Despite many fears, the nation's recent sharp decline in infant deaths has been achieved without any increase in the number of babies born with birth defects, Johns Hopkins University scientists reported today.

Starting in the 1970s, neonatal intensive care units in hospitals have been keeping scores of premature and otherwise afflicted babies alive, babies who would have died in earlier times. There have been widespread fears, and some allegations, that one result might be legions of infants with physical and mental deficiencies.

The best evidence, including a Johns Hopkins study of nearly 10,000 babies, indicates this has not happened, said Sam Shapiro, a professor of health policy and one of the country's leading medical statisticians.

The main reason, he said, probably is that infants and mothers have been receiving better care.

"No one can say" what the exact reasons are, he said, but "a reasonable inference is that many factors have come together."

He particularly credited the development of neonatal intensive care units. But also, he said, "we probably have to give some credit" to better access to health care and better nutrition for pregnant women in low-income areas; to improvements in pediatric and obstetric care, including fetal monitoring before birth to detect any difficulties; to detection of birth defects in the womb, and to abortions to prevent births of severely defective fetuses.

Shapiro and his colleagues compared two groups of year-old infants: 4,738 babies born in 1976 and 4,690 born in 1978-79. The infants were from eight areas: Arizona, Cleveland and surrounding Cuyahoga County, Dallas County; the area around Syracuse, N.Y., three parts of Los Angeles and Manhattan's upper West Side.

The babies included nearly 100 percent of each area's infants who weighed less than 3 pounds, 4 ounces, at birth, a group at high risk for defects.

Deaths of infants under 4 weeks decreased by 18 percent in the United States between 1976 and 1978-79. Yet the incidence of birth defects and delays in development in the children studied decreased by 16 percent between 1976 and 1978-79.

Shapiro did note some caveats.

For one, there have been many reports of increased numbers of newborns with defects who need care in institutions.

Only this week, in a strong criticism of the Reagan administration's "Baby Doe" rule--requiring hospitals to treat all newborns, no matter how badly handicapped--Dr. Marcia Angell, deputy editor of the New England Journal of Medicine, said in an editorial: "Handicapped children salvaged in neonatal intensive care units grow into handicapped children who often require more care and support than their families can provide. The numbers of these children are growing as the technology to save them improves."

In another, Shapiro said a Canadian study indicates that there might be an increased incidence of birth defects in the four-tenths of 1 percent of all children who weigh less than 2 pounds, 5 ounces, at birth.

Also, he said, the fact that more children are being cared for in special units may mean that more are finally receiving "appropriate care."

"There are also some studies that disagree with ours," Shapiro added. "But ours fall in line with what most other investigators say."

There also have been studies showing that greatly increased numbers of youngsters up to age 17 are being treated today for chronic illnesses or disabilities ranging from asthma and diabetes to vision defects and learning problems.

There is no agreement yet on the extent to which this represents a real increase in the numbers of such children or an increase in parents' and doctors' awareness of problems that were often ignored or unheard of in the past.