Labor, also called parturition, describes the hard work of delivering a baby! It specifically refers to the process which starts with uterine contractions which cause cervical changes which allow the fetus to be delivered vaginally, and ends with delivery of the placenta. Labor typically begins at some point when the fetus is considered full term—between 37 and 42 weeks’ gestation.

In the third trimester, before labor starts, a woman might have a plug of mucus and blood fall out of the opening to the cervix, sometimes called a “bloody show”. Other times the amniotic sac might rupture, sometimes called “water breaking”. Either of these can trigger the onset of labor and so-called true labor contractions. These guys have to be distinguished from the milder and ineffective false labor contractions, also called Braxton Hicks contractions (or sometimes called practice contractions). Once they start, true labor contractions progress in frequency, duration, and intensity, and they can feel like waves that build up to a peak intensity and then gradually decrease.

The contractions pull on the thick tissues of the cervix, causing it to efface or get thinner and also dilate or open up, so then the fetus can leave the uterus and enter the world. From the moment true contractions begin to the baby’s delivery usually takes about 12 to 18 hours for a first-time pregnancy, and about half that time for subsequent pregnancies. Although, as any mother knows, this time can vary a lot!

Even though labor is a continuous process, it can be broken down into three stages. Additionally, this first stage is subdivided into two phases. The first phase is the early phase or latent phase, and usually lasts up to 20 hours, or until the cervix dilates to 6 centimeters. At first, there are irregular contractions that occur every 5 - 30 minutes and last about 30 seconds each, causing the cervix to dilate from 0 cm to about 3 cm and efface from about 0% - 30%.Then, regular contractions follow - they happen every 3 - 5 minutes and last about a minute or more, and this causes the cervix to dilate from 3 cm to about 6 cm and efface to about 80%. This marks the beginning of the active phase of labor, during which the cervix dilates from 6 to 10 centimeters and effaces to 100%. Contractions are very intense, lasting between 60 to 90 seconds each, with only 30 seconds to 2 minutes of rest in between - so sometimes they even overlap a bit, with one contraction beginning before the previous one is done. Also, the amniotic sac often ruptures at this point if it hasn’t already.

Now that the cervix is fully dilated, we enter the second stage of labor can be thought of as the pushing stage. During this stage, the critical thing is for the baby, and in particular the baby’s head, to navigate through the maternal pelvis, and this depends on the “3 Ps” - power, passenger, and passage. Power refers to forceful uterine contractions, passenger refers to the fetus, and passage refers to the route that the fetus has to travel through the bony pelvis. In fact, the relationship between the baby’s head and the bony pelvis is so critical, that human babies have evolved with unfused skulls, just so their head can be as large as possible and still successfully and safely make that passage through the pelvis into the world.

Now there are a few factors that determine how easy this passage is for the fetus. First is fetal size, the critical factor is the size of the fetal head.

Also though there’s fetal attitude, which refers to the way that the fetal body is flexed, not its personality. When labor starts, the fetus is normally fully flexed, which means the chin is on the chest, and they have a rounded back with flexed arms and legs. In this position, the smallest diameter (which is referred to as the suboccipitobregmatic diameter) presents at the pelvic inlet.

Larger fetuses and those that aren’t completely flexed have more difficulty making it through the passage. Next there’s fetal lie, which describes how the fetus is positioned in the uterus.

A longitudinal fetal lie is ideal, where the long axis of the fetus, which is its spine, lies along the maternal long axis—the mom’s spine. The fetus can also be transverse though, where the fetal spine is perpendicular to mom’s spine, or it can be oblique, where it’s slightly at an angle, and these two positions can impede the progression of labor.

Finally, there’s fetal presentation which refers to the first fetal part, called the presenting part, to descend into the pelvic inlet. Cephalic, or head-first, is the first type, and furthermore the most common and optimal presentation for easy delivery is a type of cephalic presentation called vertex, which includes complete flexion of the head as well. Also though, there’s breech presentation (which is head up, so the bottom, feet, or knees present first), as well as shoulder (where the shoulders present first).

To make it through the passage, the fetus makes several positional changes which are called cardinal movements or mechanisms of labor. Initially there is descent, which is the downward movement of the fetus to the pelvic inlet. The degree of descent is called the fetal station, which is described in terms of the relationship of the presenting part to mom’s ischial spines.

The fetus moves from the pelvic inlet (which is about minus 5 station) down to the ischial spines (which is station 0), and this position’s called engagement.

Then there’s flexion, where the fetal chin presses against its chest as its head meets resistance from the pelvic floor.

Next there’s internal rotation, where the fetal shoulders internally rotate by 45 degrees so the widest part of the shoulders are in line with the widest part of the pelvic inlet.

After the fetal head passes under the symphysis pubis (which is at about +4 station), there’s extension, which is where the fetal head will change from flexion to extension, and then they move to about +5 station and emerge from the vagina.

After the delivery of the head, there’s restitution, where the head externally rotates so that the shoulders can pass through the pelvic outlet and under the symphysis pubis.

And finally there’s expulsion, where the anterior shoulder slips under the symphysis pubis, followed by the posterior shoulder, and then finally followed by the rest of the body. This marks the end of the second stage of labor.

The third stage of labor occurs after the baby has been delivered, and involves delivery of the placenta. After the baby is delivered, the uterus contracts firmly and the placenta begins to separate from the uterine wall. It’s then carefully removed to ensure that there are no placental remnants left in the uterus.

Sometimes the several-hours after delivery is called the “fourth stage”, because there are major physiologic changes like adaptation to the blood loss, and the start of uterine involution, where the uterus begins returning to its pre-pregnant state.

Alright, as a quick recap, labor is composed of 3 stages. The first stage starts with true labor contractions and ends when the cervix is completely effaced and dilated. The second stage is the pushing stage, which ends with the birth of the baby. The third stage ends with delivery of the placenta.

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