A normal pregnancy lasts about 40 weeks after the first day of the last period (38 weeks after fertilization). Contractions and dilation (opening) of the cervix before 37 weeks of pregnancy is called preterm labor. Babies born before 37 weeks of pregnancy are called premature babies.

The danger of preterm labor is that it will lead to the premature birth of a baby that has not fully developed, and therefore has a high risk of problems. About 10% of all pregnancies result in premature birth. The majority of serious complications or infant deaths are due to consequences of premature birth. Unfortunately, the number of babies born prematurely in the U.S. has risen in recent years.

Preterm labor can be extremely frightening, because parents-to-be quite naturally fear that their baby will be born too early and suffer the problems of prematurity. If your baby is born too soon, there is a good chance that his or her lungs will be underdeveloped. If so, he or she will need to be put onto a ventilator to help with breathing. Receiving oxygen through a ventilator can lead to complications, so doctors try to get premature infants off the ventilator as soon as possible.

Your baby may also have trouble maintaining a normal body temperature, may become hypothermic (too cold), and will need to be kept warm. If your baby is born too early to coordinate the muscles to suck and swallow, he or she will have to be fed intravenously, through a needle in a vein, or through a tube passed through the nose and throat and into the stomach.

A premature baby may also develop complications such as:

• bleeding in the brain;
• an increased risk of infections, especially meningitis and sepsis (a serious, widespread infection throughout the body);
• problems with kidney function; and
• jaundice.

Premature babies are at higher risk for long-term complications, which may include vision impairment or blindness, hearing impairment, cerebral palsy, and chronic lung problems. The earlier the baby is born, the more likely that she will have these problems.

Who Has Preterm Labor?

You are more likely to experience preterm labor or preterm birth if:

• You've had preterm labor or delivered a premature infant in the past (this is a most important risk factor).
• You're carrying more than one baby (twins or triplets).
• Your mother used the medication diethylstilbestrol (DES) while she was pregnant with you.
• You have an abnormally shaped uterus or an abnormal cervix.
• You've had a cone biopsy of your cervix.
• You're younger than 17 or older than 35.
• You got pregnant while using an IUD, and the IUD was left in place during pregnancy.
• You were seriously underweight when you became pregnant.
• You smoke or use cocaine or other street drugs.
• You've had second-trimester miscarriages during previous pregnancies, or you've had 3 or more elective abortions.
• You have preeclampsia, kidney disease, heart disease, diabetes, an infection, or other medical condition. 
• You're not receiving prenatal care from a qualified healthcare provider.
• You have a cervical infection, such as group B streptococci, gonorrhea, chlamydia, syphilis, trichomoniasis, or gardnerella.
• Your job involves extremely strenuous, physical work.

 What Causes Preterm Birth?

Premature delivery may be preceded either by contractions or premature rupture of the fetal membranes (PROM), when the water breaks before labor begins.

Although there have been many advances in caring for premature babies, there has been no improvement in solving the problem of preterm labor or PROM. Medical science doesn't completely understand why some women go into labor or break their water too early. There's no completely accurate way to predict which women will do so or prevent these women from giving birth prematurely. In some cases, an infection may be involved. In others, it may be an abnormally short cervix or a combination of factors. In about half of all cases, no cause can be found.