Heat waves may lead to poor pregnancy outcomes, a review of studies has found.

The analysis, in BMJ, included 70 studies in 24 countries, and found that over all, each one-degree Celsius (1.8 degrees Fahrenheit) increase in temperature was associated with a 5 percent increase in preterm birth and a 5 percent increase in stillbirth.

Heat waves — generally meaning a period of two days or more of abnormally high temperatures — were associated with a 16 percent increased likelihood of preterm birth and a 46 percent increased risk for stillbirth. The effects of high temperature were strongest in the last month of pregnancy. There was no effect of temperature on birth weight.

The evidence was strongest and most consistent for the ill effects of heat waves, but several studies found that large fluctuations in daytime temperature or high temperatures at night were also associated with preterm birth.

“High temperature is dangerous for health over all,” said the lead author, Matthew Francis Chersich of the University of Witwatersrand in Johannesburg. “It’s not just heatstroke in a marathon runner or someone stuck in the desert. There is a higher risk in pregnant women, and it’s much worse in poor women — anywhere people don’t have access to air-conditioning.”

Still, he said, pregnant women need not be terrified of taking a walk on a hot day. “There is a slightly higher risk if she repeatedly exposes herself to high temperatures. It’s repeated, frequent exposure that places a baby at unnecessary risk.”