A premature baby undergoes treatment at Xining Children Hospital in China.

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Going into Labor

Most women expect that it'll be obvious to them when they go into labor, but if it's their first baby, they may not be so sure. The most obvious signs are contractions. Labor contractions should be strong enough so that it's difficult do anything else, and they last about a minute. They're usually in the lower back and pelvis and feel similar to menstrual cramps. Labor contractions also fall into a pattern, which is why it's important to time them. Once they've been coming every three to five minutes for about an hour (for a first baby), then it's usually time to get going. As time goes on, they get stronger, last longer and get closer together.

The dramatic breaking of the amniotic sac, which is what people are referring to when they say a woman's water broke, means that a woman is definitely in labor.

Sometimes weeks before she goes into labor, a woman's cervix begins to open, or dilate. It must dilate to 10 centimeters for a vaginal delivery. The cervix also gets thinner, known as effacement. Neither dilation nor effacement is felt by the woman as they're happening, but doctors and midwives check for these signs during exams. She may notice the loss of her mucus plug as her cervix effaces and dilates, which is also known as bloody show because it appears as a brown, sometimes bloody discharge.

While all of this is going on, the baby is supposed to drop down into the pelvis, known as lightening, which usually happens a few weeks before labor begins. This takes pressure off the mother's stomach and lungs but can put more on her bladder. The baby should be head-down in the classic birth position. He or she is probably not moving as much, because there's not a lot of wiggle room. The mother may feel little jumps that are actually the baby's hiccups as he or she practices breathing and inhales amniotic fluid. In addition to all this, the baby is building up meconium (the first fetal bowel movement), which will be passed a couple of days after birth.

­A woman's due date is generally calculated as 280 days (40 weeks) after the first day of her last missed period. If you do the math, that would make pregnancy 10 months long, not nine. However, most months aren't four calendar weeks long; they're usually four weeks plus a few days, which accounts for the extra time. It's just simpler to say nine months because the pregnancy is divided into trimesters, or three-month periods. To further complicate matters, although a pregnancy is considered full term at 40 weeks, the due date is an estimation. Women generally give birth anywhere from week 38 to week 42.

A baby born before week 38 is considered premature, but most doctors will allow delivery at week 35 provided the baby's lungs are mature. If the mother goes into labor before this point, doctors may initially try to stop it by prescribing bed rest and IV fluids. If the cervix isn't dilated more than 3 centimeters or effaced, the next step is to administer drugs through the IV such as Yutopar or magnesium sulfate to delay labor. Rarely do these interventions bring the pregnancy to full term, but any extra time is helpful in giving the baby the best chance of survival. If it's before week 34, doctors may give the mother ­steroids to speed the development of the baby's lungs.

On the next page, we'll take a look at the early stages of labor -- and options for pain relief.