Pregnant women have been told they are safe to fly for three weeks longer into their pregnancy than previously advised.
New advice from The Royal College of Obstetricians and Gynaecologists says the safest time to fly during pregnancy is before 37 weeks or, if carrying twins, before 32 weeks, new advice 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.
However, the RCOG cautions that if a woman is more than 28 weeks pregnant she should take her medical notes and a GP's letter on board.
A leaflet issued by the Royal College says that if a pregnancy is straightforward, flying is not harmful to the woman or her baby.
It gives advice on the side-effects of flying when pregnant, incuding 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.
Long-haul flights of four hours or more can increase the risk of developing deep vein thrombosis (DVT), when a blood clot forms in the leg or pelvis, and pregnancy increases this risk even more.
The leaflet recommends that women wear loose clothing and comfortable shoes, take regular walks around the plane and do exercises in their seat every 30 minutes.
Cutting down on drinks containing alcohol or caffeine and wearing elastic compression stockings can also help.
In some circumstances, however, it is better to take the decision not to fly when pregnant.
For example, if a woman has an increased risk of going into labour before her due date, has severe anaemia, sickle cell disease, has recently had significant vaginal bleeding or has a serious heart or lung condition.
Philippa Marsden, chairwoman of the RCOG's patient information committee, said the information was relevant for pregnant women travelling both short and long-haul.
She said: "To help decide whether or not to fly, women should think about how many weeks pregnant they will be, what facilities are available at their destination and whether it will increase their risk of medical problems.
"It is important to discuss any health issues or pregnancy complications with your midwife or doctor before you fly."
Cath Broderick, chairwoman of the RCOG women's network, said the guidance offered sensible recommendations.
"There is new advice around what to take on board the flight such as your pregnancy notes, documents confirming your due dates, a European Health Insurance card and any medication you are taking."