The Reagan administration is proposing severe cuts in welfare benefits for two previously undisclosed categories -- pregnant women and the destitute aged, blind and disabled.

The welfare bill drafted by the Department of Health and Human Services would:

Bar the states from making welfare payments to pregnant women except in the last three months of the pregnancy. At present, 34 states allow a pregnant woman to collect welfare even if she is pregnant for the first time and has no children.

The payments are designed to insure better nutrition and health care, and avert a high incidence of abnormalities in children born to women who don't get proper food and care during pregnancy. Nearly all the 34 states start the payments as soon as the pregnancy is medically verified, usually many months before the last trimester. The proposal would save the United States money by delaying eligibility until the last trimester.

Interestingly, this change was foreshadowed by a regulation proposed by Ronald Reagan as governor of California years ago to cut the costs of payments to pregnant women. That regulation, later struck down by the state supreme court, declared that the body of a pregnant woman constitutes a financial resource to the unborn fetus, the value of which should be deducted from the welfare payment.

Eliminate the so-called "welfare pass-through" under the program for the aged, blind and disabled. The federal government now pays such clients $238 a month ($357 for a couple). Many states also give the beneficiaries something extra out of state funds. A 1976 provision, sponsored by the late senator Hubert H. Humphrey (D-Minn.), forbade the states to reduce their payments if the federal government increased benefits through cost-of-living increases. Otherwise, Humphrey reasoned, the state would simply take away from the welfare clients as much as the federal government added. The Reagan administration proposal would repeal the 1976 law.

About 1.6 million people receive state supplementary payments which total over $2.1 billion a year. The next federal cost-of-living increase is scheduled for July 3, when the payments will go up to $264 for a single person and $397 for a couple.

Back in the 1970s, as part of his California welfare reform package, Reagan decided to alter a 25-year-old state rule that allowed pregnant women to get welfare for themselves and costs associated with the unborn child.

The welfare department, headed by Robert Carleson, now Reagan's special assistant in the White House and chief author of the current Reagan welfare proposals, issued a regulation declaring that while payments would be made to a pregnant low-income woman for herself and her unborn child, they would be reduced by the imputed value of "in-kind" resources the child receives from the mother while insider her womb, in lieu of food, clothing and shelter.

Initially, the value of such resources was calculated to be $65 a month, but this was later reduced. In one court case challenging the concept, the judge decided the plaintiff's "unborn child is deemed to receive 'in-kind income' in the total amount of $35," and would have cash welfare benefits reduced accordingly under the regulation.

The California supreme court threw out this regulation in 1974, declaring: "The regulation before us attempts to assess the value to an unborn child of the comforts he receives in his mother's womb. We think that in the absence of express federal or state provisions on the subject, the instant regulation stretches the existing statutory concept of 'income' or 'resources' beyond the probable intent underlying those terms."

Carleson, in a phone interview, said that in the California case, Reagan was providing pregnant women with a full monthly benefit for themselves and their unborn children. It made sense, he said, to recognize that an unborn child had fewer needs than one already born and to adjust the grant downward accordingly. He also said that today, medical and nutrition needs of pregnant women are taken care of by food stamps, maternal and child health programs, and the women-infant-children food program, so they don't need cash welfare during the start of the pregnancy when they can still work.

The administration draft bill still hasn't been formally sent to Congress, but copies are circulating on Capitol Hill.