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organs. The fetal stage, from 8 weeks until birth, is characterized by
rapid growth and changes in body form caused by different rates
of growth of different parts of the body (34).
The fetus is a very small but rapidly developing human being
who is influenced by the maternal and external environment. The
mother responds to the fetus, especially when fetal movement begins. Table 8-1 summarizes some major physiologic milestones in
the sequential development of the conceptus, the new life that has
been conceived (34). Table 8-2 summarizes major physiologic
changes in the pregnant woman (34).
TABLE 8-1
Summary of the Sequence of Prenatal Development
Time Period
After Fertilization
Developmental Event
Germinal Stage
30 hours
40 hours
60 hours
3 days
4 days
4 1/2–6 days
6–7 days
7–14 days
2–8 weeks
First division or cleavage occurs.
Four-cell stage occurs.
Morula, a solid mass of 12 to 16 cells; total size of mass not changed because cells decrease in size
with each cleavage to allow morula to pass through lumen of fallopian tube. Ectopic pregnancy
within fallopian tube occurs if morula is wedged in lumen.
Zygote has divided into 32 cells; travels through fallopian tube to uterus.
Zygote contains 70 cells. Morula reaches uterus; forms a blastocyst, a fluid-filled sphere.
Blastocyst floats in utero. Embryonic disc, thickened cell mass from which baby develops, clusters on
one edge of blastocyst. Mass of cells differentiates into two layers: (1) ectoderm, outer layer of cells
that become the epidermis, nails, hair, tooth enamel, sensory organs, brain and spinal cord, cranial
nerves, peripheral nervous system, upper pharynx, nasal passages, urethra, and mammary glands;
(2) endoderm, lower layer of cells that develops into gastrointestinal system, liver, pancreas, salivary
glands, respiratory system, urinary bladder, pharynx, thyroid, tonsils, lining of urethra, and ear.
Nidation, implantation of zygote into upper portion of uterine wall, occurs.
Remainder of blastocyst develops into the following: (1) Placenta, a multipurpose organ connected to
the embryo by the umbilical cord that delivers oxygen and nourishment from the mother’s body,
absorbs the embryo’s body wastes, combats internal infection, confers immunity to the unborn
child, and produces the hormones that (a) support pregnancy, (b) prepare breasts for lactating,
and (c) stimulate uterine contractions for delivery of the baby. Placenta circulation evidenced by 11
to 12 days. (2) Umbilical cord, a structure that contains two umbilical arteries and an umbilical vein
and connects embryo to placenta. Approximately 20 inches long and 1⁄2 inch in diameter. Rapid cell
differentiation occurs. (3) Amniotic sac, a fluid-filled membrane that encases the developing baby,
protecting it and giving it room to move.
Period during which embryo firmly establishes uterus as home and undergoes rapid cellular differentiation, growth, and development of body systems. This is a critical period when embryo is most vulnerable to deleterious prenatal influences. All development birth defects occur during first trimester (3
months) of pregnancy. If embryo is unable to survive, a miscarriage or spontaneous abortion,
expulsion of conceptus from the uterus, occurs.
Embryonic Stage
15 days
16 days
19–20 days
21 days
22 days
26–27 days
28 days
Cranial end of elongated disk has begun to thicken.
Mesoderm, the middle layer, appears and develops into dermis, tooth dentin, connective tissue,
cartilage, bones, muscles, spleen, blood, gonads, uterus, and excretory and circulatory systems. Yolk
sac, which arises from ectoderm, assists transfer of nutrients from mother to embryo.
Neural fold and neural groove develop. Thyroid begins to develop.
Neural tube forms, becomes spinal cord and brain.
Heart, the first organ to function, initiates action. Eyes, ears, nose, cheeks, and upper jaw begin to
form. Cleft palate may occur if development is defective.
Cephalic portion (brain) of nervous system formed. Leg and arm buds appear. Stubby tail of spinal
cord appears.
Crown to rump length, 4–5 mm.
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TABLE 8-1
Summary of the Sequence of Prenatal Development —continued
Time Period
After Fertilization
30 days
31 days
32 days
34 days
35–38 days
40 days
42 days
50 days
55–56 days
8 weeks
Developmental Event
Rudimentary body parts formed. Limb buds appear. Cardiovascular system functioning. Heart beats 65
times per minute; blood flows through tiny arteries and veins. Lens vesicles, optic cups, and nasal
pits forming. By end of first month, new life has grown more quickly than it will at any other time in
life. Swelling in head where eyes, ears, mouth, and nose will be. Crown to rump length, 7–14 mm
(1⁄4 to 1⁄2 inch).
