The 34-year-old made the announcement in a television interview with Barbara Walters of ABC News.
Mr Beatie was born female but underwent gender reassignment and is now legally male. He kept his female reproductive organs so he could have a family.
The baby was due in early June, he told the TV host. He felt good and everything was going well.
"I had my checkups with my hormone level... everything is right on track," he said. Thomas Beatie grew up in Hawaii as Tracy Lagondin, but began to live as a man when he was in his twenties.
He had breast surgery to remove glands and flatten his chest but kept his female reproductive organs.
He has been married to his wife, Nancy, for five years and the couple bought sperm from a donor when they decided to start a family.
Mr Beatie gave birth to their baby daughter, Susan, the natural way, after a lengthy labour.
After the birth, he told Ms Walters, he did not resume taking male hormones because the couple wanted to have another baby.
The couple's second child is due on 12 June.
Beatie's wife, Nancy, 46, whom he married five years ago, was unable to conceive because of a prior hysterectomy.
He has said that is why he had a baby himself, through artificial insemination using donor sperm and Beatie's own egg.
The couple live in Bend, Oregon, and have led a quiet life since the birth of their baby girl, Susan.
Muhammad Mahtab Bashir