Facebook CEO Mark Zuckerberg and wife Priscilla Chan, through posts on Facebook and Instagram on Thursday, announced that they were expecting their second child.

“Priscilla and I are happy to share we're expecting another baby girl!” the tech mogul announced in the post that accompanied childhood pictures of himself and Priscilla, with their siblings. “I cannot think of a greater gift than having a sister and I'm so happy Max and our new child will have each other.”

Read: Facebook CEO Mark Zuckerberg’s Wife Priscilla Chan Pregnant With Second Child

Zuckerberg talked about the difficulties they faced when Chan was pregnant with their first daughter, Max, making it tough for the couple to decide on whether they wanted more children. He wrote in the post: “When Priscilla and I first found out she was pregnant again, our first hope was that the child would be healthy. My next hope was that it would be a girl. ”

The couple met during their time at Harvard University. While Zuckerberg dropped out of college to work on Facebook, Chan earned a degree in Biology, following which she taught science at the Harker School in San Jose, California.

Chan, fluent in English, Spanish and Cantonese, went on to attend the University of California, San Francisco School of Medicine, and graduated on May 14, 2012, the day Zuckerberg turned 28. The couple had been living together at Zuckerberg's $7 million Palo Alto, California house since 2010 and finally got married in a small ceremony at their home soon after, on May 19, 2012.

While Zuckerberg, as the co-founder, chairman and CEO of Facebook, currently has a net worth of $57.9 billion, according to Forbes’ real-time tracking, Chan has a reported net worth of $14 billion, according to TheRichest.com. Most of this is credited to her marriage to Zuckerberg but Chan’s initiatives in the health and medicine sphere have also earned her recognition.

Read: Billionaires Looking To Find A Cure To All Diseases In The World

The couple has been actively involved in philanthropy, recently pledging to spend $3 billion over the next decade to attempt to cure, handle or prevent all disease by the end of the century. In 2015, Zuckerberg and Chan, to celebrate Max’s birth, had announced they will be donating 99 percent of their Facebook shares to their initiative.

“Like all parents, we want you to grow up in a world better than ours today,” Zuckerberg wrote in a letter to Max, published on Facebook. “We will give 99% of our Facebook shares -- currently about $45 billion -- during our lives to advance this mission. We know this is a small contribution compared to all the resources and talents of those already working on these issues. But we want to do what we can, working alongside many others.”