This is according to Tmz
"We're told a "minor issue" surfaced, and as a result ... doctors do not feel comfortable releasing them. We do not know the nature of the issue. One source told us Beyonce also remains at the hospital, but as far as we know she's okay. The babies are expected to be okay as well.
It's unclear when everyone will be released."
The Single Ladies star, 35, and her 47-year-old rapper husband Jay Z's twins make the Carter's now a family of five alongside daughter Blue Ivy, 5.
“Bey and Jay are thrilled and have started sharing the news with their family and closest friends,” a source said.