Once the placenta separates the uterus continues to contract, possibly being felt as mild cramping or quite strong contractions. The shrinking uterus forces the placenta and membranes to fall into the lower uterine segment of the uterus, through the partially opened cervix and finally into the woman's vagina and out of her body.
The physiological or natural management of the 3rd stage means that the caregiver should interfere as little as possible. Some caregivers are inexperienced at facilitating a natural 3rd stage (because they usually actively manage it) and may find it difficult to 'sit on their hands' to allow the placenta to be delivered spontaneously.
Actions like 'rubbing up' the uterus by massaging it (in an attempt to stimulate a contraction) and / or pulling on the cord or cutting and clamping the cord before the placenta has separated (unless absolutely necessary) can interfere with the natural process, causing partial separation of the placenta and increasing the amount of blood loss for the woman.
NOTE: If the placenta has already started to separate, 'rubbing up' the uterus may be needed to stimulate a contraction to help expel a partially separated placenta and minimise bleeding.
The woman's body is usually capable of expelling the placenta unaided, but there may be some circumstances when the woman and the caregiver need to help the placenta to be fully expelled, while still facilitating a physiological 3rd stage and being careful not to interfere with the natural process.
Circumstances that may require help can include:
The placenta not separating
Breaking the cord
The placenta being held in the cervix
The membranes being held in the cervix
The placenta not separating
Occasionally the uterus takes a while to contract or needs some assistance to do so.