The actress, who found fame in the 90s playing Six, announced her exciting news earlier today, saying that she, her husband Brad and two-year-old daughter Gray are "elated" and "enchanted".
Writing her blog for People.com, Jenna said: "We will be welcoming another little girl into our family in a few months (I'm 20 weeks along), and we couldn't be more elated and enchanted.
"Our hearts are overflowing with love, and Gray is simply over the moon about becoming a big sister! She kisses my belly daily, and tells me she's "watching out for the baby."
Although the comedy first aired more than two decades ago, the Blossom cast reunited yesterday!
Jenna, 37, was joined by Mayim Bialik (Blossom), Joey Lawrence (Joey Russo) and Michael Stoyanov (Anthony Russo) to celebrate America's HUB TV network re-running all five series' throughout July.
After getting together with her childhood pals, Mayim made them pose for a group selfie before sharing the picture on Twitter.
Alongside it, she said: "Um...@joeylawrence @MikeStoyanov @JennavonOy. We still look like young whippersnappers..@hubtvnetwork #blossomreunion."
Celebrities who have announced pregnancies in 2014:
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