Armstrong broke the news on her Facebook page.
She said: "Thank you sooo much for all the love!! Will post pictures after a little rest.
"We're all doing wonderfully. It was an amazing experience and we couldn't be more thrilled with our new arrival. xoxoxoxoxo!!!!!"
Armstrong, a former O.C. alum, has reportedly named her first child Calin.
The 32-year-old actress announced the news of her pregnancy back in October.
Armstrong will next be seen in films A Winter Rose and Windsor Drive, both scheduled for release in 2013.