Miscarriage risk may be higher in the summer months, a study running from 2013 to 2020 in the US has suggested. The risk of miscarriage before eight weeks' gestation was observed to be 44 percent higher in late August compared to late February.

The study, published in the journal Epidemiology, used the Pregnancy Study Online (PRESTO), a web-based study of those trying to conceive without fertility treatment. Participants were aged 21 to 45 years old and recruited via social media advertising.

Between June 2013 and August 2020, 12,197 people enrolled and completed the first questionnaire of the study. Participants were then sent follow-up questionnaires about their pregnancy status every two months for up to 12 months. Within this time frame, 6,104 participants reported that they were pregnant. Of these, 1,188 then reported a miscarriage – 19.5 percent of participants – the median gestation time of which was six weeks.

A peak in miscarriages was observed in June, July, and August. When this data was adjusted to take into account when participants started trying to conceive, risk of miscarriage before eight weeks’ gestation peaked in mid-August. This trend was not observed in pregnancies past eight weeks, although the risk of miscarriage at any gestational age was still around 31 percent higher in late August versus late February.

“Any time you see seasonal variation in an outcome, it can give you hints about causes of that outcome,” said lead author of the study Dr Amelia Wesselink in a statement.

“We found that miscarriage risk, particularly risk of ‘early’ miscarriage before eight weeks of gestation, was highest in the summer. Now we need to dig into that more to understand what kinds of exposures are more prevalent in the summer, and which of these exposures could explain the increased risk of miscarriage.”

The study authors admit several limitations of the study, one of which being the lack of information on whether the pregnancies had typical chromosomes – a large proportion of miscarriages are caused by having too many or too few chromosomes. As daily urine samples were not taken to test for pregnancy, some early miscarriages were probably not recorded.

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 hottest,” and that this suggests that “heat may play a role.”

“We know that heat is associated with higher risk of other pregnancy outcomes, such as preterm delivery, low birth weight, and stillbirth, in particular,” said Dr Wesselink.

“Medical guidance and public health messaging – including heat action plans and climate adaptation policies – need to consider the potential effects of heat on the health of pregnant people.”