Babies conceived using fertility treatment may be somewhat smaller at birth than newborns conceived naturally, but whether that is due to the treatment or the underlying infertility is not fully clear, a US study said.

Researchers looked at nearly 2,000 women and found that babies born to those with fertility problems weighed a little less - about a third of a pound (453 grams) on average, according to results published in the journal Fertility & Sterility. They were also at somewhat greater risk of low birthweight, or under 5.5 pounds (2.49 kg).

"But it's been hard to tease out. Is it the infertility or the technology used to treat it?" said Amber Cooper, an assistant professor of obstetrics and gynecology at Washington University in St Louis, who led the study.

Studies have linked poorer fetal growth and lower birthweight to a higher risk of certain health problems in adulthood, including high blood pressure and heart disease.

Cooper's team looked at records for 461 women who came to their center with fertility problems over 10 years and ultimately had a baby. More than half underwent in vitro fertilization (IVF), while 106 were treated with fertility drugs and 104 eventually became pregnant on their own.

Those women were compared with 1,246 fertile women who gave birth during the same time period.

Overall, babies born to women with fertility problems were smaller. But there was no difference in average birthweight between women who underwent IVF and infertile women who eventually had a baby without medical help.

The biggest gap was seen in the group of women who'd been treated with fertility drugs, which spur ovulation. Their newborns were about a half-pound lighter compared with fertile mothers - a gap that Cooper said was small, but still fairly significant.

On the other hand, the increased risk of low birthweight was mainly seen in IVF babies. Twelve percent of those mothers had a low-birthweight newborn, versus just under eight percent of mothers with no fertility problems.

"The findings suggest that a large portion of this may be related to the underlying infertility," Cooper said.

Fertility drugs are often used for women whose infertility is tied to problems with ovulation - in the case of this study, nearly half of the fertility drug group.

Past research has also suggested that the longer a couple takes to conceive, the greater the odds of low birthweight or other complications. IVF may often be done only after fertility drugs or other less-exhaustive treatments fail.

Researchers were not able to pin lower birthweights to any specific causes of infertility, though, and Cooper said some effect from the technology could also not be ruled out.

Since close to two percent of US births each year are now helped along with fertility treatment, it will be important to keep studying any effects of the treatment on long-term health, Cooper said.