The tiny morsel of a person had weighed in at less than 500 grams at birth and appeared to have no chance for survival. But this was a fighter of a baby and he somehow managed to maintain just enough breath for life.

Now machines pumped blood and oxygen, and the nurses at the Children's Hospital nursery checked him against charts and graphs, monitoring temperature readings and heart beats. The newborn lay on his side with fists clinched and face wrenched, as if literally fighting to live.

In hospital jargon, this baby existed in "twilight time," that period through which it would live and prosper only by the grace of God.

The baby was what is known as a "low-weight" birth, as light as a telephone receiver, a category that includes two-thirds of the infants being cared for at the Children's Hospital nursery. Of the 179 infant deaths in the District in l983, 135 were low-weight births.

"The problem is that you never know what will happen," said Dr. Anne Fletcher, director of the nursery. "You can't predict when you'll get that 'miracle baby,' that 500-gram about a pound baby that makes it. But for every one you get, 85 or 90 will die."

Juxtaposed against the advances in medical technology, the statistics make no sense. There is extra-corporeal- membrane-oxygenation, which adds oxygen to the blood and removes carbon dioxide, allowing sick lungs to rest and heal. There are ventilators that breathe for the baby, scanners and monitors for blood pressure and thermal regulation and pumps that provide essential fluids for life. There are the best doctors and the most dedicated nurses -- still babies die.

As difficult a problem as it is to solve, there is clear indication of its cause: Many of the babies -- like the little fighter who desperately clung to life -- had not received good prenatal care. Not one visit to a clinic or doctor.

Although blacks have a much higher rate of low-weight births than whites, says Fletcher, "Pound for pound, the low-weight black baby is more mature at birth than a low-weight white baby and has a better chance of survival."

If given proper prenatal care.

But despite medical advances that have expanded enormously a baby's chances of surviving birth and infancy, hundreds of pregnant women in this city have either no access to them or do not take advantage of them. The result is that babies still are being born at home in beds, still being delivered in emergency rooms, still being delivered only weeks after the mother learned that the bulge in her stomach was not just fat.

According to Marian Wright Edelman, director of the Children's Defense Fund, the number of women in this city seeking prenatal care is declining. More women, she said, are opting for late care or no care during pregnancy.

Says Fletcher: "Many mothers simply haven't given adequate thought about having the baby. With young kids in this city, there is a tendency to have a baby so they can have someone to play with, someone to call their own. There are no plans made for caring for a child either before birth or for the next 21 years of childhood."

From a technological point of view, the expertise exists to ensure the best of care at birth. But it is that critical period before a child is born that is all too often ignored.

The scene around the city in this season of passion tells the story in stark terms as young mothers-to-be proudly sashay along the streets. Ignorant of the physiology of the fetuses they carry, they appear to relish the warmth that is the "miracle of life," all the while munching on candy bars and puffing cigarettes.

Sometimes, Fletcher said, she has to tell mothers of babies struggling in the incubators, "If I was God, I'd make your baby better."

But there was no such end to the twilight for the little fighter at Children's Hospital last week, for miracles are no substitute for care.