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updated 2/17/2011 10:48:56 AM ET 2011-02-17T15:48:56

By the time women turn to in-vitro fertilization to conceive, any pregnancy sounds like a good pregnancy. As a result, women often choose to have multiple embryos transferred into their wombs in order to increase their chances of having at least one baby.

But, according to a new study, the likelihood of having a healthy, full-term infant is five times higher in women who have just one embryo transferred than in women who opt for putting in two at once -- at least for certain groups.

That's mostly because doubling the embryo number vastly increases the chance of having twins, which carry far more complications than singletons do.

And even though transferring two embryos increased the overall chance of having a baby, the study found that the difference just about disappeared when women followed a failed round of IVF with another attempt using a single frozen embryo.

As a growing number of people turn to assisted reproductive technologies, the new findings suggest that -- putting financial complications aside -- the safest strategy for many moms and babies is to try IVF with just one embryo at a time.

"The Human Fertilisation and Embryology Authority in the U.K. and The Society of Obstetricians and Gynaecologists of Canada both advocate the use of single-embryo transfer," said David McLernon, a medical statistician with a focus on population health at the University of Aberdeen in the United Kingdom. "This evidence we find in our paper basically solidified that."

Ever since the first IVF baby was born in 1978, the biggest risk to the procedure has been the high chance of having multiples. Carrying twins or more increases rates of pregnancy complications, premature births, low-birth weights, developmental issues, certain birth defects, and even death for the mother.

Today, the chance of having multiples with IVF in the United States ranges from 25 to 35 percent, compared to a less than 2 percent chance of conceiving twins naturally.

"The gold-standard outcome for IVF should be a singleton," said Aaron K. Styer, a reproductive endocrinologist at Massachusetts General Hospital Fertility Center in Boston. "A twin pregnancy is a complication of IVF, and is something we are all working diligently to reduce."

With that goal in mind, clinics around the world have reduced the average number of embryos they transfer during IVF from three or more to two. Starting in Scandinavia, there has been a more recent push to do just one at a time.

To see how the strategies compare in the kinds of outcomes they produce, McLernon and colleagues collaborated with eight European fertility clinics that had conducted trials in which women were assigned to get either one or two embryos during an IVF procedure.

The researchers combined all of the results into one big data set that included more than 1,300 women, allowing them to make the most detailed comparison yet between single and double transfers.

When it came to simply having a baby on the first try, women who had two embryos fared better, the researchers report today in the British medical journal BMJ. Forty-two percent of women gave birth after a double transfer, compared with 27 percent who received just one embryo.

But when the single transfer didn't work on the first try and women went through another round with just one frozen embryo, their ultimate rate of giving birth ended up being about the same as those who received two embryos on the first round.

What's more, the chance of having a healthy, full-term single baby was five times higher in women who got one embryo compared to women who got two.

The findings highlight the benefits of transferring just one embryo at a time, Styer said, but the strategy might not be best for all women. In particular, putting in multiple embryos might actually be best for women who are older than 35, have already tried more than once and failed to conceive with IVF, or whose embryo quality isn’t great, among other situations.

In the United States, money also plays a big role in the kinds of decisions people make about IVF. With the exception of a few states, most governments and insurance companies here refuse to pay for infertility treatments, meaning that women often feel like they have just one expensive, but all-important shot to start a family.

Ultimately, Styer said, clinics need to start using singletons as their measure of success, rather than all pregnancies. And doctors and counselors need to work closely with patients to explain all of the benefits and risks involved, depending on the woman’s personal situation.

In healthy women under 35, he added, the chances of conceiving are the same whether women have one embryo transplanted or two.

"The majority of twin pregnancies deliver well, but there is a proportion that do not, and some patients do not understand that," Styer said. "Everyone thinks they're going to come out on the good end of the deal, but there's really no way to predict that."

© 2012 Discovery Channel

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