What's going on: Your immune system hunts down and destroys invading organisms. So why does it leave your fetus (which, genetically speaking, is half foreign) unharmed? Because the placenta -- the organ that delivers oxygen and nutrients to your growing baby -- cranks out an enzyme that stops the "natural killer" cells circulating in your blood and lymphatic system from attacking. Does this downshifting of the immune system mean that expectant moms are more prone to colds and flus? Paradoxically, the opposite may be true: Pregnant women seem to be less likely to catch them. "We're not sure how this happens, but immunity to viruses revs up in pregnancy," says Roberta Ness, M.D., an epidemiologist at the University of Pittsburgh School of Public Health.
What to do about it: You may be less prone to illness now, but you can still get sick. So keep bugs at bay by washing your hands religiously and avoiding sick people like, well, the plague. Cutting down on stress, getting lots of rest, and eating well will also keep your defenses up. If you're in your second or third trimester during flu season, consider getting a flu shot, too.