Screening unborn children for genetic defects appears to reduce the chances of a healthy pregnancy and live birth, new research suggests.
The findings, published in the current issue of the New England Journal of Medicine, suggest that checking embryos before using them in in vitro fertilization, and implanting the ones that seem more genetically and structurally healthy, may actually lead to lower rates of pregnancy.
Ironically, this technique -- known as preimplantation genetic screening, or PGS -- is aimed at finding the healthiest embryos. Most experts thought it would not change the odds for women trying to get pregnant, and the technique has even become routine for women over 35 who wish to get pregnant.
"In contrast to what would be expected or what many people are saying who are currently offering this technique, PGS actually decreases pregnancy chances in these groups of women who are 35 years or older," said Sjoerd Repping, director of the IVF lab at the Academic Medical Center in Amsterdam and one of the main authors of the study.
Many doctors expressed surprise at the findings.
"This is the first study that suggests that not only does it not improve the outcome, but it may actually be harmful," said Dr. Steven Ory, president of the American Society for Reproductive Medicine.
"I'm a little bit surprised to see a decreased rate," said Dr. Marcelle Cedars, director of the division of reproductive endocrinology and infertility at the University of California at San Francisco. "But I'm not really surprised to see an absence of a positive effect."
PGS is often done as a part of in vitro fertilization.
In IVF, a sperm and an egg are joined together outside a woman's body. As the newly formed embryo grows, it splits into more and more cells.
PGS involves taking a single cell from the growing embryo and, in this study, looking to see if it has the proper complement of two of each chromosome -- one from mom and one from dad.
Embryos that fit the bill are then implanted in the mother's womb.
Researchers in Amsterdam wanted to study the effect of this screen on rates of pregnancy in women over 35, dubbed "advanced maternal age." About half of all women undergoing IVF fit into this category.
"As women got older, we would see a reduction in their pregnancy rate and that we could tie this to the quality of the egg," said Dr. William Gibbons, a current IVF practitioner and former president of the Society for Assisted Reproductive Technology.
In women 35 and older, an abnormal number of chromosomes in their eggs is thought to be a leading cause of miscarriage. It stands to reason, then, that implanting embryos that have been checked and do not have this problem should lead to more full-term pregnancies.
In this study, conducted by researchers in Amsterdam, 206 women had their embryos screened before they were implanted for IVF treatment, and 202 women underwent IVF without having their embryos screened.
When the two groups were compared, both were found to have the same rate of miscarriage.
The differences between the groups came from the number of women who had viable pregnancies at 12 weeks after treatment. In the control group, it was 37 percent, and in the group whose embryos were screened, it was only 25 percent.