Pregnant women are 44% more likely to have a miscarriage in summer than they are in winter

Women are more likely to suffer from a miscarriage during the dog days of summer, and the sweltering heat in many parts of the U.S. may be to blame, a new study finds.

A research team led by Boston University found that American women are 44 percent more likely to suffer a miscarriage in late-August than they are in late-February – a drastic difference across a six month period.

While the exact reason for this disparity has yet to have been determined, researchers speculate that it is likely due to the extreme heat as states with more severe summer weather exhibited the trend the most.

They also note that many other potential birth issues, like stillbirths and low birth weight are more common in summer as well.

Researchers, who published their findings in the journal Epidemiology, gathered data from the the SPH-based Pregnancy Study Online, which is funded by the National Institutes of Health.

The survey includes women that are actively trying to conceive, and follows them through the process of their pregnancy through the first six months after the baby is born.

For this study, data from 6,104 participants who conceived a child within 12 months on enrolling were included.

Researchers gathered data on potential negative birth outcomes – like a miscarriage – across all parts of the year.

After controlling for outside factors that could impact miscarriage rates, like maternal risk factors, race or socioeconomic status, they found that the rate of miscarriages was surging during summer months.

The largest gap was found between the months of August and February – which is the most brutal winter period and most brutal part of summer in some parts of America.

In August overall, there was a 31 percent increased risk of suffering a miscarriage than in February.

When isolating the data down to just the final week of both months, researchers found that there was a 44 percent difference.

‘Any time you see seasonal variation in an outcome, it can give you hints about causes of that outcome,’ Dr Amelia Wesselink, co-author of the study and epidemiology professor at Boston said in a statement.

While researchers could not pin-point a mechanism that causes a surge in miscarriages during certain parts of the year, hear is the main suspect.

The disparity in miscarriages was the largest in the South and Midwest, states that experience the hottest summers on average – further adding to evidence that weather is the main factor at play.

‘We found that miscarriage risk, particularly risk of ‘early’ miscarriage before eight weeks of gestation, was highest in the summer,’ Wesselink said, noting that further research would be needed.

‘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.’ 

Extreme heat leading to poor maternal birth outcomes would not be a totally out-of-the-blue concept, though. Researchers note that hotter weather has already been linked to some issues.

‘We know that heat is associated with higher risk of other pregnancy outcomes, such as preterm delivery, low birth weight, and stillbirth, in particular,’ Wesselink says. 

‘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 and their babies.’ 

Experts at the University of Michigan have previously noted that the body has to work harder when it is pregnant, because it is responsible for two organisms.

A woman is more likely to get dehydrated as her body does more work, and the wear and tear the hot weather can have on both her and the unborn child can lead to severe negative consequences.