NEW YORK (Reuters Health) - A new study confirms that children conceived via infertility treatment may have a higher-than-average risk of cerebral palsy -- explained largely by their higher rates of multiple births and preterm delivery.
The study, of nearly 590,000 children born in Denmark between 1995 and 2003, found that those conceived through assisted reproduction were about twice as likely to be diagnosed with cerebral palsy as children who were conceived naturally.
The findings, reported in the journal Human Reproduction, confirm those from a number of past studies. They also suggest that the increased risk of cerebral palsy can be largely attributed to the heightened odds of twin or higher-order births, as well as preterm delivery, with assisted reproduction.
However, the absolute risk of having a baby with cerebral palsy is still quite low for couples undergoing infertility treatment.
In the U.S. and Europe, it's estimated that two or three of every 1,000 babies are affected by the disorder. So even with a relatively increased risk, the vast majority of children born via assisted reproduction techniques will not have cerebral palsy.
Still, researchers say their findings offer an argument for implanting women with only one embryo at a time, in order to cut the odds of multiple births and preterm delivery.
Cerebral palsy refers to a group of conditions, usually present at birth, that permanently impair movement, balance and posture. The impairments range from mild -- some children have only relatively minor problems with movement -- to more severe, with some children being unable to walk or having additional impairments, such as mental retardation or vision and hearing problems.
The precise cause of cerebral palsy is unknown, but it is believed to involve a disruption in normal fetal brain development. Premature and low-birthweight infants are known to have a higher risk than full-term, normal-weight babies.
For the new study, researchers led by Dr. Dorte Hvidtjorn, of the University of Aarhus in Denmark, examined national data on all 588,967 children born in the country between 1995 and 2003. That included 33,139 children conceived via in-vitro fertilization (IVF) or with the help of fertility drugs to stimulate the ovaries to produce eggs.
IVF, the most technologically advanced of assisted reproductive technologies, involves removing an egg cell from a woman's body, fertilizing it in the lab, and placing it in the woman's womb. It can cost up to $15,000 per "cycle" of medications and procedures, with successful pregnancies often requiring several cycles.
Overall, 0.2 percent of all children were diagnosed with cerebral palsy. The risk was more than doubled among children in the IVF group, and 55 percent higher among those conceived using fertility drugs, compared with children conceived naturally.
When the researchers factored in the effects of multiple births and preterm delivery, the link between fertility treatment and cerebral palsy disappeared -- indicating that those two factors are likely responsible for the connection.
In fact, the researchers found no increased risk of cerebral palsy among singletons born via IVF.
The findings, according to Hvidtjorn's team, indicate that the risk of cerebral palsy among children conceived through assisted reproduction is "strongly associated" with their high proportion of multiple births and premature deliveries. In this study, 63 percent of children conceived through assisted reproduction were premature, versus 33 percent of those conceived naturally.
Similarly, 54 percent of births in the assisted-reproduction group were twins or triplets, compared with 7 percent of births among children conceived naturally.
As it stands, doctors often transfer more than one embryo to a woman's uterus during an IVF procedure, with the goal of boosting the chances that at least one will successfully implant and result in a birth.
However, Hvidtjorn's team writes, the current findings make an argument for transferring only one embryo at a time.
Recent research also suggests that women implanted with one embryo during IVF have a similar success rate as those who receive two embryos. A Swedish study published last year in the New England Journal of Medicine found that among 661 women implanted with either one or two embryos during their first IVF attempt, 53 percent and 57 percent gave birth, respectively.
Human Reproduction, online June 16, 2010.