The Duchess of Cambridge left her residence at Kensington Palace's Nottingham Cottage, which has no air conditioning, for the cooler climes of Carol and Michael Middleton's house on Friday.
"It was just too warm in Nott Cott," an insider said. "She decided to spend the weekend with mum and dad. Much more comfortable."
The Bucklebury, Berkshire residence has a pool and a patio -- plus air conditioning -- where the duchess can relax while she awaits labor. London temperatures hit 89.4 degrees Saturday, the hottest of the year so far.
Princes William and Harry competed in a charity polo match Sunday, and a source said the first time dad is remarkably calm.
"William is relaxed," a royal aide said, adding that he planned to "drive to Bucklebury and stay the night at the Middleton's house with Kate" after the match.
A helicopter is reportedly waiting for her there, so she can be transported to St. Mary's Hospital in London. The duchess is due to give birth in the private Lindo Wing where both William and Harry were born.