Her baby isn't due until early September, but reports suggest that Reese Witherspoon could give birth any day now.
RadarOnline reported that Reese, 36, was taken into hospital with complications on Tuesday night, but was later released.
She then returned to hospital on Wednesday morning and doctors now plan to keep her in for a few nights in the hope that they can delay labour.
A source told RadarOnline: "Doctors have sedated Reese and they're keen on keeping her from going into an early labour so the baby's development will go to full term."
They added: "Reese's husband Jim hasn't left her side and he doesn't plan to. Jim is really worried about the baby. He's a first-time father so this is all really new to him and he just wants everything to be ok.
"He's a bundle of nerves. Meanwhile, this will be Reese's third child, so she's been through this before and she's trying to keep him calm."
The source claimed: "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."
Reese already has a 12-year-old daughter, Ava, and a son, Deacon, nine, from her marriage to Ryan Phillippe. She married Jim Toth at her ranch in Ojai, California, in March 2011.