Implanting single IVF embryos may be best

2019-03-02 08:03:09

By Michael Le Page, Copenhagen The case for transferring only single embryos during IVF is even stronger, according to two new studies. The first study shows that when only a single rather than multiple embryos are transferred to the womb, the risk of problems at birth drops to normal levels. And a second study suggests why – because it reduces the chance of having a “vanishing twin”. When IVF was first introduced, it was usual to transfer three or more embryos into the woman’s womb. The result was a much higher than normal number of twins and triplets, with all the associated problems: greater risks for the mother and babies during pregnancy and birth, greater stress on parents and lifelong health effects for the children as a result of their lower birth weight. This had already persuaded some European countries to restrict the number of embryos that can be transferred to just two, or even one. Studies suggest that when done by good clinics this makes little difference to a woman’s chance of becoming pregnant. Now the two studies, presented at a meeting of the European Society for Human Reproduction and Development in Copenhagen, Denmark on Tuesday, suggest single embryo transfer (SET) is inherently safer, even when the transfer of multiple embryos results in a singleton birth. Diane De Neubourg’s team at the Centre for Reproductive Medicine in Antwerp, Belgium, compared 251 babies born due to SET with singletons born after natural conception. “We found very little difference between the SET babies and those conceived naturally,” she says. “We were not expecting this result.” Their birth weights and the risk of premature birth were almost identical, for example. By contrast, many studies have shown that singletons born after multiple transfers face higher risks – they are more likely to have a low weight and to be born prematurely, among other things. De Neubourg points out that one limitation of this study is that the women who had SET were those with the best chance of a good outcome: young women with high quality embryos. It is possible that if older women with poor quality embryos had SET, more problems would be seen at birth compared with spontaneous conceptions. But a major question is why there should any difference in risks if multiple embryos are transferred in cases where only one baby is born. Part of the answer, according to Anja Pinborg of the Fertility Clinic at the Rigshospitalet at the University of Copenhagen, is that one in 10 IVF singletons actually start life as a twin. And the later in pregnancy that one of the twins dies, the higher the risk of neurological problems for the surviving twin, she says. Pinborg estimates that vanishing twins account for around one-third of the greater risk of defects seen in IVF children. The other two-thirds could be related to the infertility problems of the parents, she suggests. Her findings are based on a study of nearly 10,000 children born through IVF and ICSI (where a sperm is injected directly into an egg) in Denmark between 1995 and 2001, with ultrasound scans at eight weeks enabling the detection of twin pregnancies. Vanishing twins do occur after normal conception, but the rate is far lower. The results might help persuade more countries to limit the number of embryos that can be transferred, or at the very least stipulate that clinics “inform their patients that vanishing twins are a considerable risk factor in IVF twin pregnancies,” Pinborg says. She believes SET should be recommended to all women under 37,