Eye and nasal pit developing. Primitive mouth present.
Paddle-shaped hands. Lens vesicles and optic cups formed.
Head is much larger relative to trunk. Digital rays present in hands. Feet are paddle-shaped. Crown to
rump length, 11–14 mm.
Olfactory pit, eye, maxillary process, oral cavity, and mandibular process developing. Brain has divided
into three parts. Limbs growing. Beginning of all major external and internal structures. Crown to
rump length, 15–16 mm.
Elbows and knees apparent. Fingers and toes distinct but webbed. Yolk sac continues to (1) provide
embryologic blood cells during third through sixth weeks until liver, spleen, and bone marrow
assume function; (2) provide lining cells for respiratory and digestive tracts; (3) provide cells that
migrate to gonads to become primordial germ cells.
Crown to rump length, 21–23 mm.
All internal and external structures present. External genitalia present but sex not discernible; yolk sac
disappears, incorporated into embryo; limbs, hands, feet formed. Nerve cells in brain connected.
Eye, nostril, globular process, maxilla, and mandible almost completely formed. Ear beginning to
develop.
Stubby end of spinal cord disappears. Distinct human characteristics. Head accounts for half of total
embryo length. Brain impulses coordinate function of organ systems. Facial parts formed, with
tongue and teeth buds. Stomach produces digestive juices. Liver produces blood cells. Kidney
removes uric acid from blood. Some movement by limbs. Weight, 1 g. Length, 1–1 1⁄2 inches
(2.5–3.75 cm).
Fetal Stage
9–40 weeks
9–12 weeks
13–16 weeks
17–20 weeks
21–24 weeks
Remainder of intrauterine period spent in growth and refinement of body tissues and organs.
Eyelids fused. Nail beds formed. Teeth and bones begin to appear. Ribs and vertebrae are cartilage.
Kidneys function. Urinates occasionally. Some respiratory-like movements exhibited. Begins to
swallow amniotic fluid. Grasp, sucking, and withdrawal reflexes present. Sucks fingers and toes in
utero. Makes specialized responses to touch. Moves easily but movement not felt by mother. Reproduction organs have primitive egg or sperm cells. Sex distinguishable. Head one third of body
length. Weight, 30 g (1 oz). Length, 3–3 1⁄2 inches at 12 weeks.
Ova formed in female. Much spontaneous movement. Sex determination possible. Quickening, fetal
kicking or movement, may be felt by mother. Moro reflex present. Rapid skeletal development.
Meconium present. Uterine development in female fetus. Lanugo, downy hair, appears on body.
Head one fourth of total length. Weight, 120–150 g (4–6 oz). Length, 8–10 inches. Fetus frowns,
moves lips, turns head; hands grasp, feet kick. First hair appears. Umbilical cord as long as fetus.
Placenta fully developed 16 weeks.
New cells exchanged for old, especially in skin. Quickening occurs by 17 weeks. Vernix caseosa
appears. Eyebrows, eyelashes, and head hair appear. Sweat and sebaceous glands begin to function.
Skeleton begins to harden. Grasp reflex present and strong. Permanent teeth buds appear. Fetal
heart sounds can be heard with stethoscope. Weight, 360–450 g (12 oz–1 lb). Length, 12 inches.
Extrauterine life, life outside uterus, is possible but difficult because of immature respiratory system.
Fetus looks like miniature baby. Mother may note jarring but rhythmic movements of infant, indicative of hiccups. Body becomes straight at times. Fingernails present. Skin has wrinkled, red appearance. Alternate periods of sleep and activity. May respond to external sounds. Weight, 720 g (1 1⁄2 lb).
Length, 14 inches.
continued
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TABLE 8-1
Summary of the Sequence of Prenatal Development—continued
Time Period
After Fertilization
25–28 weeks
29–32 weeks
33–36 weeks
37–40 weeks
Gestational age—
280 days
in utero—266 days
Developmental Event
Jumps in utero in synchrony with loud noise. Eyes open and close with waking and sleeping cycles.
