Children conceived with the help of fertility drugs grow up to be shorter than those who were naturally conceived, according to a new study.

Researchers presenting at The Endocrine Society's 94th Annual Meeting in Houston said that because previous studies have suggested that in vitro fertilization (IVF)-conceived children may be taller than naturally conceived children, the latest study aimed at examining whether there was a height difference in children whose mothers used only fertility drugs, like clomiphene (Clomid), without resorting to IVF, a process which consists of fertilization and culture of embryos in a laboratory dish.

Scientists at The Liggins Institute, University of Auckland, in Auckland, New Zealand studied 84 children conceived with the help of fertility drugs alone and 258 children who were conceived naturally. All the children in the study were between 3 and 10 years old and were from a single-fetus, full-term pregnancy.

To optimize the accuracy of the study, researchers did not include children who had low-birth weight or were born prematurely because children born under these conditions usually have an increased risk of health problems.

Researchers found that although their height still within the normal range, children born with the help of fertility drugs were on average nearly an inch shorter than those conceived naturally, even when researchers controlled for parental height, the most important determining factor of a child's height.

"Reassuringly, these children remained well within the normal height range for both their sex and age," said researcher Dr. Tim Savage, a pediatrician and research fellow at the University of Auckland.

Researchers said that the height difference was more pronounced in boys than in girls, with boys in the fertility group to be more than an inch shorter than the naturally conceived boys.

Savage and his team found no significant difference in the general physical health between fertility and naturally conceived children.

Fertility drugs alone are at least twice as common as IVF, and account for about 5 percent of all live births in the developed world, according to Savage.

Scientists were unable to pinpoint the cause of the slightly shorter stature of fertility drug–conceived children, and it was unknown whether these children were able to catch up in stature when they reach their full adult height.

However they suggested that the height difference may have something to do with what happens around the point of conception.  They proposed that the shorter stature could be caused by "imprinting" variations or abnormal changes in gene expression, which is important in normal development, related to the process of ovarian stimulation.

"Fertility treatment helps millions of couples to achieve their dream of becoming parents," Savage said. "It is important to continue research in this area in order to provide medical practitioners, parents and children with valuable information."

Research findings presented at medical meetings should be considered preliminary until published in a peer-reviewed medical journal.