The two already have two daughters: Maxima, six, and August, five. Mark and Priscilla met at a Harvard University frat party in 2003.
They got engaged eight years later and married one year later. Mark has been open about his family and parenting.
Zuckerberg and Chan aren't the first couple to have children. They met in Harvard while waiting in line at a party.
The two later married in a secret backyard ceremony in Palo Alto, California, where they thought their guests were celebrating Chan's medical school graduation.
In fact, they were married on the day of Facebook's IPO, so the social networking site has always been the third wheel in their relationship.
The couple got married one day after Facebook was publicly traded, and Chan demanded that he take her out at least once a week for at least one hour.