TRENTON, N.J. (AP) _ New Jersey today becomes the first state to deny an increase in benefits to mothers who have babies while on welfare, part of a reform package intended to make poor families more self-sufficient.

Opponents of the provision say children will suffer, and they have threatened a court fight.

Under the law, a welfare recipient who has a child 10 months or more after enrolling in the federal-state Aid to Families with Dependent Children program no longer will get the usual $64 monthly increase in benefits.

''I don't think that's right. Some people need that help,'' Barbara Johnson, a 21-year-old welfare recipient in Trenton, said as she waited to see a caseworker.

''Sixty-four dollars can buy Pampers. Sixty-four dollars can by T-shirts for my child. For somebody that don't have a job, $64 is a lot better than none.''

Johnson, who has a 4-year-old daughter, is two months pregnant but will not be affected because the state is allowing a 10-month grace period before it begins withholding benefits.

The freeze is part of a law that federal and state officials say will help break the cycle of poverty.

The law allows a welfare recipient who has a child to work and earn an amount equivalent to half her monthly grant without losing benefits. It also allows mothers to marry without losing benefits for their children.

The typical AFDC family in New Jersey, a mother and two children, receives $424 per month.

Other provisions of the Family Development Program, as the package is called, begin today in only three counties - Camden, Essex and Hudson - where more than half of New Jersey's 130,000 welfare families live. The provisions will be phased in statewide over three years.

These provisions are aimed at linking families with the services they need to become self-sufficient, including education, child care, health care and job training.

Women on welfare will be required to go to school or receive job training, or risk losing some or all of their benefits.

''We need to work with all the issues that affect a family,'' said Aletha Wright, a Human Services Department official responsible for the program. ''Before, we focused a lot on the adult, and now we're looking at the adult and the family.''

The law was signed by Gov. Jim Florio in January and received the necessary approval from the Bush administration in July.

President Bush has promised to help states that try new welfare approaches, and reforms are being considered in such states as Michigan, Maryland and Wisconsin. More than 40 states have cut or frozen welfare benefits in the past two years.

New Jersey's denial of benefits for additional children has been the focus of controversy.

''It's safe to say there'll be some legal challenge,'' said Martha Davis, attorney for the NOW Legal Defense and Education Fund in New York City.

Opponent have argued that children will suffer if the state withholds the $64. The youngsters will continue to receive food stamps and Medicaid coverage.

Supporters said the law was written to allow welfare recipients to make decisions about their lives and to foster a feeling of self-sufficiency.

''Most recipients don't want to be on the system. Most people like to pay their own way,'' Wright said.

Major changes also confront Michigan welfare recipients beginning today. They will be asked to sign a ''social contract'' binding them to work, go to school or volunteer 20 hours a week.

Those who refuse will be referred to a job training program that is being expanded to include community service. Their benefits can be cut if if they fail to cooperate.

Other changes include more generous incentives to work part time, beefed-up child-support collections, tougher penalties for welfare fraud and expanded programs to keep families together and children out of foster homes.