When Katie Young was pregnant with her third child, she went to the hospital twice within a couple of weeks of her due date, thinking she was in labor, only to discover that she was having very strong Braxton Hicks contractions.

"You would think after three kids I would know the difference, but they were so powerful I just kept thinking maybe it was a different kind of labor, less painful since it was my third child," says Young. "Of course, when real labor started, I had no doubts about what was going on."

When they're pregnant, many women notice irregular, generally painless contractions of their abdomen. Sometimes these contractions are almost unnoticeable, but sometimes they can be so intense that the entire abdomen will harden to the point of discomfort. These contractions were first described by British gynecologist John Braxton Hicks in 1872. Research indicates that women notice them more in subsequent pregnancies than in their first pregnancy.

No one really knows why they occur, says Dr. Gerard M. DiLeo, author of The Anxious Parent's Guide to Pregnancy (McGraw-Hill, 2002), but the theory is that it is a natural reaction of the uterus.

"The uterus is a muscular chamber that basically doesn't like to have anything in it," says Dr. DiLeo. "Braxton Hicks are disorganized contractions that are due to the irritability of the uterus. It's just reacting to what is in it. The difference is that Braxton Hicks have many different vector forces in many different directions. In real labor, the vector forces are all in the same direction and are working in a very targeted way to empty the uterus."

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