The couple is already parents to daughter Max, and Zuckerberg wrote about his joy in learning their second baby would be another girl.
"I cannot think of a greater gift than having a sister," he wrote on Facebook. "I'm so happy Max and our new child will have each other."
Pediatrician and philanthropist Chan and social media mogul Zuckerberg both grew up with sisters, and they shared their own childhood memories in the announcement.
"They taught me to learn from smart, strong women. They weren't just my sisters but some of my best friends," Zuckerberg said of his three sibs.
Chan's sisters "taught her the importance of family, caring for others and hard work," the post reads, telling how the Chan siblings supported each other throughout their lives as first-generation college students and into their professional lives. "They have so many inside jokes—the kind only siblings can understand," Zuckerberg writes.
Clearly Zuckerberg Baby #2 will have some sibling bonding to look forward to with a mom and dad who so clearly value their families.
"We are all better people because of the strong women in our lives—sisters, mothers and friends," Zuckerberg wrote. "We can't wait to welcome our new little one and do our best to raise another strong woman."
Libby Ryan is Editorial Assistant for Parents.com.