Stages of labor

Stages of labor


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Stages of labor

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Increased estrogen levels during the last few weeks of pregnancy leads to (more/fewer) oxytocin receptors on the myometrial cells of the uterus.


USMLE® Step 2 style questions USMLE

1 questions

A pregnant 26-year-old woman presents at 40 weeks' gestation in active labor. She requests labor pain relief and is given a combined spinal-epidural (CSE). Between which two anatomic layers is she administered a continuous infusion of local anesthetic?

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Content Reviewers:

Rishi Desai, MD, MPH


Tanner Marshall, MS

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.

  1. "Medical Physiology" Elsevier (2016)
  2. "Physiology" Elsevier (2017)
  3. "Human Anatomy & Physiology" Pearson (2018)
  4. "Principles of Anatomy and Physiology" Wiley (2014)
  5. "Reassessing the labor curve in nulliparous women" American Journal of Obstetrics and Gynecology (2002)
  6. "Second-stage labor duration in nulliparous women: relationship to maternal and perinatal outcomes" American Journal of Obstetrics and Gynecology (2009)
  7. "A comparison of active management and expectant management of the third stage of labour: a Swedish randomised controlled trial" BJOG (2011)