(someone) is on maternity leave
"Maternity leave" is time that you are allowed to take off of work after you have a baby. The length you are allowed to take depends on the country and your company's policies.
The word "maternity" means "being a mother". So "maternity leave" is time that a new mother takes off of work. Many companies in the U.S. also allow fathers to take time off. Because of this, the phrase "parental leave" can also be used for time taken off by either mothers or fathers. This term isn't as common, though.