Avoid non-pressurized, high-altitude flights. As you probably know, most airline cabins are pressurized to compensate for the lower levels of oxygen available at high altitudes. Once you get to 7,000 feet above sea level, oxygen levels decrease as altitude increases. When you make your reservations, be sure you're getting a plane with a pressurized cabin. Be especially careful with commuter flights, as they are not pressurized, since they usually fly at low altitudes. While a short time spent in an unpressurized cabin above 7,000 feet is unlikely to harm your baby (baby's oxygen level in the womb is already lower than mother's), it can reduce the oxygen in your blood, causing you to feel lightheaded and impair your thinking and ability to move. (Pregnant women should avoid vacationing at altitudes greater than 7,000 feet. Some studies show a statistical correlation between living in high altitudes and having lower birth weight babies.)