BOSTON — In the five years since the “Octomom” case, big multiple births have gone way down but the twin rate has barely budged. Now fertility experts are pushing a new goal: one.

A growing number of couples are attempting pregnancy with just a single embryo, helped by new ways to pick the ones most likely to succeed. New guidelines urge doctors to stress this approach.

Twins aren’t always twice as nice; they have much higher risks of prematurity and serious health problems. Nearly half of all babies born with advanced fertility help are multiple births, new federal numbers show.

Abigail and Ken Ernst of Oldwick, N.J., used the one-embryo approach to conceive Lucy, a daughter born in September. It “just seemed the most normal, the most natural way” to conceive and avoid a high-risk twin pregnancy, the new mom said.

Not all couples feel that way, though. Some can only afford one try with in vitro fertilization, or IVF, so they insist that at least two embryos be used to boost their odds and view twins as two for the price of one.

Many patients “are telling their physicians ‘I want twins,'” said Barbara Collura, president of Resolve, a support and advocacy group. “We as a society think twins are healthy and always come out great. There’s very little reality” about the increased medical risks for babies and moms, she said.

The 2009 case of a California woman who had octuplets using IVF focused attention on the issue of big multiple births, and the numbers have dropped, except for twins.

The Centers for Disease Control and Prevention’s most recent numbers show that 46 percent of IVF babies are multiples— mostly twins —and 37 percent are born premature. By comparison, only 3 percent of babies born without fertility help are twins and about 12 percent are preterm.

It’s mostly an American problem — some European countries that pay for fertility treatments require using one embryo at a time.

The American Society for Reproductive Medicine is trying to make it the norm in the U.S., too. Its guidelines, updated earlier this year, say that for women with reasonable medical odds of success, those under 35 should be offered single embryo transfer and no more than two at a time. The number rises with age, to two or three embryos for women up to 40, since older women have more trouble conceiving.

To add heft to the advice, the guidelines say women should be counseled on the risks of multiple births and embryo transfers and that this discussion should be noted in their medical records.

“In 2014, our goal is really to minimize twins,” said Dr. Alan Copperman, medical director of Reproductive Medicine Associates of New York, a Manhattan fertility clinic. “This year I’m talking about two versus one. Several years ago I was talking about three versus two” embryos.

The one-at-a-time idea is catching on. Only 4 percent of women under 35 used single embryos in 2007 but nearly 12 percent did in 2011. It’s less common among older women, who account for fewer IVF pregnancies, but it is gaining among them, too.

“Patients don’t really want multiples. What they want is high delivery rates,” said Dr. Richard T. Scott Jr., scientific director for Reproductive Medicine Associates of New Jersey, which has seven clinics in that state.

Better ways to screen embryos can make success rates for single embryos nearly as good as when two or more are used, he contends. The new techniques include maturing the embryos a few days longer. That improves viability and allows cells to be sampled for chromosome screening. Embryos can be frozen to allow test results to come back and more precisely time the transfer to the womb.

Taking these steps with single embryos results in fewer miscarriages and tubal pregnancies, healthier babies with fewer genetic defects and lower hospital bills from birth complications, many fertility specialists say.

Multiple studies back this up. In May, doctors from the New Jersey clinics did the kind of research considered a gold standard. They randomly assigned 175 women to have either a single embryo transferred after chromosome screening or two embryos with no screening, as is done in most IVF attempts now. Delivery rates were roughly equivalent — 61 percent with single embryos and 65 percent with doubles.

More than half of the double transfers produced twins but none of the single ones did. Babies from double transfers were more likely to be premature; more than one-third spent time in a neonatal intensive care unit versus 8 percent of the others.

– Marilynn Marchione, Associated Press Chief Medical Writer