Experts are warning mums-to-be to keep out of the hot sun in the early stages of pregnancy in order to protect their unborn babies.
Research has found an increased sensitivity to high temperatures can also make expectant mothers feel ill.
The warning, from the Royal College of Obstetricians and Gynaecologists (RCOG), comes as temperatures are starting to edge up across the UK.
Some studies have suggested that babies can be affected by heat during the first 12 weeks of pregnancy.
One study, published in the journal BJOG in 2005, found that exposure to high temperatures in the first three months can lead to slightly lower birth weights.
Exposure to low temperatures in the last three months of pregnancy also had a similar effect.
The researchers, from the University of Bristol, added that an "increasing occurrence of temperature extremes, in particular, heat waves" could have important public health implications.
The RCOG said babies develop quickest in the first three months in the womb and pregnant women are also more sensitive to high temperatures, which can make them feel unwell.
The College stressed that women should not panic, but take steps to protect themselves.
RCOG spokeswoman Maggie Blott said: "Women in their first stages of pregnancy in the summer should be aware of the health risk surrounding increases in temperature.
"Pregnant women should stay out of the sun, wear loose clothing, keep well hydrated and eat healthy food little and often."
Royal College of Obstetricians and Gynaecologists
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