You’ve planned your trip in advance and have just found out that you’re pregnant. You have heard that you should not fly after a certain time. What is correct? What are the potential risks?

The average pregnancy lasts 40 weeks. According to The American College of Obstetricians and Gynecologists, the safest time for travel is in the second trimester, between 18 and 24 weeks. Before 18 weeks you run the risk of miscarriage or ectopic (tubal) pregnancy. This is not necessarily induced by travel, but you would require medical help if it happened. After 24 weeks there is increased risk of pre-term labor or high blood pressure.

Many airlines have restrictions for traveling while pregnant. For instance, British Airways allows you to travel at up to 36 weeks of gestation with an uncomplicated single pregnancy. If you are carrying twins, triplets or more, you can only fly on their airline until 32 weeks.

After 28 weeks you are required to have a written letter from your doctor or midwife stating that your pregnancy is uncomplicated, and your estimated due date. Your doctor should affirm that you are in good health and that it is OK for you to fly. British Airways even has belt extensions, if needed.

United Airlines has no restrictions during the first eight months. At nine months a written letter from your doctor is required.

Generally, airlines allow you to travel up to 36 weeks for domestic flights and up to 32 weeks for international flights. While it is best to call the airlines, the website www.babycenter.com has some great information on airline travel and pregnancy.

Many women may worry about harm to the baby from the cabin pressure in the airplane. Since the air pressure is less than at lower altitudes, it will increase your heart rate and blood pressure, due to the body’s demand for more oxygen. For healthy women, this should not be a problem for you or your baby.

According to the Centers for Disease Control and Prevention (CDC), pregnant women with complications such as high blood pressure, diabetes, toxemia or multiple gestations (twins) may be advised not to fly. Always check with your doctor. (Flying in an unpressurized small plane is not advised.)

As pregnant women are at an increased risk for blood clots in their legs, make sure to get up and walk and move your legs during the flight. Drink plenty of fluids to avoid dehydration.