DISCOVERY DEVELOPMENT

Now that you're pregnant, you're dutifully attending your childbirth education classes, and reading everything you can get your hands on about the stages of labor and birth.

But classes and books tend to give you the big picture, not the surprising, and sometimes embarrassing, details. Because knowing what to expect makes for a less stressful delivery, we have asked the experts to spill the beans on the following common (yet seldom-talked-about) scenarios.

- You throw up. Who knew that vomiting during labor is normal? I certainly didn't, until the birth of my first daughter nearly three years ago. One reason it happens: Epidurals can cause hypotension, a sudden drop in blood pressure. "An early sign of this is nausea and vomiting," says David Birnbach, M.D., spokesperson for the American Society of Anesthesiologists. But throwing up can occur even if you haven't been given an epidural, either because of the pain you're experiencing or as a result of food sitting in your stomach (digestion usually stops during labor). To keep vomiting to a minimum, eat only light foods during the earliest stages of labor, and stop eating completely, and drink only clear liquids, once you're in active labor.

- Your teeth chatter. "Nearly 50 percent of women complain of shivering and chattering teeth," Dr. Birnbach says. It has nothing to do with being cold. (In fact, your body temperature may rise a degree or two during labor, making you feel hot.) The jury's still out on what exactly causes this, but the latest evidence points to blood incompatibility. "During labor, a small amount of fetal blood crosses into the mother's bloodstream," says Henry Klapholz, M.D., an associate professor of obstetrics and gynecology at Harvard Medical School. "Studies show that when there's an incompatibility in blood type between mother and baby, for example, your blood is type A and your baby's is type B, the mother shakes, shivers and get chills," says Dr. Klapholz.

- You make nasty noises. As a baby descends through the birth canal, air gets forced out the anus, so be prepared to pass gas. This is especially likely, if you've had an epidural, which paralyzes the anal sphincter. Another unpleasant side effect of childbirth: You may have a bowel movement right on the delivery table. "It's purely a space issue," says Arianna Sholes-Douglas, M.D., director of the High-Risk Pregnancy Center at Mercy Medical Center, in Baltimore. "As your baby's head makes its way through the birth canal, the rectum gets flattened and its contents pushed out." In any event, don't worry. "These bodily functions happen all the time, there's very little we haven't seen or heard before," says Deborah Robbins, R.N., program manager of parent and childbirth education at Brigham and Women's Hospital in Boston.

- You act like a lunatic. During labor, especially if you haven't been given pain medication, you may find yourself screaming, crying, even swearing at your husband or doctor. Or you may strip. "I've had patients who were so uncomfortable that they pulled off their gowns and delivered naked," says Lisa Fraine, a certified nurse-midwife in Allentown, Pennsylvania. All of these reactions are common; they're simply a response to pain and exhaustion. You can also blame hormones: "Labor causes a shift in your estrogen and progesterone levels, which is akin to a major case of PMS," Dr. Klapholz says. If you do lose it, don't feel bad. Doctors and nurses are used to these reactions. (Still, it doesn't hurt to apologize afterward.) But if you're uncomfortable with the idea of such a display, be prepared for your delivery. "Women who take childbirth classes tend to stay calmer than those who don't," Dr. Klapholz says.

- Your mind goes blank. In the heat of the moment, it can be easy to forget what they taught you in childbirth class. "I couldn't remember the various positions I was supposed to get into to ease labor pains," says Elizabeth Estes Niven. "Instead, I stayed flat on my back, gripping the bed railing." You're also likely to forget many of the details of the birth. So be sure your partner takes plenty of photos or captures it all on videotape.

Sandra Gordon, who has written for Parents, American Baby, and Child magazines and is the co-author of Consumer Reports Best Baby Products (2004), is a New England-based freelance writer.