Or, maybe such a signal predisposes the child to make more fat-storing cells, others said.
It's still not clear just what in the womb could create such effects—high levels of blood sugar and certain fatty acids are some leading candidates.
Waterland has found evidence it may have to do with how critical genes are regulated. Chemical tags attach to the chromosomes and act like dimmer switches to modulate how hard certain genes work.
Waterland studied mice genetically prone to porkiness and found the fatter the mom, the heavier her offspring tended to be. But that effect was blocked when researchers fed pregnant mice a cocktail of substances that encourage the chemical tags to attach to the chromosomes.
What does that suggest? Maybe a mom's obesity somehow interferes with the regulation of certain genes, and the chemical cocktail overcame that, Waterland says.
Those genes might affect the offspring's long-term weight if they're involved in the brain's regulation of appetite and activity levels, Waterland proposes. He also says it's too soon to tell whether an obesity-blocking supplement could work in women as well as in the mice.
Once scientists identify the obesity signal, they may be able to recommend ways to suppress it, perhaps through diet or behavioral strategies.
In the meantime, experts say, obese women can take their own steps.
Dr. Laura Riley of Massachusetts General Hospital in Boston said she gets her patients' attention when she tells them their obesity could promote the same problem in their children.
"I'm a mother," Riley added. "Believe me, it caught my eye."