuses estimated at ≤−22% fetal weight deviation (i.e. true SGA and false positive SGA).
Management of SGA pregnancies
Women with eSGA were examined by means of umbilical artery blood flow measurements. Umbilical artery pulsative index (PI) was estimated, and a PI <mean +2 SD above the mean was considered normal, that is blood flow class (BFC) 0. PI ≥+2 SD was classified as BFC I and women with PI above 3 SD as BFC II. The absence of blood flow velocity throughout diastole or reversed diastolic flow was classified as BFC III. BFC III was always regarded as an indication for caesarean delivery. BFC II was often regarded as an indication for induction. Cases with BFC I were regarded as a relative indication for induction. Cervical ripening status together with obstetric history (earlier vaginal delivery or caesarean) could affect the mode of delivery at the discretion of the clinician on duty. Pregnancies with eSGA were not scheduled to continue into 42 weeks of gestation. During a vaginal delivery, fetuses considered eSGA were regarded as at high risk and were scheduled for continuous cardiotocography (CTG) throughout delivery.
Management of post-term pregnancy in Sweden
Management of post-term pregnancies differs in Sweden, partly due to a lack of knowledge. There is an overall acceptance that if pregnancies are allowed to go beyond 42 + 0 weeks, surveillance is needed. Most units practise planned inductions from around 42 + 0 gestational weeks. Routine scan at 41 weeks is not routine and is considered controversial. There are no other differences in protocols between the two compared units.
Construction of study groups and details on retrospective assignment
We followed a predetermined sequence in our investigation. First, the local registry was searched (both the computerised delivery system, and the records at our neonatal ward).
All relevant pregnancies were scrutinised and missing data regarding birth weight, Apgar score, pH, and gestational age at birth was entered. As far as possible, this work was conducted blind as to severe adverse fetal outcomes and delivery site.
The expected birth weights and birth weight deviations were calculated using the Swedish reference algorithm.
Local data on stillbirth were complemented by information in the National Birth Registry, where over 99% of all deliveries in Sweden are recorded.
In order to determine deaths during the first year after delivery, we searched the local registry at the delivery and neonatal units. In addition, using birth date within the study period, we also searched the National Death Registry, where all deaths in Sweden are registered. By this means, all infants who died during the first year of life were identified and cross-matched with those born during the study period.
The National Discharge Registry was cross-matched in order to identify all infants diagnosed with cerebral palsy.
We calculated the main outcome variable risk of severe adverse fetal outcomes as a composite variable of at least one of the following: cerebral damage (hypoxic ischaemic encephalopathy grades 2 or 3, intracranial haemorrhage, cerebral palsy), severe asphyxia (Apgar score <4 at 5 minutes, neonatal seizures, umbilical blood pH ≤7.0), or death (stillborn or death during first year of life). One pregnancy with one or several adverse outcomes was included as one severe adverse fetal outcome.
Finally, data on marital status, education, country of birth, and disposable income were entered from Statistics Sweden.
The main outcome was the difference in risk of a ‘severe adverse fetal outcome’ in post-terms pregnancies, adjusted for possible confounders such as maternal age, BMI, parity, marital status, education, country of birth, and disposable income.
The ICD9 or corresponding ICD10 diagnosis numbers defined as severe adverse fetal outcome were; cerebral palsy (G80* and G81*), intraventricular haemorrhage (772B, 772C), subdural haemorrhage (767A), neonatal seizures (779A), hypoxic ischaemic encephalopathy grades II or III (779 B, C), stillbirth (656E, 768A, 768B), or deaths during the first year of life. Data on parity were those obtained at a woman's first visit to the Maternal Health Service, usually at about 12–13 weeks of gestation. Data regarding maternal age, Apgar, umbilical pH, mode of delivery, and gestational age were routinely obtained at birth. Data regarding marital status, education, country of birth, and disposable income were gathered from Statistics Sweden for the year before delivery.
Stillborn was defined as intrauterine or intrapartum death. Postpartum death was defined as death after delivery but prior to 1 year of age (neonatal death or infant death). Marital status was dichotomised into married or not married. Income was categorised into as high, median, or low. Education was categorised as ≤9 years, >9 and ≤12 years, or >12 years. Country of birth was categorised as Nordic, European, or non-European.
