Evidence summary
Association between higher maternal BMI and increased risk for stillbirth
The purpose of antenatal testing is to decrease the risk for stillbirth between visits. Because of the resources involved and the risk for false-positives when testing low-risk patients, antenatal testing is reserved for pregnant people with higher risk for stillbirth.
In a retrospective cohort study of more than 2.8 million singleton births including 9030 stillbirths, pregnant people with an elevated BMI had an increased risk for stillbirth compared to those with a normal BMI. The adjusted hazard ratio was 1.71 (95% CI, 1.62-1.83) for those with a BMI of 30.0 to 34.9; 2.04 (95% CI, 1.8-2.21) for those with a BMI of 35.0 to 39.9; and 2.50 (95% CI, 2.28-2.74) for those with a BMI ≥ 40.1
A meta-analysis of 38 studies, which included data on 16,274 stillbirths, found that a 5-unit increase in BMI was associated with an increased risk for stillbirth (relative risk, 1.24; 95% CI, 1.18-1.30).2
Another meta-analysis included 6 cohort studies involving more than 1 million pregnancies and 3 case-control studies involving 2530 stillbirths and 2837 controls from 1980-2005. There was an association between increasing BMI and stillbirth: the odds ratio (OR) was 1.47 (95% CI, 1.08-1.94) for those with a BMI of 25.0 to 29.9 and 2.07 (95% CI, 1.59-2.74) for those with a BMI ≥ 30.0, compared to those with a normal BMI.3
However, a retrospective cohort study of 182,362 singleton births including 442 stillbirths found no association between stillbirth and increasing BMI. The OR was 1.10 (95% CI, 0.90-1.36) for those with a BMI of 25.0 to 29.9 and 1.09 (95% CI, 0.87-1.37) for those with a BMI ≥ 30.0, compared to those with a normal BMI.4 However, this cohort study may have been underpowered to detect an association between stillbirth and BMI.
Recommendations from others
In 2021, ACOG suggested that weekly antenatal testing may be considered from 34w0d for pregnant people with a BMI ≥ 40.0 and from 37w0d for pregnant people with a BMI between 35.0 and 39.9.5 The 2021 ACOG Practice Bulletin on Obesity in Pregnancy rates this recommendation as Level C—based primarily on consensus and expert opinion.6
A 2018 Royal College of Obstetricians and Gynecologists Green-top Guideline recognizes “definitive recommendations for fetal surveillance are hampered by the lack of randomized controlled trials demonstrating that antepartum fetal surveillance decreases perinatal morbidity or mortality in late-term and post-term gestations…. There are no definitive studies determining the optimal type or frequency of such testing and no evidence specific for women with obesity.”7
A 2019 Society of Obstetricians and Gynecologists of Canada practice guideline states “stillbirth is more common with maternal obesity” and recommends “increased fetal surveillance … in the third trimester if reduced fetal movements are reported.” The guideline notes “the role for non-stress tests … in surveillance of well-being in this population is uncertain.” Also, for pregnant people with a BMI > 30, “assessment of fetal well-being is … recommended weekly from 37 weeks until delivery.” Finally, increased fetal surveillance is recommended in the setting of increased BMI and an abnormal pulsatility index of the umbilical artery and/or maternal uterine artery.8
Editor’s takeaway
Evidence demonstrates that increased maternal BMI is associated with increased stillbirths. However, evidence has not shown that third-trimester antenatal testing decreases this morbidity and mortality. Expert opinion varies, with ACOG recommending weekly antenatal testing from 34 and 37 weeks for pregnant people with a BMI ≥ 40 and of 35 to 39.9, respectively.