BOSTON (AP) _ Multiple births linked to test-tube technology are declining nationally, largely because doctors are implanting fewer embryos during each attempt to make a woman pregnant, a study suggests.


Doctors routinely place several embryos in the womb at once to improve the odds of producing a baby _ a technique that sometimes works all too well and leads to twins, triplets or other multiple births.


But technical advances and the advent of professional guidelines appear to have led to more sparing use of embryos, the study's researchers reported in Thursday's New England Journal of Medicine.


The findings are likely to stoke the debate over whether the government should put a cap on the number of embryos that can be used for each attempt.


``It's so rapidly evolving that, to put it in the hands of legislation, is clearly to temper and limit progress,'' said Dr. Robert Rebar, director of the American Society for Reproductive Medicine. ``The guidelines are working.''


Researchers at Boston's Brigham and Women's Hospital analyzed federal data on in-vitro fertilization cases in which women had their own eggs fertilized with sperm in the laboratory and then had them implanted. The technique accounts for the vast majority of laboratory-assisted fertility procedures.


The average number of embryos implanted per attempt dropped from four to three between 1995 to 2001, the last year of federal data. In the last five of those years, triplets and greater multiple pregnancies fell from 11 percent to 7 percent of all in-vitro pregnancies. The rate of twins held steady among in-vitro pregnancies, and actually rose among all births in the general population.


Many doctors and parents-to-be hope to avoid multiple births, especially triplets or higher. Such babies are often born dangerously premature and underweight. Such pregnancies can also raise the risk of bleeding, high blood pressure and other complications in mothers.


The 7-year-old voluntary guidelines issued by fertility specialists recommend two to five embryos per in-vitro attempt.


In recent years, doctors have been able to cut down the number of embryos per attempt with improved drug regimens for getting women to produce eggs, more skillful methods of fertilization, and better cultures for growing embryos.


Even with the reduction in the average number of embryos used, the success rate for in-vitro attempts rose from 20 percent to 27 percent from 1995 to 2001.


``It took a while, I think, for physician practice to catch up with the results of our own techniques. All of a sudden, it's working really, really well,'' said Pamela Madsen, director of the American Infertility Association. She is mother of two single-birth children by in-vitro fertilization.


Many countries, including Britain and Brazil, have limited the number of embryos to two to four per attempt. So far, the United States has left the decision up to doctors and parents.


The Boston researchers said the findings show that the voluntary guidelines are working and federal law is not needed to control multiple births.


The study's senior author, Dr. Mark Hornstein, sits on the executive council of the Society for Assisted Reproductive Technology. It co-publishes the professional guidelines.


Despite the latest findings, some specialists favor a federal limit.


Dr. Frank Mannino, who specializes in newborn care at University of California-San Diego, said the rate of twins remains alarmingly high. He said two embryos per in-vitro attempt might be a sensible limit.


``I think we are dangerously close to needing legislation if people do not adhere to reasonable guidelines,'' he said.