There is a period of 6 weeks after birth called puerperium – this is the time during which the mother’s uterus (and other reproductive organs) return to their original non-pregnant condition. During this period mothers can get contractions. There are a number of terms for these contractions – afterbirth pains, afterpains, postpartum cramps, postpartum pains, uterine cramps, postpartum contractions, postpartum surges and the most accurate; uterine involution.
Involution means the shrinkage of an organ and uterine means of the uterus. This literally means a shrinking uterus.
The uterus shrinks back to its original size by way of contractions, similar to those experienced in childbirth, and because of this, they can be effected by the same hormones that affect labour and birth.
Oxytocin (more info in this post) probably has the most profound effect during labour (it stimulates contractions for one) and it also effects uterine involution, causing contractions and possibly speeding up the process of involution. Its effects can be heightened when breastfeeding, where oxytocin plays a key role – it causes the muscles around the milk-making glands in a mothers breast to contract, causing the let-down reflex (a release of milk).
Many first-time mothers experience far fewer contractions after birth, because their uterine muscle tone is at its best. When they experience a contraction the uterus will tend to contract and stay contracted, rather than intermittently relaxing and then contracting again – like non-first-timers. The more births a mother has, the worse her muscle tone gets and the number of contractions needed to return the uterus to its pre-pregnancy state increases. This is why postpartum contractions are known to become worse with each successive pregnancy and last for up to 6 weeks.