Mistakenly let the news slip during live TV interview with Jimmy Fallon
Jessican is pregnant with her second baby, having given birth to daughter Maxwell in May 2012.
She confirmed she was expecting again in December and had insisted she wouldn't be discussing the sex of the new arrival.
However, while talking about how difficult the pregnancy has been this time around during a TV interview last night, Jessica mistakenly let the news slip.
"It's a totally different pregnancy. I just feel awful. I've been vomiting. The crazy thing is I never knew that a wiener could actually make me nauseous," she told talk show host Jimmy Fallon, before covering her mouth and widening her eyes.
"Oh shoot! Well, I guess I just told the world that I'm having a boy!"
Jimmy tried to make light of the announcement after he realised Jessica hadn't meant to say anything.
"I didn't even know what was happening," he said. "I thought maybe you were having a girl with a penis. I don't know. Well, congratulations."
He then attempted to get an even bigger scoop from the 32-year-old star. There has been speculation Jessica and her fiancé Eric Johnson are planning to call their new baby Ace, which she neither confirmed nor denied.
"Let's hope it's a boy. Could Ace be a girl?" Jessica said. "I kind of just outed that I'm having a boy, right? I can't believe I did that. That was not planned."
The star continued to berate herself for her slip of the tongue. After an advert break, making the announcement was still on her mind.
"I just did it so crude too. I feel awful. I talked about wieners," she sighed.
Jessica has previously admitted neither of her pregnancies was planned, joking her latest one happened after a "kind of like a one-night-stand". She and Eric have put their wedding plans on hold because she is expecting again and she is trying to come up with ways to ensure they do eventually tie the knot.
"I keep planning on getting married but I keep getting knocked up," she giggled. "It's just one of those things... Maybe I should just completely reverse and just stop having sex all together until Eric and I actually say 'I do.'"