As your due date nears, you might be wondering how your body decides one day that it is time to give birth. What exactly kicks things off, and why do some women give birth on their due date while others seem to stay pregnant forever until they are induced?

The exact physiology behind labor and how it occurs isn’t totally understood. But we do know a few key facts about how labor can start, and why it isn’t the same for everyone.

During your pregnancy, your body is actively trying to prevent an early delivery. Certain hormones like progesterone, nitric oxide, and a few others keep your uterus from contracting and delivering too early. Some things can override this and activate labor prematurely. These include certain infections, a severe illness in the mother such as the flu, and bleeding from a placental abruption. Still others deliver early because their cervix is too short or weak, and for some women who are prone to preterm delivery, we have no good reason to explain why it happens.

We have a theory that labor begins with an activation phase. Think of this as your body’s switch being given the green light to go into labor. Estrogen is thought to play a part in this: it makes the uterine cells that contract activate and be put on high alert. These cells are also now receptive to oxytocin, which is the hormone that makes them contract.

The next phase is called the stimulation phase. This is when labor really gets going. Uterine contractions now happen because the hormones oxytocin and prostaglandins are released in your body telling the contraction cells in the uterus to get to work—and voila!—contractions begin!

Your baby might have a role in starting labor too, though we aren’t completely sure. Certain hormones released by your baby eventually get converted to a form of estrogen, which helps with the activation phase mentioned earlier. In animals such as sheep, the role of the baby is much more certain when it comes to starting labor, but in humans we still need to do more studies.

So while we might not be totally sure when labor will begin and why it starts so early for some women and later in others, a few clues exist that show the important role of certain hormones. Another interesting fact is more women go into labor at night. It has been proposed that this is a holdover from our distant ancestors. Think of the cavewoman in labor. It is much easier for her to hide away and give birth (when she is much more vulnerable to an attack) in the dark of night. Therefore, something associated with the circadian rhythms of our bodies may be the reason labor starts more often when it is dark out!

Reviewed by Dr. Jen Lincoln, April 2020


  • We don’t totally understand what starts labor, but hormones definitely play a role.
  • Your baby may also send a message (via hormones or something else released) that causes your uterus to contract and labor to begin.
  • In humans and most animal species, labor often begins at night.


  1. Gabbe SG et al. Obstetrics: Normal and Problem Pregnancies. 5th ed. Normal labor and delivery.


  1. Amazing how our bodies work!

    1. I agree! I am amazed how we can grow a human, give birth, and then go on to make milk to feed that human. So awesome!

  2. Going into labor will be the scariest part of having a baby!

    1. I think one of the best ways is to try and know what to expect, whether it be by classes or books or reading articles online. For many women, the idea of going into labor is scary but once it happens they are super excited – you are finally about to meet your baby! For me, the scariest part was when they told us we could take a new human home – now THAT worried me!!

  3. I agree- this is a very interesting article. I would much rather go into labor at night when I am usually at home.

  4. Very interesting article. I just learned so much and I’m a geek so I’ll save and reference again when my wife goes into labor in a few weeks! A bit odd about the night thing though. I thought the reason we hunker down at night and either find shelter or build fire is because the predators often hunt at night and we can’t see them coming so we hide or keep them away with fire. Circadian rhythm sounds right and that the body is more relaxed when sleeping maybe?

  5. The thought about why many women go into labor at night is fascinating!


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