Genetic Screening May Endanger Fetuses

As for the number of women who gave birth to live babies, the rates were 24 percent in the women whose embryos were screened, compared with 35 percent in women who didn't have the screening.

Mysteries Remain

There are many potential explanations for why the pregnancy and birth rate are lower after screening.

One is the physical impact of taking out a part of the embryo in order to screen it, or doing a biopsy.

"If you do a biopsy from an embryo -- you have to remove a single cell to know about the genetic makeup -- that may be detrimental to the embryo's ability to implant," said Repping.

"The current technology for the biopsy can slow embryo growth and hinder implantation, by about 10 percent or so," said Dr. Robert Stillman, medical director at Shady Grove Fertility Center, the largest IVF clinic in the country. "There's a risk to a biopsy, whether you're doing it in the breast or the lung or an embryo."

Another reason the screen reduced pregnancy rates may be mosaicism, which occurs when "one cell is not like the rest of the embryo, like a mosaic tile," according to Stillman.

"I think the fact that one cell from an embryo is not representative of all cells from an embryo is very important," Repping said. Mosaicism could lead scientists to implant an embryo in which one cell tested normal but the remaining embryo was unhealthy.

Finally, many experts point to the fact that the limits of the current technology allow the screen to check only eight of the 23 pairs of human chromosomes. Abnormalities present in one of the other 15 pairs could have considerable sway on the chances of pregnancy.

"Next year or the year after, we will be able to screen all 23 chromosome pairs, and we can do it with safer biopsy techniques," Stillman said.

Even then, some experts don't think that genetics can tell the whole story behind which women will get pregnant using these therapies.

Cedars of UCSF recommends that women talk to their doctors about what the data means for them. Her advice is "to be cautious and to not too quickly look for the magic bullet."

"Technology does not always have what we wish it did."

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