A black baby born in the United States is nearly twice as likely as a white baby to die before its first birthday. And in the District, a black baby is more than three times as likely as a white baby to die in its first year of life.

These stark disparities persist despite steady progress in reducing overall infant mortality rates over the past two decades.

Improved medical care alone cannot close the gap in infant mortality between blacks and whites, which experts agree results from a complex interaction of social, economic, environmental and medical factors.

"We should be talking about the ecology of pregnancy," said Dr. Lennox Westney, vice chairman of the obstetrics and gynecology department at Howard University College of Medicine. "It's not an obstetrical problem, it's not a medical problem. The problem is the life style of the pregnant mother, the family, the community, the nation."

Westney and other experts discussed the issue last week at Howard University during a conference on "Factors Affecting Pregnancy Outcomes in Black Populations."

Although better prenatal care and increasingly sophisticated obstetric and perinatal ("around birth") care have nearly halved infant mortality rates since 1970, the decline has slowed since 1980 and even reversed in some areas. Preliminary figures for the District, released in July, showed a 16 percent jump in the infant mortality rate last year.

In 1982, the most recent year for which complete figures are available, the infant mortality rate was 11.5 per 1,000 live births for the total U.S. population, 10.1 for whites, 17.3 for all nonwhites and 19.6 for blacks.

In the District, the infant mortality rate is higher than in any state, and the gap between whites and nonwhites is even greater than in the nation as a whole. The District's rate in 1982 was 21.2 for the entire population, 7.5 for whites, 24.3 for all nonwhites and 24.1 for blacks.

In Boston, a new study shows that despite virtually universal access to medical centers with the highest level of obstetrical and pediatric care, death rates still vary widely by race and income. Mortality rates there are higher among poor families, both black and white, The New England Journal of Medicine reported last month. But in all income groups, the black infant mortality rate exceeds that for whites.

Infant mortality is a widely used barometer of local and national health (the United States ranks 12th, behind Japan, Canada, Australia and eight European countries). But infant mortality is just "one aspect of a more general problem," said Dr. David Rush, head of pediatric and perinatal epidemiology at Albert Einstein College of Medicine in New York. The racial disparity in mortality rates begins at birth and continues into old age, Rush said.

While the total infant mortality rate has fallen by nearly 80 percent since 1935, he said, the ratio of nonwhite-to-white mortality "has hardly changed at all."

And the racial gap in the mortality rate for mothers at time of delivery, far from narrowing, actually has widened since 1935, he said. Nonwhite mothers are more than three times as likely to die in childbirth as white mothers -- a fact that Rush called "one of the worst discrepancies by race in any of the survival statistics I know of."

One of the most important ways to decrease infant mortality and increase the chance of a normal-weight, healthy birth is to pay attention to nutrition -- not only during but also before pregnancy.

"The prepregnancy time is now becoming known to be crucial in terms of the outcome of pregnancy," said Dr. Myron Winick, director of the Institute of Human Nutrition at Columbia University's College of Physicians and Surgeons.

Nutrition during pregnancy has long been known to affect the health of the fetus. A classic study of Dutch women in the winter famine of 1944-45 found that the birthweight of their babies dropped precipitously. The women, all previously well nourished, lost an average of 4 percent of their weight during the famine, but their babies weighed an average of 10 percent less than babies delivered by comparable women before the famine.

This means that the mother's body, "under conditions of severe malnutrition, will tend to protect herself at the expense of fetal growth," Winick said. During pregnancy, the body of a malnourished woman will build up its lactational stores -- the capacity to breast-feed later -- in anticipation of possible scarcity of food after the baby's birth. This happens even if that build-up lowers the baby's birthweight.

During pregnancy, a woman's blood volume rises in order to boost cardiac output and increase the flow of blood to key organs, including the uterus and the placenta. But in a malnourished woman -- even one who gets enough calories -- that increase doesn't occur properly, which hinders the transfer of nutrients from mother to fetus.

"The problem is not a lack of food but a deficient delivery system," Winick said.

A federal program that provides supplemental food for needy women, infants and children (WIC) who are "nutritionally at risk" was recently evaluated by a team of investigators, including Rush at Einstein College of Medicine. The WIC program, started in 1974, has had "a very strong and significant effect" in improving maternal and child health, Rush said. High-risk women who get WIC benefits are more likely to get the prenatal care they need to lower the risk of having a premature, malnourished or low-birthweight baby, he said.

Among both whites and blacks, Rush said, the WIC program lowered the rate of premature births and low birthweight -- two of the biggest risk factors for infant mortality.

Poor nutrition is not the only cause of poor delivery of nutrients to the fetus. Cigarette smoke and other environmental pollutants such as lead, Winick said, cause toxemia that can stunt fetal growth "in very much the same way."

Smoking just one cigarette per day, he said, has been shown to constrict the blood vessels and reduce blood flow to the uterus and the placenta. Similarly, heavy consumption of alcohol -- even before pregnancy -- increases the risk of birth defects.

The time for a woman to begin maximizing the chance of having a healthy baby is before she becomes pregnant, Winick warned.

"One cannot cope with a pregnancy and change a longstanding habit easily," he said. "If a woman is a heavy smoker, the time to cut back or, hopefully, cut it out is before she becomes pregnant. And this is especially true of alcohol use ."

Most obstetricians recommend that a woman reach 120 percent of her ideal prepregnancy weight during pregnancy. To avoid a huge weight gain during pregnancy, therefore, she should get as close as possible to her ideal weight before she becomes pregnant. (For the average, adequately nourished woman, the American College of Obstetricians and Gynecologists recommends about a 25-pound gain during pregnancy.)

And women whose diets are deficient in key nutrients such as iron, zinc, calcium or folic acid should begin building them up before they become pregnant if possible, he said.

But poor nutrition is just one factor in pregnancy outcome. Experts at the Howard conference warned that an all-out attack on infant mortality must deal with a range of social and economic stresses -- poverty, crowded housing, chemical pollution and poor education -- that can affect pregnancy.

"We are living in the wealthiest country in the world," Rush said, "in which we continue to have discrepancies in the well-being of our citizens which seem to me intolerable."