The `Water for Elephants` actress - who already has daughter Ava, 13, and son Deacon, nine, from her first marriage to Ryan Phillippe - is thought to be expecting a son with her second husband Jim Toth.
A source told America`s Star magazine: "They`re both really happy to be having a son, but especially Jim as it`s his first. He`s over the moon about it."
Reese was rushed to hospital with pregnancy complications in August but it was just a scare and mother and baby are doing fine, although Jim is still nervous.
The source added: "Jim`s still nervous after Reese`s scare and hospital stay. But he just can`t wait to hold his son in his arms."
The 36-year-old Hollywood star spent a couple of days in a Los Angeles medical facility after it was feared she was going into early labour.
After being discharged doctors told her to take it easy at home and get plenty of rest.