Frozen embryos better than fresh ones
For women seeking help to become mothers through in vitro fertilization (IVF), frozen embryos might be a better choice than fresh ones.
Women who use frozen embryos are somewhat less likely to give birth prematurely, compared to children conceived from an egg that is removed, fertilised and implanted fresh within the same cycle. The role of frozen embryos has grown over the last few years.
Researchers studied data from 2,293 children conceived with frozen embryos, 4,151 conceived with fresh embryos and 31,946 pregnancies that did not require IVF or other fertility treatment in Finland. It was found that overall, 258 (about one in 11) babies from the fresh embryo transfer group were born prematurely, compared to 120 (one in 16) in the frozen embryo transfer group. Frozen embryos were also less likely to be linked to low birth weight and being small for the length of the pregnancy.
Similar differences were seen between the fresh and frozen embryos in regards to low birth weight -180 (6 percent) versus 76 (4.2 percent) - and being small for the length of the pregnancy - 91 (3.1 percent) versus 28 (1.5 percent). These relationships held after the researchers took various factors such as the mother's age and socioeconomic status into account. The only potential negative effect of being born from frozen embryos was that on average, the birth weight of children born from frozen embryos was 134 grams (0.3 pounds) greater than a baby born from a fresh embryo. Although the increased weight is unlikely to lead to any complications, having much larger babies can increase the risk of requiring a caesarean section. When researchers compared frozen embryo outcomes to those from natural conceptions, they found more premature births among the frozen embryos. However, there were no significant differences in fetal or infant mortality among any of the groups.
Why frozen embryos result in more favourable outcomes is still unclear. The freezing and thawing process could filter out the weak embryos, leaving only the good quality embryos. Frozen embryo transfer also allows doctors to time a woman's hormone cycles to more closely mimic natural conception. High oestrogen levels and lingering stress from the egg collection procedure performed just a few days before IVF could impair implantation, as well as increase the risk of an unhealthy birth.
Considering effectiveness, price and safety, the researchers suggested that the best IVF option might be transferring one or two fresh embryos, followed by freezing the rest for future implantation, which could be done later.