The warning, from the Royal College of Obstetricians and Gynaecologists (RCOG), comes as temperatures begin to rise 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 could lead to slightly lower birth weight.

The researchers, from the University of Bristol, concluded that an "increasing occurrence of temperature extremes, in particular, heat waves" could have important public health implications.

The RCOG made its comments in anticipation of a heatwave this summer. It said babies develop most rapidly in the first three months of pregnancy 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: "Pregnant women should stay out of the sun, wear loose clothing, keep well hydrated and eat healthy food little and often."