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The highest risk of stillbirth occurred when pregnancies went beyond 41 weeks -- then the odds rose by 87%, the British researchers said.
They stressed, however, that women who are 41 weeks' pregnant should not be alarmed. That's because the absolute risk of stillbirth is still very low -- equivalent to one additional stillbirth for every 1,449 pregnancies, compared to delivering at 40 weeks.
The new study "is the largest study of its kind, and finally provides precise estimates of potential risks of stillbirth," said study author Shakila Thangaratinam, from Queen Mary University in London.
"Now that we understand the extent to which stillbirth risks increase with each week of pregnancy, we should be incorporating this information in all discussions around delivery plans in pregnant women at term," Thangaratinam suggested. "So, this is all about helping women make informed decisions on timing of delivery."
The researchers also unearthed an unexpected finding.
"We were surprised to see how much poorer pregnancy outcomes were for black women -- they were up to twice more likely to experience stillbirth than white women," Thangaratinam added in a university news release. "Health care professionals need to take these added risks into account when developing care plans for these women."
Of the roughly 3,000 stillbirths that occur in the United Kingdom each year, one-third appeared healthy at 37 weeks' gestation, prior to their deaths, according to the researchers.
For the study, the investigators analyzed data on over 15 million pregnancies and nearly 18,000 stillbirths. The risk of stillbirth increased from 0.1 per 1,000 pregnancies at 37 weeks to 3.2 per 1,000 at 42 weeks.
Another jump in risk occurred between weeks 40 and 41, when the risk of stillbirth rose 64%. And it jumped by 87% between weeks 41 and 42, the findings showed.
"Prolonged pregnancy is a known risk factor for stillbirth and women are routinely offered induction of labor after 41 weeks' gestation," Thangaratinam explained. "This recommendation is based on evidence of increased stillbirth risk beyond 41 weeks. However, one in three women with a stillbirth at term loses her baby before this period. The magnitude of this risk is not routinely communicated due to a previous lack of robust evidence."
Two U.S. obstetricians who weren't involved in the study said the findings aren't surprising, but neither should they cause undue alarm for pregnant women at or slightly beyond term.
"The important point to take away from this study is that there is a small increased risk of stillbirth, but the overall risk is still very small," said Dr. Jennifer Wu, an obstetrician-gynecologist at Lenox Hill Hospital in New York City.
Dr. Mitchell Kramer directs obstetrics and gynecology at Northwell Health's Huntington Hospital in Huntington, N.Y. He said that the data on a heightened risk to black women who carry past term is new and "certainly requires more study."
But for all women, "studies like this let patients know the importance of delivering no later than 41 weeks," Kramer said. He added that "induction of labor is safe and can decrease the likelihood of a catastrophic occurrence like stillbirth."
The findings were published July 2 in the journal PLOS Medicine.
-- Robert Preidt
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