Cody and Robyn Brown announced the sex of their unborn child during last night's episode of TLC's "Sister Wives," People reported. The couple is expecting a baby girl, which marks the second child for Cody and Robyn and adds baby number 18 to the polygamist Brown family brood.

"The whole family is so excited for our new baby girl," Robyn told People. "So many of them guessed ahead of time it was a girl."

The couple made the big announcement during the episode, which featured a clip where Cody accompanied Robyn, one of his four wives, to her ultrasound appointment.

"No, no, I want you to tell me, we want to know today," Robyn told her ultrasound technician in a clip from the episode. Cody agreed, but said he wanted to try to guess the baby's sex first.

"Yeah, I knew with Truely, I felt like we knew with Solomon and I feel like I know now," the dad of 18 said in the couple's joint on-camera interview. "So I think I know the gender of the baby."

Cody and Robyn waited until the last day of the family's vacation to Alaska for the big reveal, and they had the family's two youngest children, 5-year-old Truely and 4-year-old Solomon, open up presents that each held a girl doll.

"It's time for a girl in the family, everybody's excited," Cody said in the family's on-camera interview. In a TLC exclusive interview, the family opened up about how they're preparing for the arrival of their new baby girl, who is due in January.

"I was looking for shoes, socks, clothes, headbands to put on her," little Truely said.

Watch TLC's exclusive interview with the family below.