1086
UNIT 5
Continuity
28
1
Dilation stage.
Te
dilation stage
is the time from labor’s
onset until the cervix is fully dilated by the baby’s head
(about 10 cm in diameter). As labor starts, weak but regu-
lar contractions begin in the upper part of the uterus and
move toward the vagina. At first, only the superior uterine
muscle is active; contractions are 15–30 minutes apart, and
last for 10–30 seconds. As labor progresses, the contrac-
tions become more vigorous and rapid, and the lower part
of the uterus gets involved. As the infant’s head is forced
against the cervix with each contraction, the cervix sof-
tens and thins (
effaces
), and dilates. Eventually the amnion
ruptures, releasing the amniotic fluid, an event commonly
called “breaking the water.”
Te dilation stage is the longest part of labor, lasting
6–12 hours or more. Several events happen during this
phase.
Engagement
occurs when the infant’s head enters the
true pelvis. As descent continues through the birth canal,
the baby’s head rotates so that its greatest dimension is in
the anteroposterior line, which allows it to navigate the nar-
row dimensions of the pelvic outlet (Figure 28.18
1b
).
2
Expulsion stage.
Te
expulsion stage
lasts from full dila-
tion to delivery of the infant, or actual childbirth. By the
time the cervix is fully dilated, strong contractions occur
every 2–3 minutes and last about 1 minute. In this stage, a
mother undergoing labor without local anesthesia has an
increasing urge to push or bear down with the abdominal
muscles. Although this phase may last 2 hours, it is typically
50 minutes in a first birth and around 20 minutes in subse-
quent births.
Crowning
occurs when the largest dimension of the ba-
by’s head distends the vulva. At this point, an
episiotomy
(e-piz
0
e-ot
9
o-me) may be done to reduce tissue tearing. An
episiotomy is an incision made to widen the vaginal orifice.
Te baby’s neck extends as the head exits from the peri-
neum, and once the head has been delivered, the rest of the
baby’s body is delivered much more easily. A±er birth, the
umbilical cord is clamped and cut.
When the infant is in the usual
vertex
, or head-first,
pre-
sentation
, the skull (its largest diameter) acts as a wedge to
dilate the cervix. Te head-first presentation also allows the
baby to be suctioned free of mucus and to breathe even be-
fore it has completely exited from the birth canal. In
breech
(buttock-first) and other nonvertex presentations, these ad-
vantages are lost and delivery is much more difficult, o±en
requiring the use of forceps, or a C-section (see below).
Homeostatic Imbalance
28.3
If a woman has a deformed or malelike pelvis, labor may be
prolonged and difficult. Tis condition is called
dystocia
(dis-
to
9
se-ah;
dys
5
difficult;
toc
5
birth). Besides extreme maternal
fatigue, another possible consequence of dystocia is fetal brain
damage, resulting in cerebral palsy or epilepsy. ²o prevent these
outcomes, a
cesarean
(
C-
)
section
(se-sa
9
re-an) is performed in
many such cases. A C-section is delivery of the infant through
an incision made through the abdominal and uterine walls.
Umbilical cord
Uterus
Cervix
Vagina
Placenta
Pubic
symphysis
Sacrum
Perineum
Uterus
Placenta
(detaching)
Umbilical
cord
1a
Early dilation.
Baby’s head
engaged; widest
dimension is
along left-right
axis.
1b
Late dilation.
Baby’s head
rotates so widest
dimension is in
anteroposterior
axis (of pelvic
outlet). Dilation
nearly complete.
3
Placental stage.
After baby is
delivered, the
placenta detaches
and is removed.
2
Expulsion.
Baby’s head extends
as it is delivered.
Figure 28.18
Parturition.
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