Coronavirus: Catching the infection after 34 weeks of pregnancy could raise the risk of a premature birth seven-fold

Testing positive for the coronavirus in the last three months of pregnancy may double the risk of a premature birth, rising to a seven-fold increased risk if infected after 34 weeks

By Clare Wilson

Having covid-19 after 34 weeks of pregnancy has been linked with a seven-fold higher risk of giving birth prematurely.

In a study of more than 5000 pregnant women, 9.1 per cent of those who had a positive covid-19 test after 34 weeks went on to give birth prematurely, defined as less than 37 weeks into the pregnancy. This is compared with 1.4 per cent of the women who didn’t test positive while pregnant. Trans men weren’t included in the study.

Throughout the pandemic, different studies have thrown up varying results when it comes to the risks related to having covid-19 during pregnancy. Some have linked the infection to a higher risk of preterm birth, but it was unclear at what point during pregnancy having covid-19 carried the highest risk.

To learn more, Tal Patalon at Maccabi Healthcare Services in Tel Aviv, Israel, and her colleagues tracked the outcomes of 2753 women who had a positive covid-19 test at any stage of their pregnancy, compared with the same number of pregnant women who didn’t.

During the first six months – about 27 weeks – of pregnancy, having covid-19 wasn’t linked with a rise in premature birth risk.

But women who were infected in the last three months – week 28 onwards – were more than twice as likely to have a premature birth than those who didn’t test positive. Having covid-19 after week 34 was linked with a seven-fold increased risk, regardless of the severity of the infection. The researchers didn’t account for the women’s covid-19 vaccination status.

In the study, having covid-19 wasn’t linked with a higher rate of miscarriages or stillbirths, or babies being born small for their gestational age, contradicting some previous research.

“Women after 34 weeks of gestation should practice social distancing and respiratory protection,” the authors write in their paper.

The results may be different if someone is infected with the omicron variant, which is currently dominant, says Patalon. The Israeli study was done when the delta variant, which has been linked with more severe illness, was circulating.

“This study adds to the robust evidence we have about the risks of catching covid-19 in pregnancy, and how crucial it is that pregnant women are vaccinated against the virus,” says Pat O’Brien at the Royal College of Obstetricians and Gynaecologists in the UK.

“We know that babies do not have any increased risk of adverse long-term outcomes if delivered after 34 weeks, and this is supported by the study, which did not notice any difference in adverse outcomes for the babies,” he says.

Journal reference: PLoS One, DOI: 10.1371/journal.pone.0270893