Stages of labor – physiology

Stages of labor – physiology

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of pharmacology and clinical reasoning topics. Try it free today. 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’re 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 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’s 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 adaption
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
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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