Actor Dax Shepard has become an expert on baby gender thanks to a crazy theory about a mom-to-be's sex drive.

His wife Kristen Bell reveals her husband loves to confront pregnant strangers and ask them about intimacy - just to prove he can guess the sex of their child.

And he gets it right way more than he's wrong.

"He has asked pretty much every pregnant women he has ever come in contact with how horny they are on a scale of one to 10," Kristen explained during a visit to TV show The Talk on Thursday (29Sep16), "and then he will deduce how much testosterone he thinks you're producing.

"He's got a pretty decent track record."

But his overly forward questions have left his wife more than a little embarrassed at times.

"(He's like), 'I can tell you the sex of your baby. How often do you wanna have sex with your husband?' They're immediately offended, but if they don't get up from the table and they actually answer he usually gets it right."

"Even if you're not pregnant the chances are he will ask you about your sex life," she joked.