Marinoni is the birth-mom, actress Nixon's rep told People on Tuesday, and "Christine and baby are doing great."
The two have been together since 2004. The actress announced their engagement while speaking at a rally for same-sex marriage equality in New York in May 2009. Marinoni is a stay-at-home mom to Nixon's other children from a previous relationship, according to the Advocate; Samantha was born in 1995, and Charles was born in 2002.
"We could go to Canada, and we could go to Connecticut, and we could even go to Iowa, but we don't want to," she said at the rally. "We want to get married right here in New York City, where we live, where our kids live."
This is child No. 3 for the "Sex and the City" star, who had her daughter and son with photographer Danny Mozes. Nixon and Mozes split in 2003.
— Christie D'Zurilla
Reuters contributed to this report.
Photo: Cynthia Nixon in "Sex and the City 2." Credit: Craig Blankenhorn / New Line Pictures.