Seven out of every 100 infants in the United States is underweight at birth and faces a higher risk of dying within the first month of life than a baby of normal weight.

Being underweight also means these children have an increased chance of developing disease and being handicapped, problems that extract heavy emotional and financial tolls for their families.

The solution, according to a National Academy of Sciences (NAS) report released Monday, is a combination of better prenatal health care programs, health education and family planning. Every dollar spent on prenatal care alone "would save $3.38 in the total cost of caring for low birthweight infants," the NAS report says.

"It's hard to find any reason not to do it," says Dr. Richard E. Behrman, dean of Case Western University's School of Medicine and chairman of the Institute of Medicine committee that conducted the study for the NAS.

Yet the report notes that the number of expectant mothers seeking prenatal care during the first three months of pregnancy "has leveled off and possibly reversed since 1981" -- a pattern that is most evident among women with the greatest risk of having an underweight child. To change that trend, the report calls for a "broad national commitment to ensuring that all pregnant women, especially those at socioeconomic or medical risk, receive high quality prenatal care."

Making the recommendations a reality, however, may be difficult. The NAS committee advocates improving health education in the schools aimed at preventing teen-age pregnancy and encouraging family planning. "We have to make sure that we provide sensible reproductive education and early prenatal care for all mothers . . . because the price is paid by all citizens, either by taxes or increased insurance rates," says Behrman.

Moreover, the NAS committee urges liberalization of Medicaid eligibility requirements for pregnant women -- just at the time the Reagan administration is seeking caps on Medicaid spending.

"We've been cutting back on the exact care that would be most necessary for these kinds of babies," says Dr. Gordon Avery, director of neonatology at Children's Hospital and professor of child health at George Washington University Medical School. "The Reagan administration is wanting to have it both ways right now. They're mandating that every baby, regardless of prognosis, be given full-court press in treatment , and yet at the same time, they're putting the squeeze on resources to care for these babies" and prevent underweight births.

While the problem of underweight infants cuts across all socioeconomic, racial and religious lines, those at greatest risk of having a low-birth-weight baby are teen-agers, who are usually unwed, poor, and often black. Also at increased risk of producing an underweight child -- though not quite as high as teen-age mothers -- are women age 35 and above.

Beginning prenatal care in the first trimester increases the chances of detecting medical problems for both mother and child and helps ensure better nutrition and adequate weight gain during pregnancy, all of which is important in producing a normal baby, the report says.

"We need to look at prenatal care the way we look at immunizations," Behrman says. "We know that we will reduce the incidence of diseases."

Low-birth-weight babies are infants who weigh less than 2,500 grams, or about 5.5 pounds, at birth. At even greater risk are very-low-birth-weight infants, weighing 3.5 pounds or less. "The lower the birth weight, the greater the danger to the child," says Dr. Heinz Berendes of the National Institute of Child Health and Human Development.

Many underweight newborns are often premature, less than 37 weeks gestation, and must make up growing time missed in the womb. Usually that means an expensive stay in the neonatal intensive care unit. But some low-birth-weight babies are full-term; they just never received enough nourishment to reach normal weight.

While the exact cause of low birth weight is still unknown, research shows certain diseases, such as high blood pressure, increase the chances of having an underweight baby. A multiple birth also raises the risk, as does short spacing between pregnancies. The NAS report notes that women who have six months or less between pregnancies are more likely to deliver an underweight infant.

But the report also notes that these steps can minimize the incidence of underweight births: Early prenatal care, beginning within the first three months of pregnancy and continuing with regular checkups until delivery. Stopping cigarette smoking. Estimates are that 25 percent of low-weight births could be prevented if pregnant women didn't smoke. "The average smoker during pregnancy lowers a baby's birth weight by half a pound," says Dr. Duane Alexander, acting director of the National Institute of Child Health and Human Development. "Heavier smokers reduce a baby's weight even more." Minimizing alcohol intake during pregnancy. Studies suggest that even one to two ounces of alcohol per day decreases a baby's birth weight. About 5 percent of underweight births are linked to alcohol consumption during pregnancy. Maintaining good nutrition. Expectant mothers who gain 25 pounds during pregnancy increase the likelihood of delivering a normal weight baby.

"Those are the things that we know diminish the chances of having a low-birth-weight baby," says NICHD's Alexander. "But even after that there still remains a substantial number."

Particularly troubling to researchers is an excess number of underweight births among blacks, even when other risk factors are accounted for. Researchers don't know why.

It may turn out that some premature births are caused by symptomless uterine infections during pregnancy, which go undetected and somehow prompt an early delivery. Urinary tract infections are also cited by the NAS report as a related cause of low-birth-weight.

"There really are many different factors that relate to the size of a baby at birth," says Berendes. The mother's size and weight is very important. Women in the United States who weigh less than 100 pounds tend not to gain enough weight during pregnancy, which puts them at an increased risk of delivering a low-birth-weight child.

But the problem may be far more complicated. A recent study by Berendes, Dr. Mark Klebanoff and Barry Graubard, found that women who were low-birth-weight babies themselves were more likely to deliver an underweight baby. The study, which was published in the Journal of the American Medical Association, showed that mothers who weighed four to six pounds when they were born were three times more likely to deliver a low-birth-weight infant than women who weighed eight pounds or more.

"We really have paid very little attention to low birth weight," says Dr. Louella Klein, president of the American College of Obstetricians and Gynecologists and a member of the NAS committee. "We need to have better provision of services to patients, but women also have a responsibility to be responsible to their babies."