WASHINGTON, Nov. 2 — Under a new federal policy, children born in the United States to illegal immigrants with low incomes will no longer be automatically entitled to health insurance through Medicaid, Bush administration officials said Thursday.

Doctors and hospitals said the policy change would make it more difficult for such infants, who are United States citizens, to obtain health care needed in the first year of life.

Illegal immigrants are generally barred from Medicaid but can get coverage for treatment of emergency medical conditions, including labor and delivery.

In the past, once a woman received emergency care under Medicaid for the birth of a baby, the child was deemed eligible for coverage as well, and states had to cover the children for one year from the date of birth.

Under the new policy, an application must be filed for the child, and the parents must provide documents to prove the child’s citizenship.

The documentation requirements took effect in July, but some states have been slow to enforce them, and many doctors are only now becoming aware of the effects on newborns.

Obtaining a birth certificate can take weeks in some states, doctors said. Moreover, they said, illegal immigrant parents may be reluctant to go to a state welfare office to file applications because they fear contact with government agencies that could report their presence to immigration authorities.

Administration officials said the change was necessary under their reading of a new law, the Deficit Reduction Act, signed by President Bush in February. The law did not mention newborns, but generally tightened documentation requirements because some lawmakers were concerned that immigrants were fraudulently claiming United States citizenship to get Medicaid.

Marilyn E. Wilson, a spokeswoman for the Tennessee Medicaid program, said: “The federal government told us we have no latitude. All states must change their policies and practices. We will not be able to cover any services for the newborn until a Medicaid application is filed. That could be days, weeks or months after the child is born.”

About four million babies are born in the United States each year, and Medicaid pays for more than one-third of all births. The number involving illegal immigrant parents is unknown but is likely to be in the tens of thousands, health experts said.

Doctors and hospitals denounced the policy change and denied that it was required by the new law. Dr. Jay E. Berkelhamer, president of the American Academy of Pediatrics, said the policy “punishes babies who, according to the Constitution, are citizens because they were born here.”

Dr. Martin C. Michaels, a pediatrician in Dalton, Ga., said that continuous coverage in the first year of life was important because “newborns need care right from the start.”

“Some Americans may want to grant amnesty to undocumented immigrants, and others may want to send them home,” Dr. Michaels said. “But the children who are born here had no say in that debate.”

Under a 1984 law, infants born to pregnant women on Medicaid are in most cases deemed eligible for Medicaid for one year.

In an interview on Thursday, Leslie V. Norwalk, acting administrator of the Centers for Medicare and Medicaid Services, said the new policy “reflects what the new law says in terms of eligibility.”

“When emergency Medicaid pays for a birth,” Ms. Norwalk said, “the child is not automatically deemed eligible. But the child could apply and could qualify for Medicaid because of the family’s poverty status. If anyone knows about a child being denied care, we want to know about it. Please step up and tell us.”

Under federal law, hospitals generally have to examine and treat patients who need emergency care, regardless of their ability to pay. So the new policy is most likely to affect access to other types of care, including preventive services and treatment for infections and chronic conditions, doctors said.

Representative Charlie Norwood, Republican of Georgia, was a principal architect of the new law.

“Charlie’s intent was that every person receiving Medicaid needs to provide documentation,” said John E. Stone, a spokesman for Mr. Norwood, who is a dentist and has been active on health care issues. “With newborns, there should be no problem. All you have to do is provide a birth certificate or hospital records verifying birth.”

But Dr. Berkelhamer disagreed. Even when the children are eligible for Medicaid, he said, illegal immigrants may be afraid to apply because of “the threat of deportation.”

The new policy “will cost the health care system more in the long run,” Dr. Berkelhamer added, because children of illegal immigrants may go without immunizations, preventive care and treatments needed in the first year of life.

Doctors, children’s hospitals and advocacy groups have been urging states to preserve the old policy on Medicaid eligibility for children born to illegal immigrants.

Sara Rosenbaum, a professor of health law at George Washington University, said: “The new policy reflects a tortured reading of the new law and is contrary to the language of the 1984 statute, which Congress did not change. The whole purpose of the earlier law, passed with bipartisan support, was to make sure that a baby would not have a single day’s break in coverage from the date of birth through the first year of life.”

California has objected to the new policy. S. Kimberly Belshé, secretary of the California Health and Human Services Agency, said: “By virtue of being born in the United States, a child is a U.S. citizen. What more proof does the federal government need?”

Copyright 2006 The New York Times Company. Reprinted from The New York Times, National, of Friday, November 3, 2006.