A rep for Drew confirmed that the star is due to give birth to her and Lanfer's first child in January 2012.
Grey's Anatomy producers have "not determined" whether the actress's pregnancy will be written into the ABC series's eighth season as yet, E! News reports.
Drew plays Seattle Grace's Dr April Kepner, a virgin who has vowed to abstain from sex until she meets the right partner.
Drew's Grey's Anatomy co-star Eric Dane's wife Rebecca Gayheart is also reportedly pregnant with the couple's second child.
Grey's Anatomy returns for its eighth season on Thursday, September 22.
> Grey's Anatomy exec: 'This is not the last season'