Women who go into pregnancy slightly overweight may risk giving birth to children with lower IQs, according to a new study.

The study, published Dec. 10 in the journal Pediatrics, found that children of overweight mothers scored slightly lower on certain tests of verbal and number skills.

While the findings do not prove that maternal weight directly affected children's mental abilities, researchers say that the results do add to previous research showing that, for whatever reason, children born to heavy mothers tend to have a somewhat lower IQ than their peers.

Lead researcher Emre Basatemur, of the Institute of Child Health at University College London and colleagues studied nearly 20,000 UK children involved in an ongoing national study. The children were tested on verbal ability, number skills and reasoning skills at the age of 5 and again at the age of 7.

Researchers found that on average, children of mothers who were overweight going into pregnancy scored slightly lower on the intelligence tests.

However, researchers noted that the difference in tests scores was very small. The results showed that every 10-point increase in a mother's body-mass indexes (an extra 60 pounds for an average-height woman) was associated with a 1.5 dip in their children's IQ score.

While, the cause in the association is still unclear, researchers said that there is no way to know for sure whether the mother's extra weight is to blame.

Basatemur noted that both genetic and environmental factors could influence children's cognitive development.

"The association observed in our study accounts for a small amount of the overall variation seen in children's cognition," he said, according to HealthDay.

Other experts not involved with the study noted that observational studies like the recent one can never prove cause-and-effect.

"You can't conclude from this that women should try to attain a healthy weight before pregnancy in order to improve their child's cognition," Dr. Ryan Van Lieshout, assistant professor of psychiatry and behavioral neurosciences at McMaster University in Hamilton, Ontario, said to HealthDay.

Despite the skepticism, researchers said that most of the research so far that looked at the relationship between mothers' weight and children's mental and behavioral development suggests a link. Past studied on animals have suggested that it is possible that a mothers' excess fat can affect their offspring's fetal brain development, but the link in humans is still inconclusive.

No matter what, researchers said that there are plenty of reasons for women to be as healthy as possible when going into pregnancy, and that the current research should be interpreted alongside existing weight recommendations.

"The association observed in our study may be a further reason to follow this advice, and reinforce the need ... to reduce the prevalence of overweight and obesity in women of childbearing age," Basatemur said.

Experts note that obesity can make a woman less fertile, raise the risk of health problems in expectant mothers, like pregnancy-related diabetes and high blood pressure, and has been linked to higher risk of miscarriage, stillbirth and birth defects.