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By By Dr. Judith Reichman

Q: I am 20 weeks pregnant and want to fly across the country for my parents' anniversary. Is it safe to travel?

A: Unless you have certain underlying medical conditions or are experiencing complications during your pregnancy, air travel within the United States is safe until you're 36 weeks pregnant. The cut off for most international airlines (determined by the airline industry as well as physicians) is 35 weeks.

If your doctor says you either have an abnormal placenta that could separate or bleed, or are at a high risk for premature labor, you should not take long flights. In the case that you do have complications with your pregnancy, you want to be able to receive immediate medical attention.

It’s important to realize that long-distance commercial flights generally cruise at an altitude of 39,000 to 41,000 feet, and that the cabin air pressure is usually maintained at a pressure similar to that which you would experience at 8,000 feet. However, pregnant women may experience an increase in their heart rate and blood pressure above an altitude of just 6,000 feet. Also pregnant women’s oxygen consumption at 6,000 feet is 13 percent lower than it is at sea level. In contrast, oxygen consumption falls just 3 percent in women who aren’t pregnant.

If you’re suffering from other medical complications — underlying heart disease, hypertension that developed during your pregnancy, poorly controlled diabetes, or sickle cell disease or trait — the high altitude could exasperate these conditions and have consequences for you and your baby. If you don’t have these health issues, then there shouldn’t be any problems.

That said, you should also take into consideration that air travel (especially in coach) inevitably means that you won’t be able to move around. Most of us just sit in our seats without even stretching our legs. Especially on long flights, these cramped circumstances can lead to the formation of venous clots in the leg, and even pulmonary embolism. The risk is small, but it is relevant to all long-distance travelers. Though no published reports have shown that pregnant women are at a higher risk for this medical complication on long flights, I don’t find that reassuring. Even when they’re not flying, pregnant women are more likely to develop varicose veins, poor venous blood flow, and clots.

So once you are up in the air, and the seat belt sign is turned off, I would suggest that you very carefully move around the cabin as much as possible and consider wearing support stockings for your flight. Finally, since you don't know when turbulence will occur, while in your seat, wear your seat belt low on your hip bone, between your abdomen and pelvis.Now some tips for feeling more comfortable on your flight. Since humidity in the cabin is very low (less than 25 percent), hydrate! We do know that dehydration can lead to contractions. I recommend drinking one glass of water for every hour of your flight. Avoid gas-producing foods and drinks before boarding the plane, since trapped gas expands at higher altitudes. You don’t want to feel scary abdominal discomfort 40,000 feet up in the air and wonder if it's a contraction or gas pain.

Dr. Reichman’s Bottom Line: If you don't have an underlying medical condition and are not at risk for premature labor, you can fly on domestic flights up to 36 weeks gestation. Just make sure you stay hydrated, move your legs as much as possible, and eat appropriately before the flight.

Dr. Judith Reichman, the “Today” show's medical contributor on women's health, has practiced obstetrics and gynecology for more than 20 years. You will find many answers to your questions in her latest book, "Slow Your Clock Down: The Complete Guide to a Healthy, Younger You," which is now available in paperback. It is published by William Morrow, a division of HarperCollins.