Stages of labor

Video summary

Labor is the process by which the fetus and placenta leave a woman’s uterus, thereby ending the pregnancy. It occurs roughly 40 weeks after the beginning of the last menstrual period. Vaginal delivery involves three stages of labor: the shortening and opening of the cervix, descent and birth of the baby, and the delivery of the placenta. 
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Labor and delivery, or parturition, is the process of delivering a baby into the world, which typically occurs between 37 and 42 weeks after the first day of the last menstrual period.
Now labor occurs due to a gradual shift in hormonal activity that moves the body from maintaining pregnancy to preparing for delivery.
For most of the pregnancy, progesterone keeps the uterus calm and relaxed, providing a stable environment for the growing fetus.
But as the due date approaches, estrogen takes the lead. Rising estrogen levels shift the uterus into a more contractile state.
Estrogen also up-regulates receptors for prostaglandins and oxytocin, setting the stage for labor. Meanwhile, the posterior pituitary gland increases oxytocin release.
Together, estrogen and oxytocin stimulate the production of prostaglandins. Specifically PGE2 and PGF2 alpha.
Next, prostaglandins increase calcium levels within myometrial cells, making contractions even stronger. They also help build gap junctions between muscle cells, enabling the uterus to contract as a single powerful unit.
On top of that, they start the process of softening and thinning the cervix. At the same time, oxytocin is getting ready for its big moment.
As its levels rise and receptors increase in number, oxytocin further amplifies uterine contractions. This creates a positive feedback loop.
Contractions dilate the cervix, which boosts oxytocin release, and oxytocin makes contractions stronger. The cycle reinforces itself until the baby is born.
But before true labor starts, the uterus often goes through warm-up contractions in the last few weeks of pregnancy. These irregular, mild, usually painless contractions are known as Braxton-Hicks contractions.
Some individuals describe them as a gentle squeezing or tightening across the belly. As the due date approaches, they might become more frequent or stronger, even happening every 10 to 20 minutes.
But the main clue is that they don't cause any changes to the cervix, so they are not true labor. While the uterus is warming up, the cervix is quietly preparing in the background.
Thanks to more water content and the breakdown of collagen fibers, the cervix becomes softer and thinner. Which is a process known as effacement.
While this is happening, the mucus plug that's been sealing the cervix during pregnancy can loosen and come out. Sometimes it's mixed with a little blood, creating a blood-tinged vaginal discharge known as bloody show.
This is one of the signs that labor is getting close, but it's not go time just yet. Once all these elements fall into place, true labor begins, which unfolds in 4 stages, each with its own purpose and progression.
Let's start with the first stage, which is all about opening the cervix. This stage can last quite a while, especially for first-time birthing parents, where it can take up to 18 hours.
For those who have delivered before, it's usually shorter, around 10 hours or less. Regardless of how long it takes, this stage has two distinct phases.
First is the latent phase. This is the slow buildup.
Contractions are mild, irregular, and not very close together, happening every 5 to 30 minutes and lasting about 30 seconds.
During this time, the cervix begins to dilate from closed to 3 centimeters. At the same time, it starts thinning and shortening to around 30% shorter than its original length.
However, as labor progresses, contractions get closer together every 3 to 5 minutes and stronger, lasting closer to 1 minute each.
By the end of this phase, the cervix usually reaches a dilation of 4 to 6 centimeters and shortens to around 80% of its original length.
If you come across the word effacement, don't let it throw you off. It's just a medical way of saying the cervix is thinning out and shortening.
Then comes the active phase where things ramp up fast. Contractions are strong, regular, with very little time to rest in between, happening every 2 to 3 minutes and lasting between 60 and 90 seconds.
Sometimes they even overlap slightly. During this phase, the rate of cervical dilation is around 1 centimeter per hour, causing the cervix to rapidly dilate from 6 to 10 centimeters and shorten fully until it is very thin and has very little length.
This is called 100% effacement. If the amniotic sac hasn't ruptured already, it often does at this point.
Now that the cervix is fully dilated, we enter the second stage of labor, also known as the pushing stage. This is when the real work begins to bring the baby into the world.
For first-time birthing parents, this stage can last anywhere from 30 minutes to 3 hours. For those who've delivered a baby before, it's usually much quicker, up to 30 minutes.
During this stage, the baby has to navigate through the maternal pelvis, and whether that journey goes smoothly depends on the three P's power, passenger, and passage.
Power refers to forceful uterine contractions. Passenger refers to the fetus, and passage refers to the root that the fetus must squeeze through.
In fact, the relationship between the baby's head and the bony pelvis is so critical that human babies have evolved with skull bones that are unfused.
This allows the baby's head to mold and shift just enough to pass safely through the pelvis. Now, there are a few factors that determine how easily the baby can pass through the birth canal, and first up is fetal size.
