The 36-year-old is expecting her third chid, her first with husband Jim Toth, and RadarOnline reports that her physicians want to keep a close eye on Reese and have advised her to remain at the medical facility.
"Doctors are keen on keeping her from going into early labor so the baby's development will go to full term," a source told the site.
"Reese's husband Jim hasn't left her side and he doesn't plan to."
Reese has two other children from her seven-year marriage to Ryan Phillippe; Ava, 12, and Deacon, eight.
This is the Hollywood agent's first child and Jim is said to be anxious that everything goes well for his wife and unborn baby.
"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 okay," the insider shared.
"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 Legally Blonde star was first taken to hospital on Tuesday evening and was released that night but was then taken in again the following day.
"Unless she goes into labor, Reese will remain in the hospital under the watchful eye of the doctors," the source added.