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Conception and Fetal
Development
Chapter 12
Learning Objectives
 Summarize the process of fertilization.
 Describe the development, structure, and functions of the
placenta.
 Describe the composition and functions of amniotic fluid.
 Identify three organs or tissues arising from each of the
three primary germ layers.
 Summarize the significant changes in growth and
development of the embryo and fetus.
 Analyze the potential effects of teratogens during
vulnerable periods of embryonic and fetal development.
 Relate selected congenital defects to stage of fetal
development.
Conception
 Conception
– Union of a single egg and a single sperm
– Not an isolated event but part of a sequential
process
 Cell division
– Mitosis
– Meiosis
Conception (Cont.)
 Gametogenesis
– Oogenesis: the process of egg (ovum) formation
– Spermatogenesis: the process of sperm formation
 Ovum
– Two protective layers
– Considered fertile for about 24 hours after ovulation
 Sperm
– Capacitation: removal of sperm’s protective coating
– Acrosome: a cap on the sperm
Conception (Cont.)
 Fertilization
– Zygote: first cell of the new individual
– Morula: 16 cells
– Blastocyst: trophoblast, embryoblast
 Implantation: 6-10 days after conception
– Chorionic villi
– Decidua basalis, decidua capsularis, decidua vera
Conception of the Fetal
Development
Embryo and Fetus
 Pregnancy last 10 lunar
months, 9 calendar months,
40 weeks, or 280 days
 Length of pregnancy is
computed from the first day
of the last menstrual period
(LMP) until the day of birth
 Conception occurs
approximately two weeks
after the first day of the LMP
Conception and Fetal Development
Development of the Embryo
 Stage of the embryo last
from day 15 until
approximately 8 weeks after
conception
 Embryonic stage is the most
critical time in the
development of the organ
system and external features
 End of the eighth week, all
organs systems and external
structures, are present
Conception and Fetal Development
Development of the Embryo
 Membranes
 Two fetal membranes
 Chorion- develops from the
trophoblast and contains
the chorionic villi on the
surface
 Chorion becomes the
covering of the fetal side
of the placenta
 Contains major umbilical
blood vessels as they
branch out of the surface
of the placenta
Conception and Development
 Membranes
 Two fetal membranes
 Amnion- develops from
the interior cells of the
blastocyst
 Amnion becomes the
covering of the umbilical
cord and covers the
chorion on the fetal
surface of the placenta
 Amnion eventually
comes in the contact with
the chorion surrounding
the fetus
Conception and Development

Amniotic Fluid
 Helps maintain a constant body
temperature
 Serves as a source of oral fluids and
repository for waste
 Cushions the fetus from trauma by
blunting and dispersing outside forces
 Allows freedom of movement for
musculosketal development
 Fluid keeps the embryo from tangling
with the membranes, facilitating
symmetric growth
 Having less than 300 ml of amniotic fluid
(oligohydramnios) is associated with
fetal renal abnormalities
 Having more than 2 L of amniotic fluid
(hydramnios) is associated with
gastrointestinal and other malformations
Conception and Fetal Development
 Umbilical Cord
 Two arteries carry blood from
the embryo to the chorionic villi,
and one vein return the blood
to the embryo
 Term the cord is 2 cm in
diameter and ranges from 30 to
90 cm long (with an average of
55 cm)
 Connective tissue called
Wharton’s jelly prevents
compression of the blood
vessels and ensures continued
nourishment of the
embryo/fetus
 Cord is wrapped around the
fetal neck, it is termed nuchal
cord
Conception and Fetal Development
 Placenta
 Maternal-placental-embryonic
circulation is in place by day 17,
when the embryonic heart starts
beating
 By the end of the third week,
embryonic blood is circulating
between the embryo and the
chorionic villi
 Placenta functions as means of
metabolic exchange
 However, limb defects have
been associated with chorionic
villus sampling done before 10
weeks
Conception and Fetal Development
 Placenta
 Structure of the placenta is
completed by 12th week
 Placenta continues to grow
wider until 20 weeks, when it
covers approximately half of the
uterine surface
 Functions of the placenta
 An early function of the
placenta is as an endocrine
gland that produces four
hormones necessary to
maintain the pregnancy and
support the embryo and the
fetus
 Hormone is the basis for
pregnancy tests
Conception and Fetal Development
 Functions of the Placenta
 Miscarriages occur if the corpus
luteum stops functioning before
the placenta is producing
sufficient progesterone and
estrogen
 Amount of hCG reaches its
maximal level at 50 to 70 days
and then begins to decrease
 Hormone increases the
resistance to insulin,
facilitates glucose transfer
across the placental
membrane
 Stimulates breast
development to prepare for
lactation
Conception and Fetal Development
 Functions of the Placenta
