The risk of miscarriage may be higher in the summer months, a study conducted between 2013 and 2020 in the US has suggested. The risk of miscarriage before eight weeks of pregnancy was observed to be 44% higher in late August compared to late February.

The study, published in JOURNAL Epidemiology, used data from the Pregnancy Study Online (PRESTO), an Internet-based study of women trying to have children without fertility treatment. The participants were between the ages of 21 and 45 and were recruited through advertisements displayed on social networks.

Between June 2013 and August 2020, 12,197 people enrolled and completed the study’s first questionnaire. They were then sent follow-up questionnaires about their pregnancy status every two months for up to 12 months.

During this time frame, 6,104 women reported being pregnant. Of these, 1,188 (19.5%) then reported a miscarriage, with an average pregnancy duration of six weeks, writes IFL Science.

The risk of miscarriage is higher in the summer months

A peak of miscarriages was observed in June, July and August. When these data were adjusted to take into account when the participants started trying for children, the risk of miscarriage before eight weeks of pregnancy peaked in mid-August.

This trend was not seen in pregnancies past eight weeks, although the risk at any gestational age was still about 31% higher in late August compared to late February.

“Anytime we see seasonal variation in an outcome, it can provide clues about the causes of that outcome,” said lead study author Dr. Amelia Wesselink.

Limitations of the study

“We found that the risk of miscarriage, particularly ‘early’, before eight weeks of pregnancy, was highest during the summer. We now need to analyze this further to understand which types of exposures are more prevalent during the summer and which of these exposures might explain the increased risk during this time,” she says.

The authors acknowledge several limitations of the study, one of which was that they did not know whether the pregnancies had typical chromosomes (a large proportion of miscarriages are caused by too many or too few chromosomes being detected). Because daily urine samples were not taken to test for pregnancy, it is likely that some early miscarriages were not recorded.

The heat could be to blame

The seasonal peak in risk appeared stronger for people living in the southern or midwestern United States, and the study authors note that here, “summers tend to be very hot” and that this suggests “heat may play a role.”

“We know that increased environmental temperature is associated with a higher risk for other pregnancy outcomes, such as preterm birth, low birth weight and stillbirth,” Dr Wesselink said.

“Medical advice and public health messages, including heat action plans and climate adaptation policies, need to consider the potential effects of heat on pregnant women’s health,” the researcher concluded.

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