The 36-year-old actress is pregnant with her and husband Jim Toth's first child together and is not due until the beginning of September.
However the baby may be coming earlier than planned.
A source told website Radar Online: "Reese was actually taken to the hospital on Tuesday night with complications.
"She was released that night and then rushed back to the hospital today. At this point it looks like she might have the baby."
Reese and Jim were married in 2010 and she has two other children - Ava and Deacon - from her relationship with Ryan Phillippe.
The insider added: "Reese and Jim are a little worried about having the baby so early.
"But it's close enough to her due date that they're confident the baby will be just fine."
'Water For Elephants' star Reese, recently asked Robert Pattinson to stay at her ranch when she heard Kristen Stewart had cheated on him.
Apparently Reese gave him a place to stay and a shoulder to cry on because she knows what it's like to be cheated on.
A source said: "Reese went through a very public cheating scandal herself when Ryan Phillippe went outside their marriage and hooked up with Abbie Cornish, so her first thought when she heard about Kristen and Rob was to reach out to him and make sure he was okay."