Odds ratios (OR) were used as a measurement of relative risk. Bivariate ORs were calculated with cross-tabulation and their 95% confidence intervals (CIs). Adjusted ORs were calculated with logistic regression analysis using serious fetal adverse outcome as a dependent variable and routine/indicated scan (i.e. intention to use or not use routine ultrasound screening at 41 gestational weeks) and other possible confounders as independent variables such as maternal age, BMI, parity, marital status, education, country of birth, and disposable income. All variables with a P value <0.1 were entered in the model. With a combination of backward and forward logistic regression analysis the model was fitted. P-values <0.05 were considered statistically significant and were included in the final model. No interactions were found. The data were analysed using SPSS software (Statistical Package Social Sciences Inc., Chicago, IL, USA). P-values <0.05 were considered statistically significant.
Of the total number of 77 768 pregnancies, 69 206 fetuses (91.4%) were appropriate-for-gestational age (AGA) pregnancies; 2834 (4.4%) were SGA; and 3884 (5.0%) large-for-gestational age (LGA), according to the Swedish definition. Those who were AGA or SGA were included in the study (n = 71 040; Huddinge 33 708 and Solna 37 342).
There were significant differences between the two units for maternal age, BMI, and the percentage of primiparas (Table 1). Additionally, gestational age and birth weight differed significantly between the two delivery sites. In Table 2 the distribution of severe adverse fetal outcome between the two sites is shown. Primiparous women, maternal age >35 years, and women with elevated BMI were at increased risk of severe adverse fetal outcomes (OR 1.3, 95% CI 1.2–1.4; OR 1.3; 95% CI 1.1–1.5; and OR 1.6, 95% CI 1.2–1.9, respectively). Marital status, education and country of birth were not significantly correlated to severe adverse fetal outcome. Disposable income was significantly correlated in some comparisons and was included in all adjustments. As the effect was not linear, disposable income was categorised.
Table 1. Background characteristics of the two delivery sites at Karolinska University Hospital one using a routine scan at 41 gestational weeks and the other an indicated scan
SGA, small-for-gestational age.
Number and percentage or mean and standard deviations are given.
Body mass Index (kg/m2)
Maternal age (years)
Country of birth
Gestational age at delivery (weeks)
Preterm 32–36 + 6 (n)
Term 37–41 + 6 (n)
Post-term ≥42 + 0 (n)
Gestational age at delivery (days)
Weight deviation (%)
Table 2. Severe adverse fetal outcome for the whole study population and delivery unit and routine scan at 41 weeks versus indicated scan
Adjusted for maternal age, parity, disposable income, and BMI.
Mode of delivery
Normal vaginal delivery (n)
Instrumental vaginal delivery (n)
Caesarean delivery (n)
When comparing newborn size (AGA, 10th percentile, or SGA groups) of all pregnancies, the 10th percentile groups were related to a more than double the risk of severe adverse fetal outcomes (OR 2.3, 95% CI 1.6–3.2; and OR 2.4, 95% CI 1.7–3.3, at Huddinge and Solna, respectively). The SGA group were related to a five-fold increased risk (OR 4.8, 95% CI 3.6–6.4; and OR 5.7, 95% CI 4.4–7.3, respectively) compared with the Huddinge AGA group (Figure 1).
Compared with term pregnancies, post-term fetuses were at similar risk at the unit with a routine scan (OR 0.89, 95% CI 0.5–1.4). However, post-term pregnancies at the unit with an indicated scan were at a 48% significantly increased risk (OR 1.48, 95% CI 1.06–2.1) compared with term pregnancies (Table 3 and Figure 2). There were no significant differences in the risk of severe adverse fetal outcome when comparing the post-term periods in unit with an indicated versus a routine scan (OR 1.60, 95% CI 0.9–2.9).
Table 3. Severe adverse fetal outcome in term and post-term pregnancies in relation to routine or indicated scan
There was a 60% increased risk of delivering a SGA newborn in the post-term period at the unit with an indicated scan than with a routine scan (Table 3). However, there were no SGA cases with severe adverse fetal outcomes at either site. In absolute numbers, there were 29 fewer post-term newborns (12 versus 41) at the unit with a routine scan with severe adverse fetal outcomes, that is more than three newborns with severe adverse fetal outcomes per year. There were 95 children diagnosed as having cerebral palsy at the two delivery units combined during the study period; 47 of these were of the spastic diplegia or quadriplegia type, mostly related to birth asphyxia. In the post-term period there were no cases of cerebral palsy at the unit using a routine scan, but three at the unit using an indicated scan, of which two were of the asphyxia-related type.
It is beyond the scope of this paper to make a complete cost-effectiveness analysis of routine ultrasound screening at 41 gestational weeks. However, we have done a short calculation to assess its merits, as follows.
