SATURDAY, June 23 (HealthDay News) -- For those who need help getting pregnant, the thought of having a child who's a little shorter than other kids probably won't be much of a worry. But the question of whether infertility treatment causes unanticipated consequences remains fertile ground for researchers.

In a study scheduled for presentation Saturday at the Endocrine Society annual meeting in Houston, researchers found full-term children conceived with fertility drugs were about one inch shorter than their peers.

The researchers wanted to find out whether there was a difference in height among children whose mothers used only ovarian stimulation by fertility drugs such as Clomid (clomiphene) without in-vitro fertilization (IVF).

Children conceived with the help of ovarian stimulation alone account for about 5 percent of all births in the developed world, according to the researchers.

Previous studies have suggested that children conceived by IVF may be taller than naturally conceived kids. The researchers wanted to know if something in the process of IVF, which includes fertilization and culture of embryos in a laboratory dish, could affect stature. So they studied children conceived without IVF, but with the assistance of fertility drugs that cause ovulation.

"The challenge in doing research like this is that in general the people who come to infertility clinics have been trying to get pregnant for multiple years," said Dr. Valerie Baker, medical director at the Stanford Fertility and Reproductive Medical Center, in Palo Alto, Calif. "These people often have serious medical problems. So it's possible that the issue is not the fertility treatment, but something going on with people who have been trying to get pregnant for more than a year without success," she said. Baker was not involved in the study.

The researchers, led by Dr. Tim Savage, a pediatrician and research fellow at the Liggins Institute, University of Auckland, in New Zealand, studied 84 children conceived with the help of fertility drugs and 258 who were conceived naturally. All were between 3 and 10 years old and from a single-fetus, full-term pregnancy. None had low birth weight, a factor that can be associated with health problems.

The children conceived with the help of fertility drugs were nearly an inch shorter than the others, although still within the normal range, even with differences in their parents' height taken into account. Parental height is considered the key factor in determining a child's height.

The height difference was greater in boys, who were more than an inch shorter on average than naturally conceived boys.

There was no significant difference in general physical health between the two groups of children.

The authors speculated that the height difference may be due to something that happens around the point of conception. They suggested it could be caused by "imprinting" variations -- changes in the way genes are expressed, which could be related to the process of ovarian stimulation. The appropriate expression of genes is important in normal development.

Other experts greeted the study findings with caution.

Dr. Edward Illions, an associate professor of obstetrics and gynecology at Albert Einstein College of Medicine and a reproductive endocrinologist at Montefiore Institute for Reproductive Medicine in Hartsdale, N.Y., said any number of mechanisms, including the act of stimulation, could affect imprinting.

Illions also expressed concern with the study's limitations. "The researchers don't tell us which particular drugs the women were on, what stimulation protocols were used or the number of treatment cycles rendered to these patients," he said.

Dr. David Cohen, chief of reproductive endocrinology and infertility at the University of Chicago, said researchers need to continue studying children born with infertility technology to better understand any implications of fertility treatment. "As far as this study goes, there are no take-aways," he said.

Data and conclusions of research presented at medical meetings should be considered preliminary until published in a peer-reviewed medical journal.

More information

The U.S. National Library of Medicine sheds light on female infertility.

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