Women who become pregnant with previously frozen IVF embryos tend to have healthier babies and fewer complications than those who have fresh embryos implanted, research suggests.
Fertility doctors found that mothers had a lower risk of bleeding in pregnancy with embryos that had been frozen and thawed, and went on to have fewer pre-term and low-birthweight babies.
Fertility clinics in Britain usually transfer fresh embryos into women several days after they have been given hormone injections that stimulate their ovaries to release eggs. These are extracted and fertilised before being implanted. Any embryos that are not used can be frozen for use months or years later. The new results raise questions about the way fertility treatment is offered in the UK. If mothers and babies fare better with previously frozen IVF embryos, it may make sense to freeze more or most embryos.
The findings appear in a review of 11 published studies that covered more than 37,000 pregnancies in women who had either fresh or previously frozen IVF embryos implanted in their wombs. The doctors who led the work suspect that IVF embryos that were frozen make for healthier babies because they are implanted long after the woman’s ovaries are stimulated with drugs, so hormone levels in the womb have had time to return to normal. Another theory is that only high-quality embryos survive the freeze-and-thaw process, though survival rates for frozen embryos are now more than 90% in some clinics.
Abha Maheshwari, a senior lecturer at Aberdeen University and consultant in reproductive medicine with NHS Grampian, described the results at the British Science Festival in Aberdeen. “We have to explore further what is the cause of frozen embryos giving us better pregnancies or lesser complications in the pregnancy,” Ms. Maheshwari said. © Guardian News & Media 2012
