Impact of BMI on stillbirth risk

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In a recent study, rates of stillbirth were higher in pregnancies with increased maternal body mass index.

Impact of BMI on stillbirth risk | Image Credit: © VadimGuzhva - © VadimGuzhva - stock.adobe.com.

Impact of BMI on stillbirth risk | Image Credit: © VadimGuzhva - © VadimGuzhva - stock.adobe.com.

According to a recent study published in the American Journal of Obstetrics and Gynecology, increased maternal body mass index (BMI) is associated with increased risk of stillbirth in offspring.

There is a direct association between fetal growth and pregnancy outcomes, making it vital to recognize risk factors of fetal growth which require intervention. Population-based fetal weight charts are conventionally used to track fetal growth, but prescriptive international standards have recently been developed.

Gestation Related Optimal Weight (GROW) charts may be altered based on maternal parity, height, weight in early pregnancy, and ethnic origin, which have been associated with birthweight and fetal weight in different populations. This has made GROW a combined fetal and neonatal weight standard for assessing different population-average charts.

Comparisons of customized vs population-average standards allow observation of overlapping associations with adverse outcome. Investigators conducted a retrospective cohort study to compare GROW with Hadlock, the most used fetal weight standard, along with 2 recently developed weight standards.

Information from hospitals using the Growth Assessment Protocol in the United Kingdom from January 2015 to October 2022 was included in the analysis. Gathered data included maternal height, weight at birth, ethnic origin, parity, fetal sex, birth weight, gestational age, and pregnancy outcome.

Birthweight was used for analysis because of a lack of routinely performed estimated fetal weight scans. Small for gestational age (SGA) was determined based on the Hadlock fetal weight standard, the Intergrowth 21st fetal growth standard, the World Health Organization fetal growth standard, and the Gestation Related Optimal Weight standard.

The SGA rate and risk of stillbirth was calculated for each standard with a respective non-SGA stillbirth risk used as a reference. Analyses with varied cut-offs for each standard were also performed to ensure a 10% positive screen rate. There were 5 BMI categories, with mothers grouped based on maternal height and weight measured at the first appointment.

Participants included British-European and South Asian mothers, as these groups made up the 2 largest ethnic groups in the available population. There were 2,002,899 British European women and 265,112 South Asian women in the final cohort.

Lower average maternal height, weight, and BMI were seen in South Asian patients compared to British-European patients, along with lower gestational age at birth and average birthweight. Stillbirth was seen more often in South Asian patients, at a rate of 5.51 per 1000 pregnancies compared to 3.89 per 1000 pregnancies in British-European patients.

SGA rates were also higher in South Asian patients than British-European pregnancies when evaluated by each population-average standard. While SGA rates differed between growth standards, each standard saw a significant association between SGA cases and relative risk of stillbirth. Similar stillbirth risks were seen between standards for different ethnic groups.

The lowest stillbirth rate observed in British-Europeans was 3.60 per 1000 pregnancies, seen in women with a BMI of 18.5 to under 25. The highest observed stillbirth rate was 4.71 per 1000 pregnancies in women with a BMI of 35 or more. Similar associations were observed in South Asian women.

These results showed stillbirth risk to be inversely proportional to SGA cases. Rates of stillbirth increased with BMI, with more significant increases seen in South Asian women. This indicated a group of potentially avoidable stillbirths which could be addressed.

Reference:

Gardosi J & Hugh O.Stillbirth risk and smallness for gestational age according to Hadlock, Intergrowth-21st, WHO and GROW fetal weight standards: analysis by maternal ethnicity and body mass index. American Journal of Obstetric and Gynecology. 2023. doi:10.1016/j.ajog.2023.05.026

This article was initially published by our sister publication, Contemporary OB/GYN®.

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