A combination of a blood test and an ultrasound can tell moms-to-be their risk of carrying a baby with Down syndrome by the end of the first three months of pregnancy, several weeks earlier than conventional screening allows.

A study reported in today's issue of the New England Journal of Medicine, suggest the combination test could be routinely offered to pregnant women as a screening test for Down syndrome, the most common genetic cause of mild-to-moderate mental retardation.

Obtaining the results in the first trimester, instead of the second trimester as with existing tests, would benefit mothers who choose to terminate the pregnancy because abortions are safer in the first trimester.

Other women will use the information to begin planning for the birth of a child with special needs. But most will get reassurance that their babies are healthy.

"There are now so many tests available, we should get away from the one-size-fits-all approach," said lead researcher Dr. Fergal Malone of Dublin's Royal College of Surgeons in Ireland. "We just need to be able to figure out what is the best screening test to offer in a particular setting or particular circumstances."

The results of a study of 38,000 U.S. women conducted by the First- and Second-Trimester Evaluation of Risk, or FASTER, research consortium showed that combined screening detected 87 percent of Down syndrome cases by 11 weeks of pregnancy.

That compares with the blood tests known as quadruple or "quad" tests, which are routinely performed between 15 and 18 weeks of gestation, and pick up about 81 percent.

"If you combine the first and second trimester tests together, you can pick up as many as 95 percent or more cases of Down syndrome," Dr. Malone said.

"This form of screening clearly is an improvement over what we presently use," said Dr. Allen Hogge, chairman of obstetrics, gynecology and reproductive medicine at the University of Pittsburgh School of Medicine. He was not a member of the research team.

It will likely become the standard of care for Down syndrome screening, Dr. Hogge added.

Now, doctors routinely screen the mother's blood with the quad test during the second trimester.

If there is an abnormal result, the mother is offered amniocentesis or chorionic villus sampling, in which a needle or catheter is inserted into the uterus to get a sample of the baby's DNA to see if there is a chromosome problem.

The invasive tests could cause a miscarriage, so screening tests can help women decide whether they should take the risk of having the procedure.

Tracy Herzer, 37, of Glenshaw, had conventional screening when she was pregnant with her second child, 16-month-old Amelia. The result was on the borderline, so her doctor recommended an ultrasound, which reassured her that everything was fine.

When Mrs. Herzer was pregnant with her third child, she had the new tests, which included an ultrasound to measure the thickness of the skin at the baby's neck, called nuchal translucency.

She was told there was a 1/600 chance of the baby having Down syndrome.

"It did make me feel better once I had everything done," Mrs. Herzer said. "And it was an easy enough test."

Her son, Elijah, was born in September.

Dr. Malone knows that some are concerned that the tests will lead to more abortions.

But, he said, "Many patients will want to go through a prenatal screening or diagnostic test even if they would never terminate a pregnancy."

They might use the time to learn about Down syndrome and prepare, and doctors start watching for heart and stomach problems that go along with the disorder.

Most families prefer finding out early rather than being surprised at birth, Dr. Hogge said.