Beyonce and Jay Z's twins arrived ahead of time.
The babies were born prematurely and are being treated for jaundice, TMZ has reported.
The pair have not left the hospital since arriving eight days ago; it was previously reported doctors had opted to keep them in for a 'minor issue.'
The siblings will have spent most of their first days under 'bili lights,' phototherapy designed to eliminate high levels of bilirubin in the blood.
Jaundice is quite common among newborns, especially preterm births; twins are more susceptible since they are rarely carried to full term.
The yellow substance is created when old red blood cells are replaced by new ones; heightened levels cause a baby's skin and eyes to turn yellow.
Untreated it can be very dangerous, but once diagnosed it is easily and usually successfully treated.
On Sunday Beyonce's dad Mathew Knowles confirmed via Twitter the couple had added two more members to the family, joining five-year-old Blue Ivy.
News that the singer had given birth the day before ignited social media, with the Bey hive fan group buzzing over the yet unrevealed sexes and names of the latest members of the Carter Clan.
