It's Justin Timberlake's birthday, but it's another birth date that's on his mind — that of his future son or daughter.

The superstar shared a picture of a bulging belly — presumably belonging to wife Jessica Biel — on Instagram on Saturday, his 34th birthday. He wrote that he was getting the greatest gift ever this year and added: "CAN'T WAIT."

The sex of the baby is not yet known; the actor-singer joked that the couple is taking bets.

The Grammy-winning Timberlake married Biel in 2012. It will be the first child for both. Biel's pregnancy has been rumored for weeks but Timberlake's post is the first official confirmation.