New research shows that DNA screening is crucial

A series of groundbreaking studies has found that embryos created in fertility clinics contain far more genetic defects than previously thought, even if they are created from eggs donated by young, healthy women.

The discovery, which has alarmed fertility specialists, suggests that routine screening of embryos for defective DNA before they are implanted could dramatically reduce rates of miscarriage and multiple pregnancy.

Fertility clinics have always known that genetic damage builds up in the eggs of older women, and is carried through to their embryos, but eggs from younger women were assumed to be healthy and defect-free. The finding suggests that on average, 42 percent of eggs from all women have serious genetic defects that could prevent embryos being carried to term.

"This is a rewriting of the textbooks. These defects should not be present in such a high proportion of patients," said Peter Nagy, a fertility expert at the Atlanta-based clinic Reproductive Biology Associates.

The researchers now believe that defective eggs are common among the general population, but are rejected early on by the body if they are fertilized. Drugs used to stimulate women's ovaries to produce eggs in fertility clinics might add to the genetic damage, researchers said, a prospect that requires further investigation.

In research presented at the American Society for Reproductive Medicine in Montreal on Tuesday, Jeffrey Nelson of the Huntingdon Reproductive Centre in California used a technique called preimplantation genetic diagnosis (PGD) to screen 289 embryos created from healthy egg donors, all of whom were under 30. He found that 42 percent had damaged chromosomes, the strands of DNA that together hold the entire complement of human genes. The extent of damage ranged from 28 percent to as high as 83 percent in some women.

"We had always assumed that embryos created from eggs donated by younger women would not have these defects," Nelson said. "But just the fact that we are seeing this high rate of abnormality suggests that we should be using (PGD) more."

By screening the embryos, Nelson was able to identify the healthiest and implant them into women, leading to pregnancy in 64 percent of patients.