Your Pregnant Body
Side effects: Breathlessness, heartburn, darker nipples, lumpy tissue in armpits or on chest, skin tags
Pregnant women actually breathe easier; they just don't always realize it. Progesterone reprograms the brain so that you'll inhale 30 to 40 percent deeper to supply all that extra blood with oxygen. Some women even add inches to their rib cage as a result of increased lung size. "Your bones change in proportion to the tissue you have to support. After delivery, they'll probably return to normal," says Dr. Sears.
On one hand, this improved breathing capacity, combined with more cortisone, means about a third of asthmatics can -- with their doctor's permission -- go off their medication. (In another third, the asthma gets worse; it stays the same for everyone else.) On the other hand, many pregnant women continue to feel breathless even though they're getting plenty of air, partly because the baby is transferring more carbon dioxide to you. In the last trimester, breathlessness occurs because the growing uterus limits lung space.
Heartburn (or gastroesophageal reflux) is caused by the relaxation of intestinal muscles in the first trimester. Digestion slows, letting food sit longer and creating more acid in your stomach. At the same time, this effect loosens control of the esophagus, which separates the stomach and throat. The contents of the stomach are then able to move backward into the esophagus, or even higher in the throat.
The other chest-related changes -- larger, more sensitive breasts, and darker nipples (from extra pigment) -- are more predictable, but may come with surprises of their own. Many women have breast tissue all across their chest and in their armpits, and hormones can cause it to swell into lumps.
Hormones also stimulate mid-pregnancy growths on the chest (and elsewhere), such as tiny, harmless polyps of skin, known as skin tags, and hair where there wasn't any. When hormone levels drop after birth, these usually disappear.