Stillbirth rates are even higher than previously thought, and about three quarters of cases did not present with clinical risk factors that would have flagged more preventive efforts, according to new findings in JAMA Network Open.

The analysis of medical claims of nearly 2.8 million commercially-insured, singleton births between 2016 and 2022 showed a little over 1 in 150 ended in stillbirth vs the 1 in 175 reported by CDC.

The discrepancy may be the result of the CDC using data from fetal death certificates for its assessments, which vary in reliability, the researchers said.

“The US has among the highest rates of stillbirth among all high-income countries and there has been barely any improvement in stillbirth rates in recent years,” said Haley Sullivan, BS, a PhD candidate in Health Policy at Harvard University in Cambridge, Massachusetts, who helped author the study. “Many stillbirths are potentially preventable, and we can lower the stillbirth rate in the US but not without attention, research, and resources.”

Sullivan and her colleagues found 18,893 cases of stillbirth, defined as a fetus that died at 20 or more weeks of gestation.

Some stillbirths occurred without warning signs. For pregnancies at 40 weeks or greater gestation, 40.7% of stillbirths occurred without recorded clinical risk factors. Among stillbirths at 38 weeks gestation, 24.1 had no risk factors, and 34.2% had no risk factors at 39 weeks. A little over 27% of cases overall did not present with a risk factor.

The rate of prospective fetal mortality — the number of stillbirths that occur at a gestational week divided by live births and stillbirths that occur at or after that week — nearly doubled between 38 and 39 weeks of gestation, from 0.66 per 1000 births to 1.3.

Other studies have found the percentage of stillbirths increases with gestational age. One study found the risk at 37 weeks is approximately 0.11 per 1000 pregnancies, which rises to 3.18 per 1000 at 42 weeks.

Nearly three quarters of stillbirths in the current study were associated with at least one clinical risk factor falling under the American College of Obstetricians and Gynecologists’ indications for fetal surveillance.

Fetal anomalies (15.4 per 1000 births), oligohydramnios (15.15), and chronic hypertension (10.51) showed the highest association with stillbirth. Other factors associated with stillbirth included maternal obesity, pregnancy-related hypertension, polyhydramnios, and substance use.

“Our findings suggest a need to improve stillbirth prevention among pregnancies with identified risk factors as well as improve stillbirth risk prediction, especially later in pregnancy,” Sullivan said.

Stillbirth rates varied widely based on income and race. Pregnant patients living in neighborhoods with a high proportion of low-income households experienced a rate of 8.95 stillbirths per 1000 births compared with 5.87 per 1000 for those in higher income areas. Communities with a higher prevalence of Black patients experienced a rate of 10.05 stillbirths per 1000 births compared with 5.73 per 1000 in those with the lowest prevalence of Black patients.

Race and income-driven disparities in stillbirths are well-documented. The new research reinforces the need to target prevention efforts to these communities, Sullivan said.

No significant variations were found based on rurality or access to maternity care.

The rate of prospective fetal mortality may have doubled between weeks 38 and 39 due to placental insufficiency — increasing dysfunction or inability of the placenta to get oxygenated blood to the fetus — during that time period, said Carla Janzen, MD, PhD, a professor in the Division of Maternal-Fetal Medicine at UCLA Health, Los Angeles, who was not involved in the study.

Given this doubling of rates, the nationwide push to wait until at least the 39th week for delivery could be revisited, Janzen said.

“We can think about easing it up a little bit,” Janzen said. “So if we do have someone who, for example, has chronic hypertension other risk factors, there’s no barrier to delivering them at 38 weeks.”

Clinicians currently assess stillbirth risk with advanced imaging procedures that include biophysical profiles and fetal nonstress tests, but more research is needed to determine the best timing and frequency for when to start the screening process, Sullivan said.

Some experts have suggested implementing machine learning to signal risk for stillbirth using biomarkers such as fetal hypoxia and placental dysfunction.

The new research confirms much of what the medical community already knows, Janzen said. But the findings provide impetus for funding more research into understanding the causes of stillbirth.

“We need to understand better, for example, why the placenta fails after 38 weeks more frequently in women who, say, come from an underprivileged background.”

This study was funded by the National Science Foundation Graduate Research Fellowship Program. Jenzen reported no disclosures. Kathe Fox reported being board president of the Health Care Cost Institute, which provided the data. Other study authors reported no relevant disclosures.

Brittany Vargas is a medicine, mental health, and wellness journalist.