THIRD STAGE OF LABOUR

The period from the birth of the baby to the delivery of the placenta and membranes is the third stage of labour. It typically lasts between 5 and 15 minutes, but a period of up to 30 minutes is considered normal. While the third stage of labour is usually uneventful, significant complications can occur during this period, with the most common complication being postpartum hemorrhage (PPH).

Physiological changes during third stage of labour

Throughout pregnancy, the placenta attaches itself to the uterine wall, providing a continuous supply of nutrients and oxygen to the baby from the maternal blood. Shortly after the birth of the baby, the body expels the placenta because it is no longer needed. The natural hormone oxytocin causes the uterus to contract throughout labour, including the third stage.

Following physiological changes occurs during third stage of labour

Remarkable uterine contraction

Separation of the placenta

Separation of membranes

Expulsion of placenta

Control of bleeding after separation

Remarkable Uterine Contraction

After the baby is born, the body naturally releases a surge of oxytocin, which triggers strong uterine contractions that separate the placenta from the uterine wall. Since there is continuous blood flow to the placental site even after the placenta is expelled, another surge of oxytocin encourages the uterus to contract again, effectively clamping down on this area to prevent hemorrhage.

Once the baby is born, the uterine muscles contract to facilitate the separation of the placenta from the uterine wall. The amount of blood loss depends on how quickly this process occurs, as the uterus contracts more effectively after the placenta is expelled. If the uterus doesn’t contract normally, the blood vessels at the placental site remain open, leading to hemorrhage. Due to the estimated blood flow to the uterus being 500-800 ml per minute at term, severe postpartum hemorrhage (PPH) can occur within just a few minutes.

Separation of placenta

At the beginning of labor, the placental attachment roughly corresponds to an area of 20 cm (8 inches) in diameter. There is no appreciable reduction in the surface area of the placental attachment during the first stage. During the second stage, there is slight but progressive retraction, which reaches its peak immediately after the birth of the baby.

After the baby is born, the uterus measures approximately 20 cm (8 inches) vertically and 10 cm (4 inches) antero-posteriorly, taking on a discoid shape. The wall of the upper segment thickens significantly, while the lower segment becomes thin and flabby, forming folds. The cavity reduces in size significantly, accommodating only the afterbirth.

Mechanism of placental separation

The separation of the placenta occurs due to the contraction and retraction of the myometrium, which leads to the thickening of the uterine wall and a reduction in the size of the placental area. As the placental area decreases in size, the placenta begins to detach from the uterine wall. Unlike the uterus, the placenta is not elastic and cannot contract or retract. At the site of separation, a clot forms, known as a retro-placental clot, which accumulates between the decidua and the placenta, further facilitating separation. Subsequent uterine contractions fully detach the placenta from the uterine wall, causing it to descend into the lower uterine segment and eventually into the vagina, where it is expelled.

Two methods of placental separation, as described by Schultze and Matthews Duncan, are not under the control of the birth attendant.

1. Schultze method
2. Matthews Duncan method

1. Schultze method

The detachment of the placenta from its uterine attachment begins at the center, resulting in the opening of a few uterine sinuses and the accumulation of blood behind the placenta, forming a retro-placental hematoma. This method is considered more common.

The placenta detaches from its central point and descends into the vagina through a hole in the amniotic sac. The fetal surface becomes visible at the vulva, with the membranes trailing behind like an inverted umbrella as they peel off from the uterine wall. The maternal surface of the placenta remains concealed, and any blood clot is contained within the inverted sac.

2. Matthews Duncan method

The placenta slides down sideways and emerges through the vulva with the lateral border leading, resembling a button passing through a buttonhole. The maternal surface becomes visible, and blood is able to escape as it is not contained within a sac. This separation process takes more time, resulting in increased blood loss. It is typically accompanied by vaginal bleeding (blood from the placental site) due to the slower separation, and no retro-placental clot.

Signs of placenta separation

Signs of placental separation include the following:

1. The fundus becomes hard and globular, rising abdominally to the level of the umbilicus.
2. The cord lengthens at the vulva. When fundal pressure is applied, the uterus becomes firm, and when the pressure is released, the cord outside the vulva does not recede if the placenta has completely separated. However, it will recede if separation is not complete.
3. A gush of blood, typically around 30-60ml, exits from the vagina.
4. The woman may experience lower abdomen pain.

Separation of membranes

In the active phase, the membranes, which are loosely attached, fold into multiple folds. Those membranes attached to the lower segment have already separated due to its stretching. The separation of the membranes is facilitated partly by uterine contractions and mostly by the weight of the placenta as it descends from the active part. As the membranes separate, they carry bits and pieces of decidua vera with them, resulting in the characteristic roughness of the outer surface of the chorion.

Expulsion of placenta

After the complete separation of the placenta, it is forcefully pushed down into the flabby lower uterine segment or the upper part of the vagina through the effective contraction and retraction of the uterus. Subsequently, it is expelled through either the voluntary contraction of abdominal muscles (bearing down efforts) or manipulative procedures.

Control of bleeding after separation

After separation, the control of bleeding is crucial, as normally, 500-800 ml of blood flows through the placental site every minute. Without an effective mechanism to control bleeding after delivery, a woman could rapidly lose blood and potentially bleed to death within minutes.

The contraction and retraction of the uterine muscle, which facilitate placental separation, also exert strong pressure on the blood vessels, effectively controlling bleeding. This control is made possible by the presence of oblique muscle fibers in the upper uterine segment. Additionally, blood clots form in the torn blood vessels at the placental site, further halting blood flow.

It’s worth noting that a full bladder or any remnants left behind in the uterus after delivery, such as placental tissue, membranes, or blood clots, can interfere with the uterus’s ability to contract, leading to excessive bleeding in the woman.

Length of third stage of labour

Considerable research has examined how active management affects the third stage of labour. Investigations have found that 50% of placental deliveries occur within 5 minutes, and 90% are delivered within 15 minutes. A third stage of labour lasting longer than 18 minutes is associated with a significant risk of postpartum hemorrhage (PPH). When the third stage of labour lasts longer than 30 minutes, PPH occurs six times more often than it does among women whose third stage lasted less than 30 minutes. This remarkable process involves uterine contractions, placental separation, placental expulsion, and the control of bleeding.

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