Respiratory tidal volume increases, but residual volume
decreases. Te respiratory rate remains relatively unchanged.
Dyspnea is common.
±otal body water and blood volume increase dramatically and
cardiac output rises. Blood pressure declines in midpregnancy
and then rises to normal levels during the third trimester.
Parturition encompasses a series of events called labor.
Initiation of Labor
When estrogen levels are suﬃciently high, they induce oxytocin
receptors on the myometrial cells and inhibit progesterone’s quieting
eﬀect on uterine muscle. Weak, irregular contractions begin.
Fetal cells produce oxytocin, which stimulates prostaglandin
production by the placenta. Both hormones stimulate contraction
of uterine muscle. Increasing distension of the cervix activates
the hypothalamus, causing oxytocin release from the mother’s
posterior pituitary; this sets up a positive feedback loop resulting
in true labor.
Stages of Labor
Te dilation stage is from the onset of rhythmic, strong
contractions until the cervix is fully dilated. Te head of the fetus
rotates as it descends through the pelvic outlet.
Te expulsion stage extends from full cervical dilation until birth
of the infant.
Te placental stage is the delivery of the a²erbirth (the placenta
and attached fetal membranes).
Adjustments of the Infant to Extrauterine Life
Te infant’s Apgar score is recorded immediately a²er birth.
Taking the First Breath and Transition
A²er the umbilical cord is clamped, carbon dioxide accumulates
in the infant’s blood, causing respiratory centers in the brain to
trigger the ﬁrst inspiration.
Once the lungs are inﬂated, breathing is eased by the presence
of surfactant, which decreases the surface tension of the alveolar
During transition, the ﬁrst 8 hours a²er birth, the infant is
physiologically unstable and adjusting. A²er stabilizing, the
infant wakes approximately every 3–4 hours in response to
Occlusion of Special Fetal Blood Vessels and Vascular Shunts
Inﬂation of the lungs causes pressure changes in the circulation;
as a result, the umbilical arteries and vein, ductus venosus, and
ductus arteriosus collapse, and the foramen ovale closes. Te
occluded blood vessels are converted to ﬁbrous cords; the site of
the foramen ovale becomes the fossa ovalis.
Te breasts are prepared for lactation during pregnancy by high
blood levels of estrogen, progesterone, and human placental
Colostrum, a premilk ﬂuid, is a fat-poor ﬂuid that contains more
protein, vitamin A, and minerals than true milk. It is produced
toward the end of pregnancy and for the ﬁrst two to three days
(gastrula) containing ectoderm, mesoderm, and endoderm.
Cells that move through the midline primitive streak become
endoderm if they form the most inferior layer of the embryonic
disc and mesoderm if they ultimately occupy the middle layer.
Cells remaining on the superior surface become ectoderm.
Organogenesis: Differentiation of the Germ Layers
Endoderm forms the mucosa of the digestive and respiratory
systems, and the epithelial cells of all associated glands (thyroid,
parathyroids, thymus, liver, pancreas). It becomes a continuous
tube when the embryonic body undercuts and fuses ventrally.
Ectoderm forms the nervous system and the epidermis of the
skin and its derivatives. Te ﬁrst event of organogenesis is
neurulation, which produces the brain and spinal cord. By the
eighth week, all major brain regions are formed.
Mesoderm forms all other organ systems and tissues. It segregates
early into (1) a dorsal superior notochord, (2) paired somites
that form the vertebrae, skeletal trunk muscles, and part of the
dermis, and (3) paired masses of intermediate and lateral plate
mesoderm. Te intermediate mesoderm forms the kidneys
and gonads. Te lateral plate mesoderm forms the dermis
of skin; parietal serosa; bones and muscles of the limbs; the
cardiovascular system; and the visceral serosae.
Te fetal cardiovascular system is formed in the embryonic
period. Te umbilical vein delivers nutrient- and oxygen-rich
blood to the embryo; the paired umbilical arteries return oxygen-
poor, waste-laden blood to the placenta. Te ductus venosus
allows most of the blood to bypass the liver; the foramen ovale
and ductus arteriosus are pulmonary shunts.
Events of Fetal Development
All organ systems are laid down during the embryonic period;
growth and tissue/organ specialization are the major events of the
During the fetal period, fetal length increases from about 22 mm
to 360 mm, and weight increases from less than an ounce to 7
pounds or more.
Effects of Pregnancy on the Mother
Maternal reproductive organs and breasts become increasingly
vascularized during pregnancy, and the breasts enlarge.
Te uterus eventually occupies nearly the entire abdominopelvic
cavity. Abdominal organs are pushed superiorly and encroach on
the thoracic cavity, causing the ribs to ﬂare.
Te increased abdominal mass changes the woman’s center of
gravity; lordosis and backache are common. A waddling gait
occurs as pelvic ligaments and joints are loosened by placental
A typical weight gain during pregnancy in a woman of normal
weight is 28 pounds.
Human placental lactogen has anabolic eﬀects and promotes
glucose sparing in the mother.
Many women suﬀer morning sickness, heartburn, and
constipation during pregnancy.
Te kidneys produce more urine, and pressure on the bladder
may cause frequency, urgency, and stress incontinence.