The safest time to fly during pregnancy is before 37 weeks or, if carrying twins, before 32 weeks, new advice says.
The Royal College of Obstetricians and Gynaecologists says flying is not harmful during a low-risk pregnancy, but there may be side-effects.
If more than 28 weeks pregnant, a woman should take her medical notes and a GP’s letter.
Many airlines have their own rules on when pregnant women can fly.
Although everyone who flies is exposed to a slight increase in radiation, there is no evidence that flying causes miscarriage, early labour or a woman’s waters to break.
The changes in air pressure and the decrease in humidity on an aircraft have not been shown to have a harmful effect on pregnancy either.
If the pregnancy is straightforward, flying is not harmful to the woman or her baby, the RCOG leaflet says.
Previously, the advice for women with multiple pregnancies was that the safest time to fly was before 34 weeks, but this has been changed to 32 weeks to tie in with International Air Transport Association (IATA) recommendations.
After 37 weeks, a woman may go into labour at any time.
The leaflet gives advice on the side-effects of flying when pregnant.
These include swelling of the legs due to a build-up of fluid, nose and ear problems caused by changes in air pressure and motion sickness making any pregnancy nausea a little bit worse.