The most important part here is the size of the baby's head. A larger head can make delivery more difficult, especially if the pelvis is on the smaller side.
Next, there's the fetal lie, which describes how the baby is aligned inside the uterus. The ideal lie is longitudinal, where the fetal spine lines up with the maternal spine, meaning the fetus is lying vertically.
Less favorable options include a transverse lie where the fetus lies sideways and an oblique lie where the fetus is at an angle.
These non-longitudinal positions can complicate delivery and might require medical intervention. Then there's fetal presentation, which refers to the part of the baby that's coming out first.
Most of the time, it's the head. This is called a cephalic or vertex presentation, and that's exactly what we want.
But when the buttocks, knees, or feet come first, that's a breach presentation. This presentation can lead to more complications during delivery.
The rarest and most challenging is a shoulder presentation, where the shoulder tries to come through the birth canal first.
In that case, a vaginal delivery usually isn't safe, and a C-section is necessary. Next we've got fetal position, which refers to the direction of the back of the baby's head, called the occiput.
The most favorable position is occiput anterior, where the baby is head down, facing the maternal spine, while the occiput is facing the maternal stomach.
That's the ideal setup for an easier vaginal delivery. If the baby is facing up in occiput posterior position or sideways in occiput transverse position, labor can be longer and more difficult.
Position matters. Sometimes just a small rotation can make all the difference.
Finally, there's something called fetal attitude, and no, it's not about the baby's mood. Fetal attitude refers to how the baby's body is positioned.
The ideal attitude is full flexion, chin tucked to chest, back rounded, and limbs pulled in close. This allows the smallest part of the head, called the suboccipital regmatic diameter, to pass through the pelvis first.
If the baby isn't well flexed or is on the larger side, completing that trip through the birth canal can be a lot tougher.
Now to make it through the passage, the baby performs a series of precise moves called the cardinal movements of labor. It all begins with descent as the baby gradually moves down through the pelvis.
This process starts early before the baby's head reaches the narrowest part of the pelvis. The degree of descent is measured by fetal station, which compares the baby's presenting part to the ischial spines, which are bony landmarks deep inside the pelvis.
Initially, the baby moves from the pelvic inlet, which is about -5 station, down to the ischial spines, which is station zero.
Moreover, when the baby's head reaches the level of the ischial spines, this is called engagement, meaning the head is locked in and ready to move further down.
Next comes flexion, where the baby tucks its chin to the chest, allowing the smallest diameter of the head to lead the way.
Then comes internal rotation, where the baby's head turns to a line with the long axis of the maternal pelvis, usually rotating about 90 degrees to help navigate through the narrowest part of the birth canal.
Once the baby's head reaches the vaginal opening, extension occurs. The baby's head moves from a tucked position to one where the chin lifts, allowing the head to pass under the pubic symphysis.
After the head delivers, there's external rotation, also called restitution, where the head rotates to line up with the shoulders, which are still inside the birthing parent.
Finally, we reach expulsion where the anterior shoulder slips under the pubic bone followed by the posterior shoulder and the rest of the body.
While these movements are typically described in a specific order, they often overlap or occur together, rather than as strictly separate steps.
Still, this general sequence helps explain how a baby successfully navigates the birth canal during a vaginal birth. The 3rd stage of labor begins right after the baby is born and ends with the delivery of the placenta.
It usually doesn't take long because placental separation often occurs within 2 to 10 minutes. Before the placenta comes out, a few signs typically appear, including a small gush of blood from the vagina and the lengthening of the umbilical cord.
Additionally, the uterus rises in the abdomen and becomes firm. When these signs are present, gently pulling on the cord while placing a hand just above the pubic bone to securely support the uterus can help deliver the placenta.
Once the placenta is out, it's important to check that no fragments of the placenta are left in the uterus. Finally, the 4th stage starts after the placenta is delivered and lasts for a few hours.
During this time, oxytocin continues to trigger strong uterine contractions. Clamping down on the blood vessels and preventing blood loss known as postpartum hemorrhage.
Also, the uterus begins the process of shrinking back to its pre-pregnancy size, although full involution takes several weeks.
Alright, as a quick recap, labor and delivery, or parturition, is the process of delivering a baby into the world, typically occurring between 37 and 42 weeks after the first day of the last menstrual period.
It is triggered by a hormonal shift, particularly rising levels of estrogen and oxytocin, but also prostaglandins, which together initiate uterine contractions, cervical dilation, and ultimately birth.
Moreover, the first stage of labor involves cervical dilation. The second stage involves pushing and the delivery of the baby, during which the fetus performs the cardinal movements of labor known as descent, flexion, internal rotation, extension, external rotation, and expulsion.
The 3rd stage involves delivery of the placenta, while the 4th covers the beginning of uterine involution.