If we could avoid two cases of post-term severe adverse fetal outcomes out of 5000 deliveries per year, then we estimate that Sweden as a whole (about 100 000 deliveries per year) may be able to avoid 40 post-term cases annually. As 20% of pregnancies reach the 41st gestational week, 20 000 examinations per year would be needed in Sweden. At a cost of UK£60per routine examination (including fetometry, AFI, and umbilical artery Doppler when indicated) it would cost UK£1.2 million pounds/year.
There is a broad spectrum of severity among cases with severe adverse fetal outcomes, ranging from complete recovery to a severe case of cerebral palsy. From the distribution of cases, it appears that cerebral palsy constitutes approximately 10% of all cases with severe adverse fetal outcomes (10% of 40, four). If we were to consider the total cost of avoiding four cases of cerebral palsy per year in Sweden, the cost of each cerebral palsy case that could be avoided would be UK£0.3 million.
A typical compensation for a normal cerebral palsy case in Sweden is 8–9 million Swedish kronor (Patient Insurance LÖF) (approximately UK£0.6 million), mainly compensation for loss of future income but excluding costs that are covered by the social security system such as the need for health care, the cost of personal assistance, special education, modification of living space and transportation. The excluded costs might be considerably higher in severe cases. A case of severe cerebral palsy in the UK currently costs about UK£5 million (UK NHS Litigation Office).
At the unit using a routine 41-week scan, post-term pregnancies were at similar risk to term births; conversely, those at the unit using an indicted scan at 41 weeks were at a 48% significantly increased risk of severe adverse fetal outcome compared with term pregnancies. However, when comparing post-term periods between the two units there was not a significantly increased risk of severe adverse fetal outcome among post-term newborn at the unit using an indicated versus a routine scan (OR 1.60, 95% CI 0.9–2.9); in addition there were no differences between individual components of the composite outcome. The risk of giving birth of a newborn being SGA or in the 10th percentile group post-term was significantly increased at the unit with an indicated scan.
Small-for-gestational age fetuses at both units had a five-fold increased risk for severe adverse fetal outcome neonatal outcomes, whereas there was a two- to three-fold increased risk in the 10th percentile group was. Thus, an increased awareness of both oligohydramniosis and SGA seems to act towards normalising an increased post-term risk in the unit where scans are performed routinely. These estimations are in accordance with the south Swedish results showing normalisation of fetal outcome with antepartum awareness of SGA.
Several studies of routine ultrasound fetometry in late pregnancy have been evaluated in a Cochrane review. However, none of those studies were designed to assess differences in severe adverse neonatal outcomes, and no study focused on post-term pregnancies. In the eight studies with a total of 27 024 patients included in that meta-analysis, the authors found no difference in antenatal, obstetric or neonatal interventions or morbidity between the screened and the control groups. In a trial by McKenna and co-workers, approximately 2000 low-risk cases were randomised to either standard care or standard care plus growth scans at 31–32 and 36–37 weeks of gestation. The authors found a non-significant 17% lower risk of neonatal admissions in the intervention group (2.8 versus 3.4%; P =0.053), findings well in agreement with ours.
Strengths and limitations
The present retrospective study does not have the advantages of a prospective randomised controlled trial design, but the size of our study allowed us to have severe adverse fetal outcomes as the main outcome rather than using milder outcomes as a proxy for severe outcomes. Our findings are of epidemiological evidential type and do not show causality.
The findings may serve for power calculations in designing a prospective randomised controlled trial. Although we adjusted for several possible confounders, we cannot exclude the presence of other unmeasured bias. There was an approximately 65% higher incidence of post-term pregnancies at the unit using an indicated scan than at the unit using a routine scan (7.1 versus 4.3%, respectively), for which we have no explanation. There were no other differences in the protocols for postdate pregnancies other than the policy to use a routine/indicated scan. A strength of the study is the examination of a large population from within a single department with two delivery units, with protocols differing only in routine/indicated scan, where approximately 10% of all pregnant women in Sweden are delivered. The study was analysed as an intention to treat assessment comparing two different policies at a group level. With the use of a composite main outcome, both short- and long-term outcomes could be evaluated in combination. Using OR as a measure of relative risk might result in a somewhat too high estimate, but with the low incidence of severe adverse fetal outcome (1%) this is acceptable. One limitation of the study is that there was no strict predetermined strategy on what to do with the varying results of the routine scan at 41 gestational weeks. Further, we do not have access to answers to all ultrasound examinations and we could therefore not make valid comparisons between AFI measurements and biometry. Finally, the larger percentage of missing pH at the unit using an indicated scan might have underestimated differences between the units.
