For some women, pregnancy is the best medicine in the world – more effective than any state-of-the-art pharmaceutical.

"I suffer from severe psoriasis, but it disappeared during my pregnancies," says mom of two Nicole Salvatore. "I was essentially cured."

This doesn't surprise Jerry Bagel, dermatologist and spokesperson for the National Psoriasis Foundation. "Many women with psoriasis get better in pregnancy," he says. "The specific causes haven't been nailed down yet, but it's likely due to the steroid-like nature of some of the pregnancy hormones."

Expectant mothers with multiple sclerosis also tend to get better during those nine expectant months. "Symptoms can vastly improve or even disappear, particularly in the second and third trimesters," says Patricia O'Looney, vice president of medical research for the National Multiple Sclerosis Society. Again, researchers are looking to pregnancy's hormones for answers. "Certain hormones appear to have a protective effect," O'Looney says.

Rheumatoid arthritis is also no match for pregnancy. "More than 75 percent of patients have some relief, and some go so far as to have a complete clinical remission," says rheumatologist Beth Jonas of the Thurston Arthritis Research Center at the University of North Carolina.

Hormones probably don't play a role here the way they do with psoriasis and multiple sclerosis. Instead, one fascinating theory points to a phenomenon called "immune mediation." As Jonas explains, the baby's immune system may actually change the mother's immune response.

Unfortunately, pregnancy doesn't usually provide a permanent cure for these conditions. Symptoms typically return some time after childbirth. The silver lining is that pregnancy's effect on these illnesses may pave the way for better treatment.

"In the case of multiple sclerosis, the question becomes whether we can help patients by giving them hormones that mimic pregnancy," says O'Looney. "That research is happening right now."