Physiological Changes during Third Stage of Labour

Remarkable Uterine Contraction

The uterus turns discoid-shaped after delivery and measures around 20 cm vertically and 10 cm anteriorly.

The contraction and retraction of the uterine muscle, which thickens the wall and decreases the capacity of the upper uterine segment so that the placental site is reduced, causes the placenta to separate. The placenta cannot contract, but when its cotyledons are packed closer together, it does become thicker and more compact. The placenta will likely separate within five minutes, and this time will depend on how strong the contraction was.

When the placenta separates, the blood sinuses are torn apart, causing 30 to 60 ml of blood to collect between the maternal surface of the placenta and the decidual basalis. This retroplacental clot or haematoma is then completely detached from the uterus by a subsequent uterine contraction, at which point the placenta is forced out of the uterus.

Separation of Placenta

The surface area of the uterine cavity gradually increases during pregnancy and rapidly decreases during the second stage of labor.

• Separation involves a slow, progressive detachment from the uterine wall brought about by the physical changes which occur during and after the actual birth of the baby. The placenta is separated from the decidual attachment by the action of the uterine muscle.
  • As the baby gradually leaves the uterus, the cavity of the uterus must become progressively smaller to permit the uterine wall to remain closely approximated to the baby. This maintains the expulsive force of the muscular contractions on the body of the baby.
  • The mechanism for achieving this close approximation is through a slight shortening or retraction of the individual muscle fibers during each contraction. Slow delivery of the shoulders and body of the infant will permit the muscle fibers to retract and adjust to the reduction in a size of the cavity, thereby promoting more efficient contractions.
  • As the surface area of the interior of the uterus slightly reduces the diameters of the placental site, the placenta becomes thicker and decreases slightly in diameter.
  • Since the placenta has a limited ability to alter its surface area, it must separate if the area to which it is attached is reduced considerably in size.
  • The separation, which occurs in the outer portion of the spongy layer of the decidua basalis, begins during the later second stage of labor and is usually completed as the uterus contracts during the final expulsion of the baby.
• The effectiveness of this process depends on the extent to which the placental site is reduced. If the contractions are firm and forceful, the placenta may be expelled almost immediately after the baby is born. However, if they are less effective, complete placental separation may be delayed.
• The blood sinuses at the placental site, which have been opened by partial or complete separation of the placenta, are the source of bleeding during and after the third stage of normal labor. This bleeding is controlled initially by firm contraction of the interlacing uterine muscle bundles around the branches of the uterine arteries.
  • The vessels are compressed and kinked.
  • Bleeding from the open ends is slight.
  • If the uterine muscle is relaxed, the bleeding will be brisk and continuous.

Expulsion of Placenta

Blood builds up behind the placenta after separation, and the uterus rises in the belly. The uterus now undergoes a hard uterine contraction, changing from a flattened, soft, discoid organ to a solid, globular organ. Depending on the intensity of the uterine contractions and the position of the placenta within the uterus, this procedure might take a few minutes or much more. A lower implantation will result in softer contractions and a longer period for ejection and separation in the lower uterine section.

As the placenta is pushed down into the lower uterine segment during a uterine contraction, the flattened uterine body of the uterus becomes globular.

As the placenta leaves the upper part of the uterus, the uterus can be felt through the abdominal wall as a hard, globular mass.

• Birth of the placenta by the Schultz Mechanism
  • The fetal surface of the placenta slips through the opening in the fetal membranes and appears at the introitus.
  • The membranes then peel off the surface of the uterine cavity, uniformly and intact.
  • The liquid blood and retroplacental clots, if any, are contained within the folded placenta and are not evident until the placenta is delivered and examined.
• Birth of the placenta by the Duncan Mechanism
  • One edge of the placenta first slips through the cervix and into the vagina. The remainder of the placenta follows, and the fetal membranes are peeled from the uterus as traction is made on the edge of the placenta which follows.
  • The liquid blood and retro placental clots escape from the uterus as the maternal surface of the placenta are delivered.
  • The Duncan Mechanism is more frequently followed by retained fragments of the fetal membranes.

Control of Bleeding

Enumeration torn sinuses that have free circulation of blood from uterine and ovarian veins must be removed after placental separation. Complete retraction has an impact on the occlusion because the arterioles are actually constricted as "living ligatures" as they tortuously move through the interlacing intermediate layer of the myometrium. It is the main clotting control mechanism. However, thrombosis develops to conceal the torn sinuses, a phenomenon made easier by pregnancy's hypercoagulable state. Minimizing blood loss also benefits from resistance of the uterine walls after placenta expulsion.