Women undergoing in vitro fertilization should never have more than two embryos implanted, according to a new study out of the UK. It finds that those who are implanted with three or more embryos are not more likely to become pregnant, but they do have a greater chance of delivering pre-term, due to the increased incidence of multiple births.
In IVF, mature eggs are taken from a woman's ovaries, fertilized with sperm in a lab and implanted in a woman's uterus.
But according to the study's authors, the health risks to mother and fetus -- as well as the "associated socioeconomic costs" of multiple pregnancies -- have sparked controversy among clinicians, policymakers and couples over the proper number of eggs to be implanted, prompting several countries to contemplate the use of single-embryo transfer.
"Not all cases with the transfer of two embryos will result in a twin birth, but it is more likely when two are transferred than when one is," explained Debbie Lawlor, Ph.D, of the University of Bristol, one of the study's authors.
"Whilst most twins -- whether conceived naturally or via IVF -- are healthy, twins are more at risk of being born pre-term, including severe pre-term, and low birth weight," she said.
Researchers looked at national data from more than 120,000 IVF cycles, which resulted in more than 3,000 live births, to examine the question of whether implantation during IVF should be limited to one embryo. They compared things like the number of multiple births and preterm babies among women 40 and younger to those 40 and older, and published their results in "The Lancet."
Within both age groups, the birth rate was higher when two embryos were transferred rather than one, the researchers found. However, transferring three embryos resulted in a lower birth rate than two among women in the younger group, while it made no difference in the older women. In addition, implanting multiple embryos was linked with greater chances of adverse outcomes in the period immediately following birth. And the transfer of three embryos also resulted in greater risk of preterm birth.
"Previous research -- before more modern techniques for IVF -- still showed that implanting three [embryos] increased the likelihood of successful live birth rate, compared with the transfer of two or one," Lawlor told The Huffington Post. "Our research shows this is no longer the case."
Despite this, the authors cite data suggesting that 40 percent of the treatment cycles in the U.S. involve the transfer of at least three embryos.
Rajesh Srivastava, Ph.D, director of the IVF and andrology labs at the University of Rochester's Strong Fertility Center, explained that in the U.S., most IVF clinics are registered with the Society for Assisted Reproductive Technology, which bills itself as the primary organization dedicated to assisted reproductive technologies in the country. According to its website, it represents more than 85 percent of the assisted reproduction technology clinics in the country.
"SART has developed guidelines for the number of embryos to be transferred based on the age group," Srivastava explained. "For patients under 35 years, their recommendation is to transfer one to two embryos depending upon the patient's diagnosis and prognosis. [For] patients who are 35 to 37 years, their recommendation is two to three, and for patients who are 38 to 40, [it's to] transfer three embryos."
Srivastava explained that while most clinics adhere to these guidelines, they are merely suggestions; there are often instances in which patients want to implant more embryos in order to increase their chances of having a baby on their first try. (In the UK, guidelines on the number of embryos transferred are more stringent.)
The authors of the new study do conclude that women 40 or older have a lower birth rate than younger women, regardless of the number of embryos transferred, highlighting the importance of age when it comes to the success of IVF. They conclude that while the transfer of three or more embryos should be avoided at any age, those deciding whether to transfer one versus two should take age into account as a factor.
"With respect to why age is a major factor for the decline in IVF success rates, it is primarily related to a combination of the decline in the number of eggs that a woman has and the quality of these eggs," said co-author Scott Nelson, Ph.D, of the University of Glasgow.