MONDAY, June 28 (HealthDay News) -- Two new studies suggest there's a connection between parents who smoke and kids who are heavier or misbehave more than other children.

The researchers haven't definitively proven that lighting up puts kids at risk for bad behavior and extra pounds. In fact, it may be impossible to ever prove a cause-and-effect because it's considered unethical to assign some parents to smoke and then see what happens.

Still, the findings "tighten the link" between parents who smoke and physical and mental health problems in their kids, said Dr. Jonathan Winickoff, an associate professor of pediatrics at Massachusetts General Hospital, who co-wrote a commentary accompanying the research.

For decades, doctors have advised pregnant women to avoid smoking for fear that they would harm their unborn children; research has linked smoking in mothers to physical problems in offspring such as low birth weight. If the mother smokes during the first trimester, the effects are worse than in later trimesters, said Neil E. Grunberg, a professor of medical and clinical psychology at the Uniformed Services University of the Health Sciences, in Bethesda, Md.

But it hasn't been as clear whether there's a connection between mothers who smoke and other health problems in their kids. And the influence of fathers who smoke -- exposing their kids to secondhand smoke or perhaps affecting sperm at conception -- has also remained a mystery.

In one of the new studies, researchers examined what happened to kids whose fathers smoked but their mothers did not. Researchers from the University of Hong Kong studied 7,924 kids from that region who were born in 1997.

The researchers found that the kids who had fathers who smoked were more likely to be heavier at ages 7 or 11 after the statistics were adjusted so they wouldn't be thrown off by factors such as gender and socioeconomic status.

The study appears in the July print issue of Pediatrics, as does a study linking pregnant mothers who smoke to misbehaving kids. Both were published online June 28.

In that second study, British and Brazilian researchers studied 509 children in Brazil and 6,735 in England. After adjusting their statistics to account for possible confounding factors, they discovered that kids of mothers who smoked while pregnant were more likely to be deemed aggressive and disruptive.

This isn't the first time researchers have come to this conclusion, said Grunberg. And if smoking does cause the problems, the study doesn't say how, he added.

So, what might be the connection between parents who puff cigarettes and kids who misbehave and weigh more than others?

Winickoff, co-author of the commentary, said it's not true that smoking makes people skinnier. Instead, it boosts the weight around their bellies and hips, he explained. One theory is that secondhand smoke could do the same thing to those who are exposed, like the kids of dads who light up.

As for pregnant mothers who smoke, their bodies don't act as filters, he said. Instead, the toxins from smoking affect the fetus.

"Anyone who's been in the delivery room when a mother who smokes cigarettes delivers can attest to the state of the placenta," he explained. "In general, it's withered, discolored. It's very clear that the blood supply to the child is compromised."

More information

The U.S. National Library of Medicine looks at pregnancy and smoking and substance abuse.