your body makes a hormone called 'relaxin' which helps your joints get stretchy to allow your tummy to grow during pg and also to allow the baby to pass thru the birth canal. It also affects your baby to make them a littlemore bendy while in the womb and pass more easily thru the pelvis. It affects all the tissues in your body, so you pee super easy even before baby is putting pressure on the bladder, get heartburn, and are constipated more easily too.