 Placenta eventually produces
more of the steroid hormone
progesterone than the corpus
luteum does during the first few
months of pregnancy
 Progesterone maintains the
endometrium, decreases the
contractility of the uterus, and
stimulates maternal metabolism
and development of breast
alveoli
 Estrogen stimulates uterine
growth and uteroplacental blood
flow
 Placental estrogen production
increases greatly toward the
end of pregnancy
Conception and Fetal Development
 Functions of the Placenta
 Metabolic functions of the
placenta are respiration,
nutrition, excretion, and
storage
 Drugs also can cross the
placental membrane and
may harm the fetus
 Caffeine, alcohol, nicotine,
carbon monoxide, and other
toxic substances in cigarette
smoke, as well as
prescription and recreational
drugs (such as cocaine and
marijuana) readily cross the
placenta
Conception and Fetal Development
 Fetal Maturation
 Stage of the fetus last from 9
weeks (when the fetus
becomes recognizable as a
human being) until the
pregnancy ends
 Viability-refers to the
capability of the fetus to
survive outside the uterus
 Limitations on survival
outside of the uterus are
based on central nervous
system function and the
oxygenation capability of the
lungs
Conception and Fetal Development
 Fetal Circulatory System
 Cardiovascular system is the
first organ system to function
in the developing human
 By the end of the 3rd week,
the tubular heart begins to
beat, and the primitive
cardiovascular system links
the embryo, connecting
stalk, and yolk sac
 The 4th and 5th weeks, the
heart develops into a four
chambered organ
Conception and Fetal Development

Fetal Circulatory System
 Fetal lungs do not function for the
respiratory gas exchange, so a special
circulatory pathway, the ductus
arteriosus, bypasses the lungs
 Most of the blood passes through the
ductus venosus into the inferior vena
cava
 Most of this blood passes straight
through the right atrium and through the
foramen ovale, and opening into the left
atrium
 Three characteristics enable the fetus to
obtain sufficient oxygen from the
maternal blood:
 Fetal hemoglobin carries 20 to 30%
more oxygen than maternal
hemoglobin
 Hemoglobin concentration of the
fetus is about 50% greater than that
of the mother
 Fetal heart rate (FHT) is 110 to 160
beats/min, making the cardiac
output per unit of body weight higher
than that of an adult
Conception and Fetal Development
 Hematopoietic System
 Hematopoiesis- the formation of
blood, occurs in the yolk sac
beginning in the third week
 Gastrointestinal System
 Structures evolve during the 5th
and 6th week
 Midgut becomes the distal half
of the duodenum, jejunum and
ileum, cecum and appendix,
and proximal half of the colon
 Hindgut develops into the distal
half of the colon, the rectum
and parts of the anal canal, the
urinary bladder, and the urethra
Conception and Fetal Development
 Gastrointestinal System
 Fetus swallows amniotic
fluid beginning in the fifth
month
 Gastric emptying and
intestinal peristalsis occur
 Fetal nutrition and
elimination needs are taken
care of by the placenta
 Between 26 and 30 weeks,
the fetus begins to lay down
stores of brown fat in
preparation for extra uterine
cold stress
 Gastrointestinal system
matures by 36 weeks
Conception and Fetal Development
 Hepatic System
 Liver and biliary develop
from the foregut during
the fourth week of
gestation
 Hematopoiesis begins
during the sixth week
and requires that the liver
be large
 Glycogen is stored in the
fetal liver beginning at
the week 9 or 10
Conception and Fetal Development
 Respiratory System
 Respiratory system begins
development during embryonic life
and continues through fetal life and
into childhood until about 8 years of
age
 Development of the respiratory tract
begins in week 4 and continues
through week 17 with formation of
the larynx, trachea, bronchi, and
terminal bronchioles enlarge, and
vascular structures and primitive
alveoli are formed
 After 32 weeks, sufficient surfactant
is present in developed alveoli to
provide infants with good chance of
survival
Conception and Fetal Development
 Pulmonary Surfactants
 Detection of the presence of
pulmonary surfactants, surfaceactive phospholipids
 Lecithin (L) is the most critical
alveolar surfactant required for
postnatal lung expansion
 Thus the measure of lecithin in
relation to sphingomyelin, or the
L/S ratio is used to determine
fetal lung maturity
 The L/S ratio reaches 2:1 the
infant’s lungs are considered to
be mature
 This occurs at approximately 35
weeks of gestation
Conception and Fetal Development
 Renal System
 Kidneys from during the fifth
week and begin to function
approximately 4 weeks later
 Oligohydramnios is indicated
of renal dysfunction
 Birth, however, the kidneys
are required immediately for
excretory and acid-base
regulatory functions
 Fetal renal malformation can
be diagnosed in utero
 At the term, the fetus has
fully developed kidneys
 Most newborns void within
24 hours of birth
