Obstetrician Leonie Penna replies: Pregnancy increases the risk of blood clots in the legs (deep vein thrombosis or DVT), and lungs (pulmonary embolus or PE). Flying doesn't increase the risk any more for a pregnant woman than a non-pregnant woman, it is simply that the risk in pregnancy is higher whether you choose to fly or not. If you do decide to fly, there are several things you can do to reduce the risk of clots. Move around the cabin or do the exercises shown in the in-flight magazine, drink plenty of water and wear travel socks.
There is some evidence that an aspirin taken 12 hours before a flight may slightly reduce the risk of clots. However, it may cause indigestion, too. Individual airlines have different views about how late in pregnancy they will allow you to fly, so it's worth checking with yours. If you have had complications in this pregnancy or a previous one, it's important to consider this when making travel plans.