"Dahlia weighed 7 lbs., 3 oz.," the rep tells PEOPLE. "Mother, father and baby are doing wonderfully."
Simmons, 37, told PEOPLE at the Emmys in September that she and her husband were waiting to find out the sex.
"We'd like it to be a surprise," she said. "We have some of the nursery done, but we're going to wait until [we find out] who this little nugget is."