Risk of stillbirth after 37 weeks in pregnancies complicated by small-for-gestational-age fetuses

doi:10.1016/j.ajog.2013.02.030

American Journal of Obstetrics and Gynecology

Volume 208, Issue 5, May 2013, Pages 376.e1-376.e7

Presented in an oral format at the 33nd annual meeting of the Society of Maternal Fetal Medicine, San Francisco, CA, Feb. 11-16, 2013.

https://doi.org/10.1016/j.ajog.2013.02.030 Get rights and content

Objective

The evidence for delivering small-for-gestational-age (SGA) fetuses at 37 weeks remains conflicting. We examined the risk of stillbirth per week of gestation beyond 37 weeks for pregnancies complicated by SGA.

Study Design

Singleton pregnancies undergoing routine second trimester ultrasound from 1990-2009 were examined retrospectively. The risk of stillbirth per 10,000 ongoing SGA pregnancies with 95% confidence intervals (CIs) was calculated for each week of gestation ≥37 weeks. Using a life-table analysis with correction for censoring, conditional risks of stillbirth, cumulative risks of stillbirth per 10,000 ongoing SGA pregnancies and relative risks (RRs) were calculated with 95% CIs for each week of gestation.

Results

Among 57,195 pregnancies meeting inclusion criteria the background risk of stillbirth was 56/10,000 (95% CI, 42.3−72.7) with stillbirth risk for SGA pregnancies of 251/10,000 (95% CI, 221.2−284.5). The risk of stillbirth after the 37th week was greater compared with pregnancies delivered in the 37th week (47/10,000, 95% CI, 34.6−62.5 vs 21/10,000, 95% CI, 13.0−32.1; RR, 2.2; 95% CI, 1.3−3.7). The cumulative risk of stillbirth rose from 28/10,000 ongoing SGA pregnancies at 37 weeks to 77/10,000 at 39 weeks (RR, 2.75; 95% CI, 1.79−4.2). Among pregnancies complicated by SGA <5% the cumulative risk of stillbirth at 38 weeks was significantly greater than the risk at 37 weeks (RR, 2.3; 95% CI, 1.4−3.8).

Conclusion

There is a significantly increased risk of stillbirth in pregnancies complicated by SGA delivered after the 37th week. Given these findings, we advocate a policy of delivery of SGA pregnancies 37-38 weeks.

Section snippets

Materials and Methods

We conducted a retrospective cohort study within our prospectively collected perinatal database at Washington University School of Medicine. Before initiation of the study, we obtained approval from the institutional review board. Sociodemographic, obstetric, medical and pregnancy follow-up data were obtained via self-report questionnaires at the initiation of prenatal care, at the time of ultrasound and from the hospital medical record on delivery. Detailed patient information was entered into

Results

Among 57,195 pregnancies meeting inclusion criteria, the background risk of stillbirth was 56/10,000 (95% CI, 42.3−72.7). SGA complicated 4217 (7.4%) pregnancies, of which 3333 (5.8%) delivered ≥37 weeks. Table 1 demonstrates relevant demographic information of the study cohort including the SGA pregnancies delivered <37 weeks, the SGA pregnancies delivered ≥37 weeks and the non-SGA population. The primary concern of this investigation was the SGA group delivered ≥37 weeks of which the average

Comment

Our study demonstrates a 2-fold risk of stillbirth after the 37th week of gestation compared with pregnancies delivered in the 37th week. The cumulative risk of stillbirth for each week of gestation beyond 37 weeks increased. At 39 weeks and ≥40 weeks, the stillbirth risk was nearly 3-fold and 7-fold. The relative risk of stillbirth at 38 weeks reached statistical significance when the SGA threshold was set at the 5th percentile. Although, the risk of stillbirth in SGA pregnancies has been

