The 36-year-old was admitted on Tuesday night before being released and then rushed back to hospital on early Wednesday morning.
A source told Radar Online, "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."
The actress' first child with hubby Jim Toth is due in early September.
According to the source Jim is "really worried" and hasn’t left his wife's side since she was admitted.
"He's a first-time father so this is all really new to him and he just wants everything to be okay.
"He's a bundle of nerves," said the source.
This is the third child for Reese, who shares two kids — Ava and Deacon — with ex-husband Ryan Phillippe.
