The `This Means War` actress - who already has two children, Ava, 12, and Deacon, eight, from her first marriage to Ryan Phillippe - is expecting a baby with second husband Jim Toth after trying for some time.
A friend told Us Weekly magazine: "Reese and Jim have been trying to get pregnant. Reese is right around 12 weeks but is not planning to announce it."
Reese, 35, married CAA agent Jim, 41, in March 2011 after dating for just over a year and friends say the pair are thrilled to be adding to their family and have started to break the news to their close friends.
A source said: "The timing is right. She`s so happy!"
When previously asked about the possibility of having more children, Reese said: "I would love that. I`m really happy.
"I feel really good and I`m at a really good place in my life."
Reese also admitted she is very "proud" of how she and Ryan have raised their kids and says she will always put her family first.
She said: "My ex-husband and I co-parent really well. We really balance each other`s schedules and just communicate a lot.
"Nothing`s more important for me than family. I definitely love having a career, but I just love being a mom - it`s how I was raised."