Able to hear. Respiratory-like movements. Respiratory and central nervous systems sufficiently
developed; some babies survive with excellent and intensive care. Assumes head-down position in
uterus. Weight, 1,200 g (2 1/2 lb).
Begins to store fat and minerals. Testes descend into scrotal sac in male. Reflexes fully developed.
Thumb-sucking present. Mother may note irregular, jerky, crying-like movements. Lanugo begins to
disappear from face. Head hair continues to grow. Skin begins to lose reddish color. Can be conditioned to environmental sounds. Weight, 1,362–2,270 g (3–5 lb). Length, 16 inches.
Adipose tissue continues to be deposited over entire body. Body begins to round out. May become
more or less active because of space constriction. Increased iron storage by liver. Increased lung
development. Lanugo begins to disappear from body. Head lengthens. Brain cells number same as
at birth. Weight, 2,800 g (6 lb). Length, 18–20 inches.
Organ systems operating more efficiently. Heart rate increases. More wastes expelled. Lanugo and
vernix caseosa disappear. Skin smooth and plump. High absorption of maternal hormones. Cerebral
cortex well defined; brain wave patterns developed. Skull and other bones becoming more firm and
mineralized. Continued storage of fat and minerals. Glands produce hormones that trigger labor.
Ready for birth. Weight, 3,200–3,400 g (7–7 1⁄2 lb). Length, 20–21 inches. Baby stops growing
approximately 1 week before birth.
Prenatal Influences
on Development
Maternal and prenatal health is a major goal of the Healthy People
2010 Initiative in the United States. Refer to the Box Healthy People 2010: Examples of Objectives for Maternal and Prenatal
Health Promotion (139).
This section summarizes major health risks to the embryo and
fetus. More information is available in maternity nursing
texts. Preconception risk factors include the following diseases (23):
(a) diabetes onset before conception, (b) folic acid deficiency,
(c) hepatitis B, (d) HIV/AIDS, (e) hypothyroidism, (f ) maternal
phenylketonuria (PKU), (g) obesity, and (h) sexually transmitted
diseases such as Chlamydia trachomatis and gonorrhea. Other preconception risk factors include (23) (a) use of isotretinoins such as Accutane, (b) alcohol intake, (c) use of antineoplastic drugs, (d) folic
acid deficiency, (e) oral anticoagulant use, and (f ) smoking.
HEREDITY
Genetic information is transmitted from parents to the offspring
through a complex series of processes. The gene is the basic unit of
heredity. Relatively few people are born without abnormal chromosomes. Everyone carries abnormal genes that could produce serious diseases or handicaps in the next generation. These genes are
usually recessive or additive. Most of the 7,000 known genetic
disorders are dominant, since the effects of dominant genes are apparent in the person’s phenotype (34). Refer to Chapter 1 for additional explanation.
Congenital malformations are physical or mental disabilities
that occur before birth for a variety of reasons. However, sometimes diseases or defects are genetic, the result of dominant or recessive transmission of abnormalities in the genes or chromosomes. An
example is Fragile X syndrome, which involves an abnormal fragile section of DNA at a specific location on the X chromosome. This is
a sex-linked inherited disorder. The female is usually a carrier, and
the male is more likely to demonstrate the effects. It is estimated
that 5% to 7% of all mental retardation is caused by this syndrome. Recessive genes may cause inherited diseases, such as cystic fibrosis, sickle-cell syndrome, muscular dystrophy, PKU
disease, and Tay-Sachs disease (57).
An example of a congenital but not inherited disorder is Down
syndrome (trisomy 21), in which the child has three of the chromosome 21 instead of the normal two. Various factors contribute to the
chromosomal break. The level of mental retardation varies. An inherited predisposition to a disorder may also interact with an environmental factor before or after birth and lead to expression of a
disorder. Some abnormalities or diseases that are inherited appear
months or years later (34, 57).
Hereditary factors do not by themselves fully determine what
the person will become. Reaction range, a range of potential expression of a trait (for example, weight), depends on environmental
conditions. Then, there are some traits, such as eye color, that are
so strongly programmed by genes that they are canalized, there is
little variance in their expression (57).
For example, diabetes mellitus has a familial (hereditary) or genetic etiology although other factors contribute to the cause. Incidence in African Americans, Hispanic Americans, and many