Conception and Fetal Development
 Neurologic System
 Open neural tube forms
during the fourth week
 Forebrain develops into the
eyes (cranial nerve II) and
cerebral hemispheres
 Development of all areas of
the cerebral cortex continues
throughout fetal life and into
childhood
 Spinal cord develops from
the long end of the neural
tube
 moves all extremities, and
changes position in utero
Conception and Fetal Development
 Sensory Awareness
 Fetus responds to sound at
24 weeks
 Fetus can be soothed by the
sounds of the mother’s voice
 Acoustic stimulation can be
used to evoke a fetal heart
rate response
 Eyes have both rods and
cones in the retina by the
seventh month
 At term the fetal brain is
approximately one fourth the
size of an adult
 Neurologic development
continues
Conception and Fetal Development
 Endocrine System
 Thyroid gland develops along
with structures in the head and
neck during the third and fourth
weeks
 Secretion of thyroxine begins
during the eighth week
 Adrenal cortex is formed during
the sixth week and produces
hormones by the eighth or ninth
week
 Pancreas forms from the
foregut during the fifth through
eighth weeks
 Islets of Langerhans develop
during the twelfth week
 Insulin is produced by week 20
Conception and Fetal Development
 Reproductive Systems
 Sex differentiation begins in the
embryo during the seventh week
 Female and male genitalia are
indistinguishable until after the ninth
week
 At birth the ovaries contain the
female’s lifetime supply of ova
 Female endometrium responds to
maternal hormones, and withdrawal
bleeding or vaginal discharge
(pseudomenstruation) may occur at
birth when these hormones are lost
 High level of maternal estrogen also
stimulates mammary engorgement
and secretion of fluids (witch’s milk)
in newborn infants of both sexes
Conception and Fetal Development
 Musculoskeletal System
 Bones and muscles develop
from the mesoderm by the
fourth week of embryonic
development
 Birth connective tissue
sutures exist where the
bones of the skull meet
 Areas where more than two
bones meet (called
fontanels) are especially
prominent
 Bones of the shoulder, arms,
hips, and legs appear in the
sixth week as a continuous
skeleton with no joints
Conception and Fetal Development
 Integumentary System
 Cells of the superficial layer are sloughed
and become mixed with the sebaceous
gland secretions to form the white,
cheesy vernix caseosa, the material that
protects the skin of the fetus
 32 weeks, as subcutaneous fat is
deposited under the dermis, the skin
becomes less wrinkled and red in
appearance
 Handprints and footprints are unique to
that infant
 Very fine hair, called lanugo appear at 12
weeks on the eyebrows and upper lip
and by 20 weeks, they cover the entire
body
 Fingernails usually reach the fingertips at
32 weeks
 Toenails reach their tips by 36 weeks
Conception and Fetal Development
 Immunologic System
 During the third trimester,
albumin and globulin are
present in the fetus
 Fetus produces IgM
immunoglobulins by the end
of the first trimester
 These are produced in
response to blood group
antigens, gram-negative
enteric organisms, and some
viruses
Conception and Fetal Development
 Multifetal Pregnancy
 Two zygotes or dizygotic twinstwo mature ova are produced in
one ovarian cycle, both have the
potential to be fertilized by
separate sperm
 Identical twins, or monozygotic
twins, develop from one
fertilized ovum, which then
divides
 They are the same sex and
have the same genotype
 Triplets- occur from the division
of one zygote into two, with one
of the two dividing again and
producing identical triplets
Nongenetic Factors Influencing
Development
Congenital: the condition was present at
birth
– Environmental factors
– Poor maternal nutrition
– Teratogen exposure
– Inadequate folic acid
Key Points
 Human gestation lasts approximately 280 days
after the last menstrual period or 266 days after
conception.
 Fertilization occurs in the uterine tube within 24
hours of ovulation. The zygote undergoes mitotic
divisions, creating a 16-cell morula.
 Implantation begins 6 days after fertilization.
 The organ systems and external features develop
during the embryonic period, that is, the third to
the eighth week after fertilization.
Key Points (Cont.)
 Refinement of structure and function occurs during the fetal
period, and the fetus becomes capable of extrauterine
survival.
 During critical periods in human development, the embryo
and fetus are vulnerable to environmental teratogens.
 There has been a steady rise in the incidence of multifetal
pregnancies, which is partly due to the use of fertility drugs
and in vitro fertilization, the increasing age at which women
give birth, and ovulation-enhancing drugs.
Question
1. What is a major cause for the increase in multiple births in the
United States?
a. Women are having their children at younger ages
b. Decreased use of contraceptives
c. Obesity among women
d. Increased use of infertility treatment