Physiological (Natural) Third Stage
Delivery of the placenta
During labour the muscle fibres of the uterus cause it to contract and get smaller. The baby moves down the birth canal and once it is born, the uterus continues contracting naturally.
As the uterus gets smaller the placenta begins to detach from where it has been attached to the uterus wall throughout the pregnancy.
At the same time, the muscle fibres in the uterus close around the woman's blood vessels, effectively sealing them and reducing further bleeding. In addition, the blood clotting system is activated to limit blood loss. The flow of blood through the placenta is approximately over 500mls (more than a pint) per minute, but during third stage it reduces to virtually nothing.
Once the placenta has separated it will come out of the uterus, through the birth canal and be expelled. This may require gravity (upright positions) and possibly some maternal effort (pushing) but the contractions of the uterus themselves do most of the work. Skin-to-skin contact with the baby or breastfeeding can also make the uterus contract to help this process.
Once the placenta has been delivered, the uterus continues to contract and gradually returns to its pre-pregnancy size.
Clamping the cord
When the baby is born, the cord will still be attached to both the baby and the placenta. In a physiological third stage it is usual for the cord to remain unclamped until it has stopped pulsating. (It is possible for the cord never to be clamped and for the baby to remain attached to the placenta until the cord naturally falls off the baby - this is called a Lotus Birth http://www.sarahbuckley.com/lotus-birth-a-ritual-for-our-times)
During pregnancy the baby's blood flows through the umbilical cord and placenta, and it is estimated that one-third of the baby's blood is in the placenta at any time. Delaying the cord clamping after birth allows the baby's blood to flow away from the placenta and back into the baby, a process called placental transfusion. Studies show that delaying cord clamping for just a few minutes improves outcomes for baby, improving the amount of iron in their blood, reducing the likelihood of anaemia and helping them adjust to their surroundings better.
Experiencing a natural third stage
The whole process can last between a few minutes to an hour, and as long as there is no worry about the amount of blood loss, this can be a calm time while the mother gets to know her new baby. It is important that, during this time, the production of oxytocin (the natural hormone of third stage) is encouraged. [More information]
If blood loss is not controlled naturally, or the placenta does not come away within a specified time period (often about an hour), the healthcare professionals can intervene to help. Some experienced healthcare professionals will be relaxed about leaving the placenta for several hours if necessary, providing all is well with the mother.