This topic covers how preterm labor affects the pregnant woman.

Preterm labor is labor that comes too early—between 20 and 37 weeks of pregnancy.

In labor, the uterus contracts to open the cervix . This is the first stage of childbirth. In a full-term pregnancy, this doesn't happen until at least week 37.

Preterm labor is also called premature labor.

The earlier a baby is delivered, the higher the chances are that he or she will have serious problems. This is because many of the baby's organs—especially the heart and lungs—aren't fully grown yet.

For infants born before 24 weeks of pregnancy, the chances of survival are extremely slim. Many who do survive have long-term health problems. They may also have trouble with learning and talking and with moving their body (poor motor skills).

Causes of preterm labor include:

• The placenta separating early from the uterus. This is called placenta abruptio.
• Being pregnant with more than one baby, such as twins or triplets.
• An infection in the mother's uterus that leads to the start of labor.
• Problems with the uterus or cervix.
• Drug or alcohol use during pregnancy.
• The mother's water (amniotic fluid) breaking before contractions start.

Often the cause isn't known.

Sometimes a doctor uses medicine or other methods to start labor early because of pregnancy problems that are dangerous to the mother or her baby.

It can be hard to tell when labor starts, especially when it starts early. So watch for these symptoms:

• Regular contractions for an hour. This means about 4 or more in 20 minutes, or about 8 or more within 1 hour, even after you have had a glass of water and are resting.
• Leaking or gushing of fluid from your vagina. You may notice that it is pink or reddish. This is called a rupture of membranes, also known as your water breaking. When this happens before contractions start, it's called premature rupture of membranes, or PROM. When it happens before 37 weeks of pregnancy, it is called preterm premature rupture of membranes, or PROM.
• Pain that feels like menstrual cramps, with or without diarrhea.
• A feeling of pressure in your pelvis or lower belly.
• A dull ache in your lower back, pelvic area, lower belly, or thighs that doesn't go away.
• Not feeling well, including having a fever you can't explain and being overly tired. Your belly may hurt when you press on it.

If your contractions stop, they may have been Braxton Hicks contractions. These are a sometimes uncomfortable—but not painful—tightening of the uterus. They are like practice contractions. But sometimes it can be hard to tell the difference