The Role of Placental Progesterone in Labor Initiation and Uterine Contractions

Why placental progesterone suppressed or drop before onset of labour?

It’s important to understand the role of progesterone in pregnancy. Progesterone is a hormone produced by the placenta during pregnancy, and it plays a crucial role in maintaining the pregnancy and preparing the body for childbirth.

During most of pregnancy, progesterone levels are high and help to support the growth of the uterus, suppress uterine contractions, and maintain a relaxed state of the uterine muscles. This is important because premature contractions could potentially lead to preterm labor, which poses risks to both the mother and the baby.

As the pregnancy progresses toward its later stages, typically around the 37th to 38th week, the body needs to prepare for labor and delivery. At this point, the levels of placental progesterone start to decrease, while the levels of other hormones, such as estrogen, begin to rise.

The drop in progesterone levels is primarily triggered by the maturation of the fetal lungs. As the lungs mature, they start to produce a hormone called cortisol, which stimulates the production of enzymes capable of metabolizing progesterone. As a result, the levels of progesterone decline, and this reduction is an important mechanism that allows the uterine muscles to become more responsive to contractions.

Furthermore, progesterone also inhibits the production of certain substances called prostaglandins, which are responsible for promoting contractions of smooth muscles, including the uterine muscles. When progesterone levels fall, this inhibition is reduced, allowing prostaglandins to accumulate and stimulate uterine contractions, initiating the onset of labor.

In summary, placental progesterone levels drop before the onset of labor because the maturation of fetal lungs triggers the release of cortisol, which metabolizes progesterone and reduces its levels. This reduction in progesterone and the subsequent increase in prostaglandins help to initiate labor by promoting uterine contractions.

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