DEAR DOCTOR K:

My sister recently gave birth prematurely, at 33 weeks. Thankfully, the doctors think my new niece will do well and should not have developmental problems. I know that doctors are able to save more premature babies these days than they used to. What determines whether a "preemie" survives?

DEAR READER:

I'm glad to hear that your new niece is doing well. Being born at 33 weeks means she was born seven weeks early.

Your question reminded me of a colleague, Dr. Adam Wolfberg, who was in training here at Harvard Medical School. He wanted to become an obstetrician specializing in high-risk pregnancies. He also wanted to conduct research into ways to improve the survival and function of "preemies." He and his wife, Kelly, had two kids, and Kelly was pregnant with their third.

Suddenly and ironically, Kelly went into labor very early, at six months. Their new daughter, Larissa, weighed less than 2 pounds at birth. She clung to life for several months -- and finally made it. I asked Wolfberg about your question.

Most babies are delivered about 40 weeks after the mother's last menstrual period. Labor starts with a perfectly timed cascade of hormonal signals between the developing fetus and the mother. Contractions develop, the cervix dilates, and before long, out comes a well-developed, healthy newborn.

But sometimes labor comes early. Early labor can be caused by complications such as bleeding between the placenta and the uterus, or infection of the placenta that threatens the fetus. Sometimes, for reasons that we don't fully understand, the cervix dilates early, triggering labor.

When bleeding or infection is the trigger, labor is an adaptive response. Simply put, it is the body's attempt to end the pregnancy in an effort to save the mother's life.

Babies born prematurely at 32 weeks or beyond, like your niece, tend to do very well. Sometimes they suffer mild behavioral or learning disabilities during childhood. But the risk of serious complications is relatively low. And almost every baby survives.

Between 24 and 32 weeks of pregnancy, more than 80 percent of babies survive. With each passing week in the womb, babies become less likely to suffer major complications related to their prematurity. Wolfberg's daughter was born a little after 24 weeks, so she was at great risk of neurological and physical impairments, and of dying.

Unfortunately, babies born before 24 weeks, weighing about 1 pound, are more likely to die than survive. Those babies that do live often suffer significant complications. These include severe lung injury, cerebral palsy, mental retardation, injury to their intestines requiring emergency surgery, and any of a host of less-severe injuries. We can hope that one day research will help to improve the prospects for such severely premature babies, as it has in the past 40 years for less severely premature babies.

Dr. Wolfberg tells the story of the struggle to save his daughter, and the science behind it, in the new book "Fragile Beginnings: Discoveries and Triumphs in the Newborn ICU." You can find out more about it at my website: www.AskDoctorK.com.

(Dr. Komaroff is a physician and professor at Harvard Medical School. Go to his website to send questions and get additional information.)

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