Every time a woman on welfare has another baby after her first child, she digs herself deeper into poverty, according to a study by Population-Environment Balance, a nonprofit advocacy group for population stabilization.

The study, based on Census Bureau statistics, found that the more children a one-parent, female-headed poor family has, the further it falls below the poverty line. The average family on welfare includes two children.

The study challenges the notion that welfare provides an incentive for women to have more and more children out of wedlock in order to improve their economic status. It suggests that if the economic incentives offered by welfare were the main reason for teen-age pregnancy and out-of-wedlock births, poor women would never have more than one child out of wedlock.

It is only with the first child that a woman can improve her economic status, because it is the first child that makes her eligible for welfare payments for herself and her child.

M. Rupert Cutler, a former assistant secretary of agriculture who heads Population-Environment Balance, said some people "believe that welfare and child-support programs act as incentives that encourage poor women to have additional children. They argue that the increase in payment that a poor woman receives after having an additional child actually raises the family's standard of living." But the figures show otherwise, he said.

"These programs do not cover the cost of having additional children. In all of the 128 cities and metropolitan areas" that were analyzed, "additional children brought poor families greater poverty."

The analysis, based on 1980 local area data brought forward to 1985, covered all female-headed families with children below the official poverty level, whether or not they were on welfare.

In the District of Columbia, it found that the average female-headed poor family with one child had cash income that was $2,523 below the official poverty line for a family that size, which was $7,230 in 1985.

But District of Columbia female-headed poverty families with two children were found to be $3,047 below the poverty line; with three children, $3,684 below the poverty line; with four children, $4,189 below; with five children, $4,864 below.

The pattern held in every jurisdiction studied.

One major reason for this, according to Tom Joe, a welfare expert and director of the Center for the Study of Social Policy, is that the additional welfare payments made by Aid to Families with Dependent Children (AFDC) for each child after the first are relatively small in most jurisdictions.