Immigrants living in the U.S. have fewer preterm births

• Link to: Northwestern Now Story

• Preterm birth predicts lifelong health outcomes
• Worsening preterm birth rates in the U.S. represent a ‘key metric to target to improve overall societal health’
• Study identifies key differences among Asian and Hispanic subgroups
• Minority stress could contribute to inequities that begin at birth between populations in the U.S.

CHICAGO --- Preterm birth rates are an important marker in assessing a country’s overall health. And the United States isn’t fairing very well.

Individuals born in the U.S. had an overall higher rate (9.7%) of giving birth prematurely compared to U.S. immigrants (9%), a new Northwestern Medicine study has found.

The findings also identify that Black individuals born in the U.S. have the highest prevalence of early births (14.3%). This translates to one out of every seven Black pregnant people delivering early, which has lifelong implications for their and their baby’s health.

The study identifies who in the U.S. is more likely to have a preterm birth, connecting not only an individual’s self-identified race and ethnicity to differences in preterm birth rates, as previous research has explored, but newly explores the pregnant person’s nativity, or country of birth.

The findings speak to higher risks for pregnant people who themselves were born in the U.S., said senior author Dr. Sadiya Khan, the Magerstadt Professor of Cardiovascular Epidemiology at Northwestern University Feinberg School of Medicine. There is preliminary evidence that shows that maternal health outside the U.S. is superior, with fewer pregnant individuals experiencing complications, but the intersection of maternal nativity with race and ethnicity, on the risk for experiencing preterm birth has not been extensively studied.

The research was published today (November 13) in the journal JAMA Pediatrics.

“This is a really important area to see whether there are disparities that could potentially be a target to advance health equity not just for birth outcomes, but across the life course,” said Khan, who also is a Northwestern Medicine cardiologist and epidemiologist. “Preterm birth has been identified by the World Health Organization as a key metric that represents a score card of our overall societal health, given its impact on maternal and child health outcomes.”

The study builds on past research from the same research group that found Black pregnant people are most likely in the U.S. to experience preterm births. Here the researchers now show that Black pregnant people who themselves were born in the U.S. had the highest rates. In both studies, prematurity was described as giving birth before 37 weeks of gestation and very preterm was described as less than 32 weeks of gestation.

The new study examined data from the National Vital Statistics system operated by the Centers for Disease Control and Prevention (CDC), which collects birth registration records on all live births in the U.S. and captures gestational age at time of birth to define preterm or term birth. The authors then examined trends and looked at differences by self-reported race and ethnicity and nativity of the pregnant person between 2014 and 2019, and noted that patterns had worsened over time.

While Khan said some of the results weren’t surprising — she’s been tracking the deteriorating birth outcomes in the U.S. for some time — the degree of difference between different groups was.

The study categorizes over 8 million parents by their country of birth and self-reported ethnicity and race, more comprehensively categorizing Asian individuals as Asian Indian, Chinese, Filipino, Japanese, Korean, Vietnamese and Pacific Islander; and Hispanic individuals as Mexican, Puerto Rican, Cuban and Central or South American.

The most notable discrepancy in preterm birth rates was between Japanese people and non-Hispanic Black people, with 6.2% of pregnant people born in Japan experiencing preterm births verses 14.3% of non-Hispanic Black people born in the U.S., the study found. Exceptions to the overall finding that non-U.S.-born people had lower preterm birth rates were found among Pacific Islanders and Puerto Rican individuals, speaking to the importance of not lumping together all racial and ethnic groups into one category.

Though patterns continued throughout the study, the team found rates of preterm birth increased between 2014 and 2019.

“Birthplace is an independent and important social determinant of health to consider that uniquely contributes to an individual’s lived experiences,” said first author Jack Huang, a research assistant professor of cardiology at Feinberg.  “It is also critical that research does not group all Asian people and Hispanic people. This is not just a poor characterization, but can be misleading in terms of understanding differences in health risks related to social determinants of health.”

Next, the group hopes to understand some of the social and biological drivers of preterm birth. But they already have some theories that minority stress may be heavily at play.

“To reduce health disparities in birth outcomes and improve health across generations, we must identify and target the underlying risk factors for preterm birth,” Huang said. “This is a critical area of focus, as gaining a deeper understanding of these risk factors is essential for promoting a lifetime of health beginning at its origins.”

The research was supported by the National Heart, Lung, and Blood Institute (grant R01HL161514).