Study finds many occur with no clinical risk factors

Stillbirths occur at a higher rate in the U.S. than previously reported, according to a new study led by researchers at Harvard T.H. Chan School of Public Health and Mass General Brigham.

The researchers also found that, while most stillbirths had at least one identified clinical risk factor, a substantial share had none, particularly those occurring at 40+ weeks gestation.

“Stillbirths impact nearly 21,000 families each year in the U.S., and nearly half of those occurring at 37+ weeks are thought to be preventable. Yet there is very little research in this area,” said co-senior author Jessica Cohen, professor of health economics. “Our study highlights the pressing need to improve stillbirth risk prediction and prevention.”

The study is published in JAMA. According to the researchers, it’s one of the largest, most data-rich studies of stillbirth burden to date.

The researchers studied the outcomes of more than 2.7 million pregnancies across the U.S. between 2016 and 2022 using commercial health insurance claims and demographic data from the Health Care Cost Institute, the American Community Survey, and the March of Dimes. Among these pregnancies, 18,893 stillbirths were identified.

The researchers examined associations between these stillbirths and a variety of clinical factors, including gestational age at delivery; pregnancy risks such as obesity, pregnancy-related and chronic hypertension, gestational and pre-pregnancy diabetes, and substance use; fetal risks, such as decreased movement, growth restriction, and anomalies; and obstetric risks, such as history of stillbirth or adverse pregnancy outcomes and low or excess amniotic fluid levels.

They also considered a variety of socioeconomic factors, including rurality and area-level measures of income, race, and access to obstetric care.

The study found that more than one in 150 births end in stillbirth—a rate higher than the rate of one in 175 births that the Centers for Disease Control and Prevention has published as the national average.

The rate was even higher for families living in low-income areas, where one in every 112 births ended in stillbirth. The researchers also observed that the stillbirth rate was one in every 95 births in areas with higher proportions of Black families compared to white families. Stillbirth rates did not significantly vary according to rurality and levels of access to obstetric care.

The study also found that while 72.3% of stillbirths had at least one clinical risk factor, a sizable portion of stillbirths occurred with no identified clinical risk factor.

Across all of the stillbirths in the study, nearly 27.7% had no risk factor. Later gestational ages showed the highest rates of having no clinical risk factor: Among stillbirths that occurred at 38 weeks gestation, 24.1% had no risk factor; at 39 weeks, 34.2%; and at 40+ weeks, 40.7%. Stillbirth rates were highest among pregnancies with low amniotic fluid levels, fetal anomalies, and chronic hypertension.

“Although momentum toward improving stillbirth research and prevention efforts has increased in recent years, rates in the U.S. remain much higher than in peer countries,” said co-senior author Mark Clapp, maternal-fetal medicine provider in the Department of Obstetrics and Gynecology at Massachusetts General Hospital.

“I hope this study will inform policy, practice changes, and future research to ensure no person or family has to experience this outcome.”

The researchers also noted that the study’s findings indicate the need for further research into what’s driving socioeconomic variances of stillbirth rates—whether that be social factors, health system factors, and/or clinical risk factors.

Haley Sullivan, student in the Harvard Ph.D. Program in Health Policy, was the study’s first author. Harvard Chan’s Anna Sinaiko was also a co-author.