This week our pediatrician told us that our sweet 5-week-old daughter has a murmur called peripheral pulmonary stenosis. The doctor said not to worry--it's very likely going to go away by itself. Still, she said she'll be checking for it again next month, just in case. In case of what? Can you explain this condition to me?

Parents are often terrified when a doctor tells them that their baby has a heart or lung murmur. The word "murmur" resonates with nightmare visions about their child's health. Indeed, parental fears about murmurs can be so significant that an entire literature has developed in pediatrics about whether or when to mention murmurs to parents. The good news for you, and for most parents, is that you really don't need to worry.

It's helpful to know that, while some serious murmurs do exist, the vast majority of murmurs are harmless. Almost all--perhaps all--children have one murmur or another some time after birth. Such murmurs last from only a few hours up to six months, and they resolve themselves without any intervention.

Normally in people the amount of blood flowing through the heart is exactly right for the heart's size. The only sound that is made comes from the opening and closing of valves. Sometimes, however, the flow of blood is turbulent, rather than silent. Turbulence can occur when the heart and the blood supply are not quite matched in size: the heart has grown either more slowly or more quickly than the blood supply has. Such a mismatch frequently occurs around birth because the baby is growing so much.

A murmur may also arise at birth because so many changes take place after the umbilical cord is clamped: the direction of blood flow shifts, causing some vessels to open up while others close. Most of the time such murmurs are temporary and benign. But since a murmur can occasionally reflect an improperly functioning valve or a hole in the heart, pediatricians do continue to check on them, "just in case."

Your daughter's murmur is not actually in the heart. Peripheral pulmonary stenosis (PPS) is caused by turbulence in the arteries to the lungs. Normally these arteries are very small before birth. In utero only 15% of the baby's blood goes from the heart to the pulmonary arteries, because unborn babies do not use breathe through their lungs. . After birth all of the blood from the heart must pass through these arteries to the lungs, in order to receive oxygen. There is thus a huge increase in the blood supply passing through relatively small vessels. Turbulence arises at the fork where the main artery splits into two, with one side going to each lung. In some babies it takes time for all the arteries of the lungs to open up to the appropriate size, causing the murmur of PPS. Because PPS is caused by small vessels, it is more common in smaller babies (under 7 lbs). Over time--the amount varies with each child and may be up to six months--the arteries and the fork will grow larger. The turbulence will stop; the murmur is gone.

There are no problems associated with PPS. It is watched only to make absolutely certain that the murmur wasn't something else more serious. The more serious pulmonary murmurs are very likely to be diagnosed immediately, however, since they make an extra clicking sound that is not heard with ordinary PPS, or else they are associated with syndromes that have other noticeable symptoms. If your child's pediatrician has told you not to worry, it's because she doesn't hear the extra click, and she sees no other symptoms to be concerned about. She has only mentioned the murmur to you because of a strong ethic that patients and parents should be kept informed.