"Both mommy and baby are great!" Sigler's rep said.
The former "Sopranos" star, 32, and Dykstra, 24, a minor leaguer in the Washington Nationals organization, first went public with their relationship in March 2012.
They announced their engagement in January, but there's no word yet on when the new parents will take a trip down the aisle.
"Jamie couldn't be happier. Cutter is the perfect guy for her," a source said shortly after they got engaged. "Although they have only been together for a year, they both know that this is it for both of them. They are both on cloud nine, as are their families and friends."
Sigler was previously married to A.J. DiScala from 2003 to 2005.