The risk of stillbirth increases with every week that a pregnancy continues past term (37 weeks), according to a British analysis of more than 15 million pregnancies.
And surprisingly, black women were found to be 1.5 to two times more likely to suffer stillbirth than white women at all gestational ages.
The researchers from Queen Mary University of London suggest that there is no real cause for alarm, however, as the extra risk is quite low – equivalent to one additional stillbirth for every 1449 pregnancies for delivery at 41 weeks compared to 40 weeks.
They acknowledge some limitations in the study, but say it provides new information that can form a basis for delivery planning.
“This is the largest study of its kind, and finally provides precise estimates of potential risks of stillbirth,” says lead author Shakila Thangaratinam.
“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 and colleagues analysed data from 13 separate studies in the UK, the US, Denmark and Norway, which included 15,124,027 pregnancies, 17,830 stillbirths and 2348 newborn deaths.
Despite the stillbirth risk figures, delivery before 41 weeks did not increase the risk of newborn death (defined as a death during the first 28 days of life); the risk was unchanged for births between 38 and 41 weeks of gestation. It increased by 87% for deliveries at 42 weeks’ gestation compared to 41 weeks, however.
More than 3000 babies are stillborn each year in the UK; a third of these are term babies (37 weeks or beyond) who were considered to be healthy prior to their death.
Prolonged pregnancy is a known risk factor for stillbirth and women are routinely offered induction of labour after 41 weeks’ gestation. The researchers say this recommendation is based on evidence of increased stillbirth risk beyond 41 weeks, but that one in three women with a stillbirth at term loses her baby before this period.
The magnitude of this risk has not been routinely communicated, they suggest, because of a previous lack of robust evidence.
The findings of the new study are published in the journal PLOS Medicine.
Nick Carne is the editor of Cosmos Online and editorial manager for The Royal Institution of Australia.
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