Justin Timberlake and Jessica Biel are expecting their first child, but they have yet to find out if the baby is a boy or a girl, CNN reported.

Timberlake used Instagram to make the big announcement.

Thank you EVERYONE for the Bday wishes! This year, I'm getting the GREATEST GIFT EVER. CAN'T WAIT.#BoyOrGirl #YouNeverKnow #WeDontEvenKnow#WeAreTakingBets," the former boy band member said in the photo's caption.

Timberlake, who after his boy-band years embarked on a successful career as a solo artist and Emmy Award-winning singer, and Biel, an actress known for her decade-long participation in the family drama "7th Heaven," married in 2012 in southern Italy, according to the Los Angeles Times. 

The famous couple had largely succeeded at preserving a level of privacy, but speculation about a possible pregnancy had been ripe recently as Biel had been spotted with a baby bump. Photographed during a Dec. 16, 2014, walk in New York City, the actress was seen "cloaking her stomach area with a flowy grey coat." Celebrity watchers started to speculate about a Biel pregnancy when she was "noticeably missing her signature six-pack" while wearing a bikini during a stay in Maui, Hawaii, three months ago.

Biel's belly was on full display in Timberlake's Instagram announcement, which included a photo of the singer kissing his wife's baby bump.

Gossip magazines, meanwhile, have already turned to a new mystery: the sex of the baby. According to Life & Style, Timberlake wants the sex to be a surprise, but an unnamed acquaintance told the publication Biel had different views.

"Jessica is more practical and said it was silly to be surprised," the source said. "She wants to be completely prepared."

The announcement was one of only a few moments when Timberlake and Biel have shared a private moment of their relationship. In October, the singer had last taken to Instagram to show a picture of him and Biel sitting on a bench near a lake.

"Life moves pretty fast. If you don't stop and look around once in a while, you could miss it," he said, quoting Ferris Bueller.