Kate Middleton's pregnant sister, Pippa, remains active even when she's already in the third trimester of her pregnancy.

Pippa is due to deliver her first baby in less than three months and she still regularly works out. In her monthly column for Waitrose, she revealed that as her due date approaches, her favorite workout is swimming.

"Swimming throughout pregnancy offers a wonderful feeling of weightlessness. As the months pass, you feel heavier by the day, but being in water gives that floating, light-as-air feeling that soothes and relieves the body, counteracts increased back strain and stretches the legs from the growing bump. It’s non-impact, gentle and relaxing," Pippa wrote.

"I'd confidently say I find it has been the most enjoyable and rewarding form of exercise since I found out I was expecting," she continued. "Being in water gives that floating, light-as-air feeling that soothes and relieves the body, counteracts increased back strain and stretches the legs from the growing bump."

According to Pippa swimming is one of the most therapeutic ways to work out especially when one becomes less mobile. It also helps prevent the shoulders from rounding forwards, which is a common symptom as one's belly expands. This workout also offsets the tendency for the pelvis to be misaligned.

Pippa also admitted that she loves how working out boosts her energy and helps her clear her mind. She added that she enjoys the "endorphins and the buzz factor that exercise gives."

Pippa and Middleton have a great relationship as siblings. In fact, according to a source, the Middleton sisters wanted to get pregnant at the same time following Pippa's wedding to James Matthews.

"It's long been a dream for Kate and Pippa to be pregnant at the same time," a family friend revealed. "To share that would be amazing. Pippa's always said how great it would be if they were to be expecting together."

In related news, Middleton and Pippa's parents, Carole and Michael, were accused of cashing in on Meghan Markle's birthday. The Middletons own a company called Party Pieces and they promote birthday items in the run-up of the Duchess of Sussex's natal day.