As summer comes to a close soon in North America, many pregnant women may be at a lower risk for having a miscarriage. That’s the because your miscarriage risk may go up in the summer months, according to findings from a new study.

The report in Epidemiology correlates more miscarriages happening in the summer months (in North America) compared to the middle of winter. In fact, pregnant people in North America had a 44% higher chance of early miscarriage (that’s within eight weeks of pregnancy) in the summer months compared to winter. The highest chance may be in late August, when the risk of miscarriage at any week during pregnancy was 31% higher over the February mark.

We know there are several risk factors for pregnancy loss, but we still don’t understand the whole picture. As many as 10 to 20% of clinically recognized pregnancies and 30% to 50% of chemical pregnancies end before 20 weeks in, and the majority stem from a genetic issue like a chromosomal abnormality—not a result of heat exposure. However, the correlation between miscarriage risk and extreme heat is worth exploring.

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How heat affects pregnancy

"We know that heat is associated with higher risk of other pregnancy outcomes, such as preterm delivery, low birth weight and stillbirth, in particular," says Amelia Wesselink, study lead and corresponding author, and a research assistant professor of epidemiology at Boston University School of Public Health, in a statement. She says that medical guidance and public health messaging must consider the effects of heat on those expecting and their babies.

Wesselink’s team isn’t the only one that has assessed heat’s impacts on pregnancy. 

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A 2021 study showed that exposure to scorching temperatures during the first few weeks after conceiving raised the rate of pregnancy loss for clinically unrecognized pregnancies, while being in the cold during those first few weeks seems to lower that rate. 

And another 2021 study made some pretty powerful statements connecting heat and pregnancy. It found that preterm births went up 16% during heat waves

There’s also been quite a bit of research into the effects of climate change on pregnancy.

Correlation, but not causation

So, should you be concerned if you’re newly pregnant in the dead of summer? Yes and no, says Hugh S. Taylor, MD, an OB-GYN and reproductive endocrinologist and a professor at Yale School of Medicine, who was not part of the research team.

“It is important to keep this information in perspective,” Dr. Taylor tells Motherly. While you can do a lot to keep yourself well, staying out of the heat and being hydrated non-stop isn’t the key factor in preventing miscarriages—as many of them are not preventable in the first place.

“Miscarriages are very common in early pregnancy, however chromosomal abnormalities cause about 50% of all miscarriages in the first trimester. This is genetic and not something we have control over; a miscarriage is very rarely something that could have been prevented.” 

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Taylor notes that dehydration caused by heat exposure can affect pregnancy and fetal development. High temperature can as well, although the body is generally quite effective in regulating internal temperature, Taylor notes.

A woman’s blood volume needs to be higher in pregnancy and her heart and circulation are altered, he explains.

You should be mindful of the heat if you’re planning on getting pregnant or newly pregnant in the summer. Be extra careful or avoid being in the extreme heat and performing any strenuous activity (probably not the best idea to take a hike).

“Most importantly, stay hydrated,” Taylor says.

Sounds like following a few tips (like drinking enough water) can go a long way to help reducing your risk, but it may not be the factor that stops a miscarriage from occurring. Again, more research is needed.

Related: Yes, pregnant women can eat some types of sushi during pregnancy

Exploring connections between miscarriage risk and extreme heat

For now, researchers have more clues into what causes miscarriage—and potentially some intel on how to possibly prevent it.

"Any time you see seasonal variation in an outcome, it can give you hints about causes of that outcome," notes Wesselink. 

With a correlation established, more research needs to be done, Wesselink goes on.

"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," she adds.

Featured expert

Hugh S. Taylor, MD, is an OB-GYN and reproductive endocrinologist, a professor at Yale School of Medicine, and chief of Obstetrics and Gynecology at Yale-New Haven Hospital.

Sources

Hajdu, T., Hajdu, G. Post-conception heat exposure increases clinically unobserved pregnancy losses. Sci Rep 11, 1987 (2021). doi:10.1038/s41598-021-81496-x

National Partnership for Women & Families National Birth Equity Collaborative. (2021). Higher Temperatures Hurt Mothers and Babies. 2021.

Wesselink, Amelia K.a; Wise, Lauren A.a; Hatch, Elizabeth E.a; Mikkelsen, Ellen M.b; Savitz, David A.c; Kirwa, Kiprutod; Rothman, Kenneth J.a,e. A Prospective Cohort Study of Seasonal Variation in Spontaneous Abortion. Epidemiology: May 2022 - Volume 33 - Issue 3 - p 441-448. doi: 10.1097/EDE.0000000000001474