Chapter 28
Pregnancy and Human Development
Keeping the lungs inflated is much more difficult for pre-
mature infants (those weighing less than 2500 g, or about 5.5
lb, at birth) because surfactant production occurs during the
last months of prenatal life. Consequently, preemies are usually
put on respiratory assistance (a ventilator) until their lungs are
mature enough to function on their own.
For 6–8 hours a±er birth, infants pass through an unstable
transitional period
marked by alternating periods of increased
activity and sleep. During the activity periods, vital signs are
irregular and the baby gags frequently as it regurgitates mucus
and debris. A±er this, the infant stabilizes, with waking periods
(dictated by hunger) occurring every 3–4 hours.
Occlusion of Special Fetal Blood Vessels
and Vascular Shunts
A±er birth the special umbilical blood vessels and fetal shunts are
no longer necessary (see Figure 28.14b). Te umbilical arteries
and vein constrict and become fibrosed. Te proximal parts of the
umbilical arteries persist as the
superior vesical arteries
that supply
the urinary bladder, and their distal parts become the
medial um-
bilical ligaments
. Te remnant of the umbilical vein becomes the
round ligament of the liver
, or
ligamentum teres
, that attaches
the umbilicus to the liver. Te ductus venosus collapses as blood
stops flowing through the umbilical vein and is eventually con-
verted to the
ligamentum venosum
on the liver’s undersurface.
As the pulmonary circulation becomes functional, pressure
in the le± side of the heart increases and that in the right side of
the heart decreases, causing the pulmonary shunts to close. Te
flap of the foramen ovale is pushed to the shut position, and its
edges fuse to the septal wall. Ultimately, only a slight depres-
sion, the
fossa ovalis
, marks its position. Te ductus arteriosus
constricts and is converted to the cordlike
ligamentum arterio-
, connecting the aorta and pulmonary trunk.
Except for the foramen ovale, all of the special circulatory ad-
aptations of the fetus are functionally occluded within 30 min-
utes a±er birth. Closure of the foramen ovale is usually complete
within the year. As we described in Chapter 18, failure of the
ductus arteriosus or foramen ovale to close leads to congenital
heart defects.
Check Your Understanding
What two modifications of the fetal circulation allow most
blood to bypass parts of the heart?
What happens to the special fetal circulatory modifications
after birth?
For answers, see Appendix H.
Explain how the breasts are prepared for lactation.
is production of milk by the hormone-prepared
mammary glands. Rising levels of (placental) estrogens, pro-
gesterone, and human placental lactogen toward the end of
Placental stage.
placental stage
, or the delivery of
the placenta and its attached fetal membranes, which are
collectively called the
, is usually accomplished
within 30 minutes a±er birth of the infant. Te strong uter-
ine contractions that continue a±er birth compress uterine
blood vessels, limit bleeding, and shear the placenta off the
uterine wall (cause placental detachment). It is very impor-
tant that all placental fragments be removed to prevent con-
tinued uterine bleeding a±er birth (
postpartum bleeding
Check Your Understanding
What is a breech presentation?
What chemical is most responsible for triggering true labor?
Why does a baby turn as it travels through the birth canal?
For answers, see Appendix H.
Adjustments of the Infant
to Extrauterine Life
Outline the events leading to the first breath of a newborn.
Describe changes that occur in the fetal circulation after
neonatal period
is the four-week period immediately a±er
birth. Here we will be concerned with the events of just the first
few hours a±er birth in a normal infant. As you might suspect,
birth represents quite a shock to the infant. Exposed to physical
trauma during the birth process, it is suddenly cast out of its
watery, warm environment and its placental life supports are
severed. Now it must do for itself all that the mother had been
doing for it—respire, obtain nutrients, excrete, and maintain its
body temperature.
At 1 and 5 minutes a±er birth, the infant’s physical status is
assessed based on five signs: heart rate, respiration, color, mus-
cle tone, and reflexes (tested by response to catheter in nostril).
Each observation is given a score of 0 to 2, and the total is called
Apgar score
. An Apgar score of 8 to 10 indicates a healthy
baby. Lower scores reveal problems in one or more of the physi-
ological functions assessed.
Taking the First Breath and Transition
Te crucial first requirement is to breathe. Vasoconstriction of
the umbilical arteries, initiated when they are stretched during
birth, leads to loss of placental support. Once carbon dioxide is
no longer removed by the placenta, it accumulates in the baby’s
blood, causing central acidosis. Tis excites respiratory control
centers in the baby’s brain and triggers the first inspiration. Te
first breath requires a tremendous effort—the airways are tiny,
and the lungs are collapsed. However, once the lungs have been
inflated in full-term babies, surfactant in alveolar fluid reduces
surface tension in the alveoli, and breathing is easier. Te rate of
respiration is rapid (about 45 respirations/min) during the first
two weeks and then gradually declines.
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