It all comes down to physiology. “As your baby moves down through the pelvis, their head places significant pressure on the rectum, which sits directly behind the vagina,” said the author of The Modern Midwife’s Guide to Pregnancy, Birth and Beyond.
Article source: https://www.huffingtonpost.co.uk/entry/labour-poo-how-common_uk_6a1fff98e4b08a03cfbda730