ALL embryos created by IVF or ICSI should be screened for chromosomal abnormalities before implantation. So claimed IVF pioneer Robert Edwards and embryo-screening expert Yuri Verlinsky at a press conference in London last week to open a meeting on pre-implantation genetics.
Edwards led the team responsible for the birth of Louise Brown, the first test-tube baby, in 1978. Verlinsky is president and CEO of the Reproductive Genetics Institute in Chicago, a leading provider of pre-implantation screening services. The pair argue that screening not only avoids the transfer of many embryos that will fail to implant, but also reduces the risk of miscarriage. To back up this argument, Verlinsky announced that a study by his team had shown pre-implantation genetic screening resulted in an increase in what he calls the 鈥渢ake-home baby rate鈥� to 81 per cent for couples whose embryos were screened, compared with a take-home rate without screening of only 12 per cent.
But others question the wisdom of screening, pointing out that the benefits are nothing like as impressive as these numbers might suggest. Some subsequent reports have interpreted the announcement incorrectly to mean that screening boosts the chances of IVF succeeding to 81 per cent. In fact, the take-home baby rate is the percentage of women who, once pregnant, carry the baby to term. It does not include the many who never become pregnant.
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What鈥檚 more, the 81 per cent figure refers only to women who had screening for translocations, or misplaced pieces of chromosome, and was based on just 45 pregnancies. The results of screening for aneuploidy, the wrong number of chromosomes, were less dramatic, with the take-home baby rate from 432 pregnancies with screened embryos at 72 per cent, compared with 28 per cent without screening. One independent study published earlier this year found no evidence that aneuploidy screening boosts pregnancy rates (Fertility and Sterility, vol 83, p 393).
Peter Braude of King鈥檚 College London says that what couples undergoing IVF want to know is whether screening increases the woman鈥檚 overall chances of getting pregnant from each IVF cycle undergone. Verlinsky鈥檚 team was unable to provide these figures when asked at the meeting.
Braude says that when only a few cells in an embryo are abnormal, a condition known as mosaicism, the embryo can correct itself and develop into a healthy baby. Many of the women in Verlinsky鈥檚 study would have had no embryos transferred at all after screening revealed abnormalities, but a few of the rejected embryos might still have been capable of developing to term, Braude says.
鈥淎 few embryos, rejected when screening revealed abnormalities, might have been able to correct themselves鈥�
Many other researchers think that screening for abnormalities might benefit certain groups of women undergoing IVF, but baulk at the idea of applying it to all. Ongoing studies presented at the conference suggest that there are no more abnormalities in children born after pre-implantation screening than in IVF children overall. But because the technique usually involves removing one or two cells from early embryos, there are still fears about its safety.
For many, the bottom line is whether the benefits of screening justify the expense. 鈥淪how me the cost analysis,鈥� says Susan William, a doctor at the Reproductive Science Center of the San Francisco Bay Area in California. Screening for chromosomal abnormalities can add several thousand dollars to the cost of each IVF cycle, she adds.
