One at a time

 作者:司马综扔     |      日期:2019-03-08 09:07:03
By Nell Boyce WOMEN undergoing IVF may no longer have to run the risk of having triplets or quadruplets, thanks to improved methods of culturing embryos. IVF specialists usually put several embryos at a time into a woman’s uterus—three in Britain and often five in the US. This is because the three-day-old embryos they use have only a 10 per cent chance of implanting. While this gives a reasonable success rate, it also frequently causes multiple pregnancies. Now biologists have found a way to transfer only one or two embryos at a time and still get high pregnancy rates. Embryos normally implant when they are five days old, but IVF researchers have had little success in culturing embryos to this stage. David Gardener of the Colorado Center for Reproductive Medicine in Denver believes this is because only a single-culture medium is used. Embryos experience a variety of environments as they move from the fallopian tubes to the uterus, says Gardener, so he used a different medium on each day. After five days, the embryo becomes a blastocyst, in which some cells are already destined to become the placenta. Gardener studied 90 women receiving either 3 or 4 three-day-old embryos or 2 blastocysts. The three-day-old embryo group had a pregnancy rate of 66 per cent,