The Basics of Labor and Delivery

Labor and delivery typically occur between gestational week 37 and gestational week 42. When labor does not progress quickly enough, doctors can use prostaglandins, hormones that affect specific body tissues, to help things along. Prostaglandins are found in various parts of the body and play a key role in various processes, such as menstruation and labor, so in using prostaglandins, doctors are enhancing your body’s normal processes. In the case of labor, prostaglandins stimulate the contraction of the muscles in the uterus and help the cervix to become ready for delivery. One specific prostaglandin that is important to know about is prostaglandin E2, which can be used in the form of a pessary (a device inserted into the vagina) called dinoprostone to induce labor.

Doctors describe labor in terms of three stages:

Stage One

The first stage of labor begins with the onset of true contractions and ends when the cervix is fully dilated at 10 cm. Prior to actual labor, many pregnant women can experience what doctors call Braxton-Hicks contractions are irregular contractions of the uterus that can occur occasionally. Typically, you feel them during the second and third trimesters of pregnancy and often they are associated with temporary, irregular tightness or mild cramping in the abdomen. Braxton-Hicks contractions are not true labor contractions, however, so they do not indicate the onset of labor. Unlike the contractions of true labor, Braxton-Hicks contractions do not become more frequent, or more intense, over time and often they can be relieved by way of hydration and relaxation.

The first stage of labour is from the onset of labour (true contractions) until the cervix is fully dilated to 10cm. It involves cervical dilation (opening up) and effacement (getting thinner from front to back). The “show” refers to the mucus plug in the cervix, that prevents bacteria from entering the uterus during pregnancy, falling out and creating space for the baby to pass through.

The first stage of labor has three phases within it: the latent phase, the active phase, and the transition phase. The latent phase is the initial part of the first stage and is characterized by irregular contractions and slow cervical dilation (from 0 to 3 cm). The active phase is the later part of the first stage and is characterized by regular contractions and more rapid cervical dilation (from 3 cm to 7 cm). The transition phase is the final part of the first stage and is characterized by strong and regular contractions and the last bit of cervical dilation (from 7 cm to 10 cm).

Stage Two

The second stage begins when the cervix is fully dilated at 10 cm and ends with the delivery of the baby; and the third stage begins with the delivery of the baby and ends with the delivery of the placenta. The success of this stage depends on the “three Ps”: Power (the strength of the uterine contractions), Passenger (the characteristics of the fetus), and Passage (the size and shape of the passageway, mainly the pelvis). Obstetricians describe the ‘passenger’ (the fetus) in terms of four qualities:

Size (particularly the size of the head)

Attitude (the posture of the fetus, including how the back is rounded and how the head and limbs are flexed)

Lie: the position of the fetus in relation to the mother’s body. The lie can be vertex (straight up and down), transverse (straight side to side), or oblique lie (at an angle).

Presentation: Which part of the fetus is closest to the cervix. Presentation can be cephalic (head down), shoulder (shoulder first), or breech presentation (legs first). Breech presentations  are further categorized as complete breech (hips and knees flexed, like being in a tucked position in gymnastics), frank breech (hips flexed and knees extended, bottom first, like being piked, although not with good form, since legs are usually partly straddled and knees are a little bit bent), and footling breech (a foot is hanging through the cervix).

During labor, the baby goes through seven maneuvers during the trip through the birth canal. These are known as the cardinal movements of labor and consist of the following:

Engagement: the baby’s head engages in the pelvis.

Descent: the baby’s head moves down through the pelvis. During the descent phase of labor, obstetricians describe the position of the baby’s head in relation to the mother’s ischial spines, which are the bones in the pelvis that can be felt through the skin. Descent is measured in centimeters, with a negative number indicating that the baby’s head is high up in the pelvis (for example, -5 when the baby is around the pelvic inlet), 0 indicating that the head is at the ischial spines (which is when the head is “engaged”), and a positive number indicating that the fetal head has descended further out.

Flexion: the baby’s head flexes down to fit through the pelvis

Internal rotation: the baby rotates its head to fit through the pelvis

Extension: the baby’s head extends and comes out of the vagina

Restitution and external rotation: the baby’s head and body align and the baby rotates to face downward)

Expulsion: the baby is born.

Stage Three

The third stage of labor begins with the completion of the birth of the baby and ends with the delivery of the placenta. There are two approaches to managing the third stage of labor: physiological management and active management. In physiological management, the placenta is delivered by the mother’s effort without medications or cord traction. This approach allows the mother to deliver the placenta on her own, but it may take longer than the active management approach. Active management involves the obstetrician or midwife assisting in the delivery of the placenta. A dose of oxytocin is administered by intramuscular injection to help the uterus contract and expel the placenta. The clinician also may apply careful traction on the umbilical cord to pull the placenta out of the uterus and vagina. Active management can shorten the third stage of labor and reduce the risk of bleeding, but it may be associated with nausea and vomiting. Active management should be considered if there is a hemorrhage (excessive bleeding), or delivery of the placenta has been delayed by 60 minutes.

David Warmflash
Dr. David Warmflash is a science communicator and physician with a research background in astrobiology and space medicine. He has completed research fellowships at NASA Johnson Space Center, the University of Pennsylvania, and Brandeis University. Since 2002, he has been collaborating with The Planetary Society on experiments helping us to understand the effects of deep space radiation on life forms, and since 2011 has worked nearly full time in medical writing and science journalism. His focus area includes the emergence of new biotechnologies and their impact on biomedicine, public health, and society.

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