Two decades of progress in improving the health of poor and minority mothers and their babies has "ground to a virtual halt," according to the Children's Defense Fund.

The group said government statistics document not only a "marked slowdown" in the decline of overall infant mortality, but an "alarming" national increase in deaths among infants between one month and one year old.

In 1983, this death rate rose 3 percent nationwide. "Not since 1978 has there been a nationwide increase in the postneonatal mortality rate and not in 18 years has there been such a large one-year increase," the advocacy group said.

"This trend is disturbing," said the group's president, Marian Wright Edelman.

She said deaths among babies over one month old are "usually a reflection of environmental conditions related to poverty, such as poor nutrition, inadequate housing and sanitary conditions and lack of basic health care, which are largely preventable."

"It's disturbing until we know more about it," said Dr. Joel Kleinman of the government's National Center for Health Statistics.

But, he cautioned, "so far we haven't been able to come up with a clear explanation of what's going on" or even whether there is a trend.

He noted that while deaths among older babies have historically been attributed to environmental factors, there might be a new factor. With "neonatal intensive care units," he said, "we're seeing a greater ability to prolong life beyond the 28-day point" of babies born with severe problems who subsequently die in the first year of life.

The Children's Defense Fund report cited preliminary government numbers indicating an even larger increase in deaths among babies one month to one year old in 1984, but Kleinman said it was too early to predict with confidence.

The report also listed an increase in the rate of babies born at risk because of lower than normal birth weights, an increase in pregnant women receiving late or no prenatal care, and the "widest disparity in more than four decades between the infant mortality rates of blacks and whites."

The private group, a frequent critic of the Reagan administration, blamed cuts in government health programs and a rise in poverty in recent years.

"I think it's shameful that a black infant in Chicago, Cleveland, or Detroit was more likely to die in the first year of life than an infant born in Costa Rica," said Edelman, who expressed concern that the "insanity" of the new budget-balancing Gramm-Rudman-Hollings law will worsen child health.

"I am sad to say, as a resident of the District of Columbia, that the District tends to be almost the worst for everything," added Edelman.

The 302-page report detailed wide-ranging disparities across the country, ranking the five worst states, based on 1983 statistics. The District topped the lists of infant mortality among all races, low-birthweight babies of all races, and women not receiving early prenatal care.

The District also was in the top five, the group said, in nonwhite infant mortality (South Dakota was highest), mortality among babies of all races one month to one year old (Mississippi ranked worst), and babies born to women receiving late or no prenatal care (New Mexico was higher).