Doctors have discovered that a large number of premature babies, perhaps 45,000 a year, suffer unsuspected bleeding inside their delicate brains.

No one knows why. But the bleeding can lead to hydrocephalus (water on the brain) and retardation, and possibly other damage.

A Georgetown University medical team has begun using ultrasound to screen "very small preemies," babies who weigh less than 3.7 pounds, for this unsuspected bleeding. In many cases, this should allow doctors to detect the bleeding soon enough to take steps to prevent the devastating hydrocephalic condition, according to Dr. Edward Grant, a Georgetown radiologist.

"We always know about the small number of premature babies who have massive and obvious internal bleeding. Many of them die," Dr. Duane Alexander of the National Institutes of Health said recently.

But not until the advent of CAT scanning or computerized X-raying a few years ago did doctors realize that unsuspected bleeding afflicted many more babies. CAT scanning put the number at about 44 percent of the 59,400 babies who are born each year at under 3.7 pounds.

After recent study of 40 such infants, the Georgetown group estimates that the incidence may really be a far more disturbing 75 percent.

"A lot of people are getting worried about this," Alexander said. "Internal bleeding means potential brain injury. But we have no follow-up yet to tell us how many such children may really suffer damage."

The one known kind of damage, hydrocephalus, can affect the brain quickly. So, Grant said, "when we detect early signs of it, steps can be taken to minimize or prevent its development." The most common is insertion of an internal shunt or tube to drain the excess fluid into the abdomen.

The bleeding that causes the problem comes from a part of the developing fetal brain called the germinal matrix. By the time a full-term child is born, this part of the brain has evolved into a more mature part, the caudate nucleus.

When the baby is born too early, however, the germinal matrix may begin bleeding. "Why we don't know," Grant said. "Until the last few years we knew very little about this entire subject because so few such babies survived. Now, with neonatal intensive care units, they do survive."

This means that medicine has a good deal of study ahead. The Georgetown group has at least shown, however, that ultrasound (at about $60 and examination) can find nearly twice as many cases of unsuspected bleeding as CAT scanning (at $200 and up per examination). Also, Grant said, there is no radiation, and the ultrasound can be repeated as often as needed to detect any developing damage.