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New York (Reuters Health) - For women seeking help from in vitro fertilization (IVF) to become moms, frozen embryos might be an even better choice than fresh, according to a new study.
The Finnish study suggests that women who use frozen embryos are somewhat less likely to give birth prematurely, compared to children conceived from an egg that is removed, fertilized and implanted "fresh" within the same cycle.
The role of frozen embryos has grown over the last few years, especially in Europe where policies now favor implanting only one embryo at a time to prevent dangerous multiple pregnancies, Dr. Sari Pelkonen, of Oulu University Hospital in Finland and lead author of the study, told Reuters Health by email.
This limit leaves extra embryos available for freezing, but few studies have looked carefully at whether frozen embryos are linked to higher rates of premature babies and other complications.
For their study, published in the journal Human Reproduction, Pelkonen and her colleagues studied data from nearly 2,300 children conceived with frozen embryos, more than 4,100 born after fresh embryos were used, and 32,000 pregnancies that did not require IVF or other fertility treatments.
Overall, 258, or about one in 11, of the babies from the fresh embryo transfer group were born prematurely, compared to 120, or about 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).
Those 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 that increased weight is unlikely to lead to any complications, having much larger babies can increase the risk of requiring a cesarean section, for example.
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.
"There's a sense people have that frozen aren't as good-that freezing and thawing could harm the embryos. This quiets those concerns," Dr. Helen Kim, Director of the In Vitro Fertilization Program at The University of Chicago, told Reuters in a phone interview.
Why frozen embryos result in more favorable outcomes is still unclear. The freezing and thawing process could filter out the "weak" embryos, leaving only the good quality ones, Dr. Gordon Baker, of the University of Melbourne and The Royal Women's Hospital in Australia, told Reuters Health in an email.
Frozen embryo transfer also allows doctors to time a woman's hormone cycles to more closely mimic natural conception, according to Baker. High estrogen 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, Pelkonen 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 years later.
The average cost of a fresh in vitro fertilization cycle in the U.S. is $12,400, although the cost varies significantly among clinics. Freezing embryos for later use is extra. However, these additional services are relatively inexpensive, notes Baker, at less than one third the cost of a fresh cycle.
Kim agreed. "I always recommend that my patients freeze," she said. "It kills me to throw away perfectly good embryos."
However, a recent study of what happened to frozen embryos at one fertility clinic found that many women who successfully have a baby using donated eggs do not try to achieve a second pregnancy with the excess embryos they've chosen to store. (See Reuters Health report, February 12, 2010.)
SOURCE: Human Reproduction, February 2, 2010.