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Recent studies have demonstrated that a routine third-trimester ultrasound scan may improve the detection of small for gestational age infants when compared with clinically indicated ultrasound scans but with no reported reduction in severe perinatal morbidity. Establishing the optimal gestational age for the third-trimester examination necessitates evaluation of the ability to detect small for gestational age infants and to predict maternal and perinatal outcomes. Intrauterine growth restriction most often corresponds with small for gestational age infants associated with pathologic growth patterns.
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This was an open-label, randomized, parallel trial conducted in Upper Normandy, France, from 2012 to 2015. The study eligibility criteria were heathy, nulliparous women older than 18 years with gestational age determined using the crown-rump length at the first trimester routine scan and with no fetal malformation or suspected small for gestational age fetus at the routine second trimester scan. Pregnant women were randomly assigned to a third-trimester scan group at 31 weeks gestational age ±6 days (early ultrasound scan) or at 35 weeks gestational age ±6 days (late ultrasound scan). The primary outcome of this trial was the ability of a third trimester scan to predict small for gestational age infants (customized birth weight <10th percentile) and intrauterine growth restriction (customized birth weight <third percentile) using birth weight as the gold standard. The purpose of these adjustments was to optimize the detection of fetal weight associated with pathologic growth patterns.
It was calculated that a sample size of 3720 women would be required to obtain 80% power at a 2-sided level of 0.05 with a 15% difference in sensitivity between the 2 intervention group to detect small for gestational age fetuses in favor of the late ultrasound scan group and considering that small for gestational age infants would represent 10% of all live births.
Secondary outcomes were maternal and perinatal morbidities with interventions reported. The analysis was based on the intention-to-treat principle.
Results from 1853 women assigned to the early ultrasound scan group and 1848 women assigned to the late ultrasound scan group were analyzed. The sensitivity was found to be higher in the late ultrasound scan group than in the early ultrasound scan group, both for identifying small for gestational age infants (27%; 22%–32% vs 17%; 13%–22%; P=.004) and intrauterine growth restriction (44%; 35%–54% vs 18%; 11%–27%; P<.001). There was little difference in the specificity between the late ultrasound scan and early ultrasound scan groups in identifying cases of small for gestational age (97%; 96%–98% and 98%; 97%–99%, respectively; P=.04) and intrauterine growth restriction (96%; 95%–97% and 97%; 96%;–97%, respectively; P=.24). Overall, the maternal and neonatal outcomes were comparable between the early ultrasound scan and late ultrasound scan groups with the exception of additional (at least 1) ultrasound scans performed (25% in the early ultrasound scan group vs 19% in the late ultrasound scan group; P<.001). Rates of perinatal death (0.4% vs 0.8%; P=.12) and adverse perinatal outcomes (1.8% vs 2.7%; P=.08) were comparable between the early ultrasound scan and late ultrasound scan assigned groups, and the overall sensitivity to detect small for gestational age infants and intrauterine growth restriction, including in the last ultrasound scan performed before delivery, were also similar (30%; 25%–36% vs 26%; 21%–31%; P=.23; and 50%; 40%–60% vs 38%; 28%–48%; P=.07).
A late ultrasound scan performed in the third trimester increases the probability of detecting small for gestational age infants and intrauterine growth restriction with fewer additional scans reported than for the early ultrasound scan group. The overall perinatal outcome risk was comparable between the 2 groups. However, the overall sensitivity for detecting small for gestational age fetuses and intrauterine growth restriction, including in the last ultrasound scan performed before delivery, remains comparable between the late ultrasound scan and early ultrasound scan groups.
Risk factors for stillbirth at term: an Italian area-based, prospective cohort study
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Stillbirth at term has great emotional impact on both parents and professionals. In developed countries, efforts to identify risk factors are mandatory to plan area-specific prevention strategies.
The aim of the study was to identify independent risk factors that contribute to stillbirth at 37 weeks’ gestation or later.
This was an area-based, prospective cohort study on pregnancy at term with enrolled from 2014 to 2021 in Emilia-Romagna, a north Italian region. Data were retrieved from both birth certificates and the Stillbirth Surveillance system database. To identify independent risk factors, a multivariate analysis using logistic regression was performed. A descriptive analysis of the causes of stillbirth is also reported.
