Kate is due to give birth to her first child this month.
"It was just too warm in Nott Cott," an insider told UsMagazine.com, referring to the couple's Nottingham Cottage residence, which has no air conditioning. "She decided to spend the weekend with mum and dad. Much more comfortable."
England's highest 2013 temperature of 89.4 was reached Saturday.
The report did not say when the duchess plans to return home to London.