Hart, who announced her pregnancy in April, told People that she has never wanted to know the gender of her children before they were born.
"We always wanted the mystery the other two times around, but the first time we gave into pressure from everyone and curiosity," she said.
"And the second time our ultrasound technician gave it away before we had a chance to stop them."
Hart, who revealed that she is "already in love" with her baby, recently celebrated her pregnancy with a baby shower but asked her guests to donate gifts to the non-profit organisation Operation Shower.
"After two precious babies and a successful career, I didn't want people to spend money on me and the baby," she explained. "We wanted to help out families that needed a hand and decided to donate baby goods to military families."
Hart previously revealed that she prepared for "about a year" before getting pregnant for the third time.