In Western Europe, where some countries pay for in-vitro fertilization, or IVF, many authorities recommend a single embryo transfer for women under 37 and a maximum of two embryos for women 37 to 40. For women over 40, three is often the limit by law.

In the United States, there are relatively lax guidelines and a lack of regulation. That country has seen a rise in multiple births, including the highly publicized case of Nadya Suleman, labeled the “octomom.’’ She had octuplets in 2009 after her doctor transferred 12 embryos. She already had six children through IVF.

Most fertility treatment in the U.S. is paid for privately which experts believe encourages the use of more embryos in the belief it will improve odds.

The American Society for Reproductive Medicine recommends single embryo transfers for women under 35, but there is no enforcement of that. For women 35 to 37, they advise two to three embryos, and three to four embryos for women ages 38 to 40.

In their study, Nelson and Debbie Lawlor, of the University of Bristol, analyzed data for all 124,000 IVF cycles in the U.K. between 2003 and 2007, resulting in more than 33,500 live births. The women were 18 to 50 and had varying histories of infertility. During IVF, eggs are fertilized with sperm in a lab dish and then put in the womb.

For women under 40 who had two embryos transferred, the live birth rate was 33 percent overall. With three embryos, that dropped to 25 percent, though researchers weren’t sure why. Nelson said it might be due to the higher risk of miscarriage in a multiple pregnancy and that miscarrying one fetus would jeopardize the entire pregnancy.

For women over 40, the live birth rate was 13 percent whether they had two or three embryos transferred.

Nelson said patients going through IVF may pressure doctors to transfer more embryos, believing it may boost their chances of having a baby and avoiding more treatment.