The retired adult movie star announced last month she was expecting her first child with her boyfriend, mixed martial arts fighter Tito Ortiz, and the pair are reportedly "thrilled" after discovering they will be welcoming two babies into the world.
A source told gossip blogger Perez Hilton: "They're having twins. Jenna and Tito just found out. They are beyond thrilled."
The pair have recently invested in a new, bigger property near the beach in Los Angeles as they prepare for their new arrivals.
Speaking about her pregnancy last month, Jenna admitted she was initially "shocked" to discover she was expecting.
She said: "It's still early, so I'm being cautious. I'm resting as much as possible. I'm so happy! We are still in a state of shock."
Jenna, who has been dating Tito for two years, wants to remain as healthy as she can until she gives birth.
She added: "I'm just staying super-healthy. I have moved down to the beach with Tito - I love being by the beach."