We speculate that the most plausible explanation for the difference found is that fewer cases at increased risk (SGA, 10th percentile groups, and presumably those with AFI <50 mm) entered the post-term period, which might lower the post-term risk. Unfortunately, we were not able to differentiate between the effect of AFI measurement and biometry. As there were no cases in the post-term period among SGA newborns at either unit, we regarded this as a success for both units. These data raise the question of which definition of SGA to use. In many countries a definition of SGA as the 10th percentile group is applied, which might be more appropriate.
A basic economic evaluation suggests that ultrasound screening at 41 weeks of gestation may constitute a cost-effective procedure for units that accept deliveries extending into the post-term period, even without taking 90% of the adverse fetal outcomes into consideration. In addition, when comparing the two models, one should subtract costs for the ultrasound examinations on clinical indication (not considered in our calculation).
A policy of routine screening ultrasound at 41 weeks of gestation seems to avoid the increased post-term risk of severe adverse fetal outcome and lower the presence of SGA fetuses, without increasing the risk of operative delivery. Bearing in mind the high personal and economic consequences of cases of severe adverse fetal outcome, more knowledge is needed on how to reduce the risk of severe delivery complications.
Disclosure of interests
The authors declare that they have no competing interests to disclose.
Contribution to authorship
PLT, AM, and KP designed the study. MK, PLT and LN extracted data. AM and PLT conducted the literature search and drafted the article. All authors contributed to the final text on the interpretation of data and critical review.
Details of ethics approval
The study was approved by the Ethics Committee of Karolinska University Hospital (No. 2010/998).
The study was funded by Clintec, Karolinska Institute, Stockholm, and by SLL Stockholm county Council.
Routine ultrasound examination at 41 weeks of gestation does not improve perinatal outcomes
Washington University School of Medicine, St. Louis, MO, USA
Mini commentary on ‘Routine ultrasound examination at 41 weeks of gestation and risk of postterm severe adverse fetal outcome’
The debate on the potential benefits of routine ultrasound at any gestational age remains unresolved despite almost 40 years of using this technology in prenatal care. In theory, routine ultrasound can detect not only fetal structural abnormalities but also suboptimal growth and lead to interventions that aim at preventing adverse perinatal outcomes.
The results of the largest randomised trial on the effect of routine ultrasound on perinatal outcomes were disappointing (Ewigman et al. N Engl J Med 1993;329: 821–7). This was the Randomized antenatal Diagnostic Ultrasound Study group (RADIUS), which showed no difference in perinatal outcomes between the group that received routine ultrasound and the control group that had selective use of ultrasound. The low incidence of detected fetal anomalies in the RADIUS report was concerning and other limitations of the study resulted in a low clinical impact.
The last Cochrane database review on this subject evaluated eight clinical trials including 27 024 women investigating the role of routine scanning after 24 weeks. The review found that routine ultrasound in low-risk or unselected populations conferred no antenatal or neonatal benefit (Bricker et al. Cochrane Database Syst Rev. 2008: CD001451). However, the review was not focused on the gestational age period of 41 weeks or more, as did the present study by Lindqvist et al.
The main findings from the current study – that routine ultrasound at 41 weeks resulted in a lower incidence of small for gestational age (SGA) fetuses and lower adverse fetal outcome – should be interpreted with some caution. Importantly, it is a retrospective study design with its inherent predisposition to undocumented biases. For example, the lower rate of SGA pregnancies may be secondary to earlier interventions such as induction of labour by providers in cases with estimated fetal weight trending towards the SGA range. This is illustrated by the finding of a higher incidence of post-term pregnancies in the centre using indicated scanning compared with the routine ultrasound group (7.1 versus 4.3%, respectively). In addition, the system of categorising the Doppler abnormalities and the existing protocol for interventions (cesarean delivery versus labour induction) in both the centres compared should be considered when trying to generalise the findings to other populations. Although another potential value of routine ultrasound is the potential detection of macrosomia, that was not the focus of this study.
The authors identified other areas for potential bias and discussed limitations of the study, and they should be commended for their effort. It is very easy to suggest a randomised controlled trial devoted to this specific question as the most informative clinically, but given the large number of subjects that would need to be enrolled for any meaningful results, and the potential cost of such an endeavour, studies such as this one are reasonable surrogates. This approach, where centres offering routine scanning at 41 weeks are compared, resulting in a quasi-randomised study, may be a reasonable alternative and could help determine the need for – and design of – individual or cluster randomised trials.