Her arrival at King Edward VII hospital – where Middleton was taken earlier for severe morning sickness – is a precautionary measure after the monarch, 86, fell ill Friday night, says a palace spokesman.
"She was not taken into the hospital immediately after feeling the symptoms," adds the spokesman. "This is simply to enable doctors to better assess her."
The Queen is expected to remain under care at the hospital for about two days; all official engagements for this week, including her trip to Rome, will be either canceled or postponed as she recovers from her stomach bug.
The royal was last hospitalized for a minor knee operation about 10 years ago, according to the BBC.
So far, no visitors have been observed arriving at King Edward VII.