By Christine Haran

Few sights are as heart wrenching as that of a tiny premature baby hooked up to a web of plastic medical tubing. Although more and more of these babies will survive, many will have physical and developmental disabilities.

The only way to prevent these lifelong problems is to prevent premature birth. But premature delivery is on the rise in the United States. According to the March of Dimes, the rate of premature birth increased 21 percent between 1981 and 2001. While some risk factors for premature delivery are beyond our control, prenatal care and lifestyles changes can help women lower their risk of having a premature delivery.

Attention to the early warning signs of premature labor may also help delay premature births. In fact, the American College of Gynecologists has released new guidelines on the use of the hormone progesterone to prevent premature labor in certain women. Below, Nancy Green, MD, medical director of the March of Dimes and an associate professor of pediatrics at the Albert Einstein College of Medicine in New York City, discusses causes of premature birth and how women can help increase the likelihood that their pregnancies will be full-term.and their babies will be healthy.

How does the body know when to go into labor?

Normal labor and delivery are presumably triggered by a complex set of physical and chemical processes. Hormonal levels, which help to maintain pregnancy, are known to change dramatically as labor approaches. Other potential triggers for delivery include signals from the uterus due to stretching as the baby grows. These processes may be the same or may be different for preterm delivery.

What is the normal length of gestation?

The average length is 40 weeks, and normal is considered 38 to 42 weeks. The birth is considered preterm if it is less than 37 completed weeks of gestation.

How big a problem is preterm delivery in the US?

Preterm delivery is an enormous problem in the United States. As of 2002, 12 percent of all deliveries were preterm. That makes the United States about 27th or 28th of industrialized nations in terms of adequate birth weights and gestational lengths. So we're pretty far down there -- around Cuba -- and the problem is getting worse annually over the last 20 years, not better.

What are the causes of preterm delivery?

About half of all the spontaneous deliveries happen for unknown reasons. So when you ask about, "What are the causes?" I can only address that half that we know about. Most preterm deliveries are due to spontaneous preterm labor, but about 10 percent of the total are due to spontaneous rupturing of the membranes, meaning the sac in the uterus that holds the fetus breaks too soon.

About 20 percent or 25 percent of premature deliveries are so-called "medically indicated." For example, inducing preterm labor and delivery is the best treatment for preeclampsia, which is a condition that characterized by high blood pressure, weight gain and swelling. With improved technology, with maternal monitoring and fetal monitoring, those pregnancies are intentionally delivered early for the sake of the health of the mother and baby.

What increases the risk for a preterm birth?

If you've already had a preterm birth you are at about 30 percent increased risk of having another one. If you've had a history of two previous preterm births, then your risk of having a third preterm birth is up to 70 percent.

Twins have about a 50 percent chance of being born premature, and for triplets, it's about an 83 percent chance of being born premature. The rate of twins in the US population has been increasing steadily, in part, due to the increase in the average age of mothers, as well as the use of hormones and other forms of reproductive technologies. The 2001 data, which is the most current for the United States, find that 3.2 percent of all US births are multiples. As the rate of multiples has increased that has, in part, contributed to the rise in preterm births.

Other risk factors are infections in the mom and certain structural defects in the uterus and the cervix. Then there are a bunch of risk factors that are associated with preterm birth but don't necessarily cause preterm birth. Those include women who smoke during pregnancy, women who are at the extremes of weight (either underweight or very overweight), African-American women (for reasons which I wish we knew but don't), women of low socioeconomic status, and women with certain unhealthy lifestyles that involves alcohol abuse and illicit drug use.

Are underlying health problems associated with preterm delivery?

Certainly any underlying health problem can increase risk; hypertension and diabetes are the most common conditions associated with preterm birth, but then other things like autoimmune disorders are associated with an increased risk of preterm birth. Then there are less common problems, like women who have specific bleeding disorders.

Is there anything women can do to reduce risk for preterm birth?

There's a lot that a woman can do to reduce her risk of having a premature baby, even though that doesn't guarantee she won't have one. Certainly before pregnancy is the best time to detect and treat any kind of underlying health problem, such as diabetes, weight problems, smoking or hypertension. So "preconception" health and health care is very important. Once a woman is pregnant, she can maintain a healthy lifestyle, have good prenatal care, be screened for underlying medical conditions.