Childbirth: The Role of Hormones in Labor and Delivery

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  • 0:05 Hormones of Labor
  • 1:06 Initiation of Labor
  • 3:46 Continuation of Labor
  • 5:24 End of Labor
  • 6:01 Lesson Summary
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Lesson Transcript
Instructor: Heather Adewale

Heather has taught reproductive biology and has researched neuro, repro and endocrinology. She has a PhD in Zoology/Biology.

How does a mother's body know when it is time to give birth? How does the labor process begin and end? Learn the answers to these questions in this lesson on the hormones involved in childbirth.

Hormones of Labor

Ever wonder how the body knows it's time to give birth? Does the baby tell the mother's body it's ready, or does the mother's body just know? What goes on in the body to initiate the birthing process?

Well, as you may have guessed, the baby and body use signals to communicate. The baby communicates to the mother's uterus when it is ready to be born by sending out signals in the form of hormones, those chemical messengers the body uses to relay instructions from one area to another.

The baby communicates when it is ready to be born by sending out signals in the form of hormones.
Hormones Trigger Labor

These hormones help to coordinate the process of labor, which is the expulsion of the fetus from the mother's uterus. They tell the mother's body when the baby is ready to be born. They help make sure the smooth muscles in the uterus are working together to create synchronized contractions. They tell the cervix to dilate, making room for the baby to pass through, and they prepare the mother's body for nursing.

Initiation of Labor

So, what are some of these hormones? Well, during pregnancy, the hormone progesterone has been one of the main players. High levels of progesterone prevent uterine contractions while the baby is still developing. Therefore, our first step is to somehow override or decrease progesterone so that labor can occur.

This process is initiated by the baby itself. As the baby enters the final stretch of growth, it stretches the uterus to its limits! This puts a physical stress, or strain, on both the baby and the uterus. This physical stress causes the release of stress hormones from both the baby and the uterus. These hormones, corticotropin-releasing hormone (CRH) and cortisol, increase during the last few days and weeks of gestation.

The rise in stress hormones triggers a rise in the steroid hormone estriol. Estriol is a form of estrogen that is predominant during childbirth. As estriol rises, it inhibits the synthesis of progesterone by the placenta and prepares the smooth muscles of the uterus for labor. Estriol and other estrogens increase the sensitivity of smooth muscles in the uterine wall to the hormones that will stimulate uterine contractions. This also helps to coordinate uterine contractions.

At the end of gestation, CRH and cortisol increase and trigger a rise in estriol.

Now, remember what progesterone does? Progesterone was the hormone that prevented the uterus from contracting, right? So, if progesterone is inhibited, that means the smooth muscles of the uterus can begin to contract! But, that's not all that happens.

As estrogen begins to stimulate uterine contractions, the uterus also produces hormones called prostaglandins. These also contribute to a decrease in progesterone levels. The release of prostaglandins helps initiate labor and, along with another hormone called relaxin, relaxes the muscles of the cervix. This is important because the cervix must be relaxed if it is to stretch (or dilate) enough to allow the baby to pass through.

Relaxin is produced by the ovaries, and it is also important in relaxing the muscles of the pelvis so that the baby can pass through the mother's hips.

Continuation of Labor

Okay, so now that we know what initiates the process of labor, what about the signals that keep the uterine contractions going? How does the body know when to stop? That is where our next hormone, oxytocin, comes in. Oxytocin plays roles in childbirth, nursing and in the bonding between mother and child. Its main role in childbirth is to continue the process of uterine contractions using something called a positive feedback loop.

As the baby pushes against the cervix, the pituitary gland of the mother releases oxytocin.

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