In the observation period, 246,437 babies born at term (including 260 stillbirths, giving a rate of 1.06/1000) were considered. The risk factors independently associated with stillbirth were small for gestational age babies (odds ratio, 2.58; 95% confidence interval, 1.88–3.53), pregnancy achieved though fertility treatments (odds ratio, 2.01; 95% confidence interval, 1.15–3.51), and delayed access to pregnancy services (odds ratio, 1.56; 95% confidence interval, 1.10–2.22). In multipara, the presence of a previous stillbirth (odds ratio, 3.91; 95% confidence interval, 1.98–7.72) was also associated with an increased risk for recurrence. Early- rather than late-term was an additional risk factor. The most frequent causes of death were placental and cord disorders (61/260 and 56/260, respectively). However, 28.1% of cases remain unexplained.
The risks for stillbirth at term are known early in pregnancy or could be identified through tailored antenatal management, allowing effective preventive strategies to reduce preventable cases.
Risks of stillbirth, neonatal mortality, and severe neonatal morbidity by birthweight centiles associated with expectant management at term
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Determining the optimal time of birth at term is challenging given the ongoing risks of stillbirth with increasing gestation vs the risks of significant neonatal morbidity at early-term gestations. These risks are more pronounced in small infants.
This study aimed to evaluate the risks of stillbirth, neonatal mortality, and severe neonatal morbidity by comparing expectant management with delivery from 37⁺⁰ weeks of gestation.
This was a retrospective cohort study evaluating women with singleton, nonanomalous pregnancies at 37⁺⁰ to 40⁺⁶ weeks’ gestation in Queensland, Australia, delivered from 2000 to 2018. Rates of stillbirth, neonatal death, and severe neonatal morbidity were calculated for <3rd, 3rd to <10th, 10th to <25th, 25th to <90th, and ≥90th birthweight centiles. The composite risk of mortality with expectant management for an additional week in utero was compared with rates of neonatal mortality and severe neonatal morbidity.
Of 948,895 singleton, term nonanomalous births, 813,077 occurred at 37⁺⁰ to 40⁺⁶ weeks’ gestation. Rates of stillbirth increased with gestational age, with the highest rate observed in infants with birthweight below the third centile: 10.0 per 10,000 (95% confidence interval, 6.2–15.3) at 37⁺⁰ to 37⁺⁶ weeks, rising to 106.4 per 10,000 (95% confidence interval, 74.6–146.9) at 40⁺⁰ to 40⁺⁶ weeks’ gestation. The rate of neonatal mortality was highest at 37⁺⁰ to 37⁺⁶ weeks for all birthweight centiles. The composite risk of expectant management rose sharply after 39⁺⁰ to 39⁺⁶ weeks, and was highest in infants with birthweight below the third centile (125.2/10,000; 95% confidence interval, 118.4–132.3) at 40⁺⁰ to 40⁺⁶ weeks’ gestation. Balancing the risk of expectant management and delivery (neonatal mortality), the optimal timing of delivery for each birthweight centile was evaluated on the basis of relative risk differences. The rate of severe neonatal morbidity sharply decreased in the period between 37⁺⁰ to 37⁺⁶ and 38⁺⁰ to 38⁺⁶ weeks, particularly for infants with birthweight below the third centile.
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Our finding did not support the hypothesis that 1 universal standard birthweight curve can be applied to all populations.
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Antenatal identification of pregnancies at high risk of delivering small for gestational age neonates may improve the management of the condition and reduce the associated adverse perinatal outcomes. In a series of publications, we have developed a new competing-risks model for small for gestational age prediction, and we demonstrated that the new approach has a superior performance to that of the traditional methods. The next step in shaping the appropriate management of small for gestational age is the timely assessment of these high-risk pregnancies according to an antenatal stratification plan.
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This was a prospective observational study of 96,678 singleton pregnancies undergoing routine ultrasound examination at 19 to 24 weeks of gestation, which included recording of estimated fetal weight and measurement of uterine artery pulsatility index. The competing-risks model for small for gestational age was used to create a patient-specific stratification curve capable to define a specific timing for a repeated ultrasound examination after 24 weeks. We examined different stratification plans with the intention of detecting approximately 80%, 85%, 90%, and 95% of small for gestational age neonates with birthweight <3rd and <10th percentiles at any gestational age at delivery until 36 weeks; all pregnancies would be offered a routine ultrasound examination at 36 weeks.
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Cesarean delivery rates increased starting at 40 0/7 weeks of gestation in ongoing pregnancies regardless of the mode of labor onset.

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: The authors report no conflict of interest.

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: Cite this article as: Trudell AS, Cahill AG, Tuuli MG, et al. Risk of stillbirth after 37 weeks in pregnancies complicated by small-for-gestational-age fetuses. Am J Obstet Gynecol 2013;208:376.e1-7.

: Reprints not available from the authors.

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ResearchObstetricsRisk of stillbirth after 37 weeks in pregnancies complicated by small-for-gestational-age fetuses

Objective

Study Design

Results

Conclusion

Section snippets

Materials and Methods

Results

Comment

Lancet

Lancet

Lancet

Lancet

Obstet Gynecol

Am J Obstet Gynecol

Research
Obstetrics
Risk of stillbirth after 37 weeks in pregnancies complicated by small-for-gestational-age fetuses