Chapter 4: Prenatal Development
Prenatal development: All development that occurs between the moment of
conception to the beginning of the birthing process, along with the things
that can go wrong.
Zygote (also known as the germinal stage): Lasts from conception
until implantation in the uterus wall. Usually lasts from ten to
Embryo: Lasts from the beginning to the third week to the end of
the eighth week. During this time, virtually all major organs are
formed and the heart begins to beat.
Fetus: Lasts from the ninth week of pregnancy until the baby is
born. All the major organ systems begin to function and the
organism develops rapidly. Period of growth.
Period of the Zygote:
Blastocyst: The ball of cells that is formed when the fertilized egg
first begins to divide. Will contain sixty to eighty cells within four
days of conception. Inner layers of the blastocyst become the
embryo, while the outer layers will develop into tissues that protect
and nourish the embryo.
Implantation: Six to ten days after conception, the zygote reaches
the uterus and burrows small tendrils into the wall, tapping into the
mothers blood supply. Takes about 48 hours to complete, occurring
7-10 days after ovulation. Nearly three out of four zygotes fail to
survive the initial implantation.
Four major support structures to nourish the developing organism:
o Amnion: Watertight sac that fills with fluid from mothers
tissues. Cushion from blows, regulates temperature, provides
a weightless environment making it easier for the embryo to
o Chorion: a membrane that becomes attached to the uterine
tissues to gather nutrition for the embryo.
o Placenta: An organ, formed from the lining of the uterus and
the chorion, that provides respiration and nourishment of the
unborn child and the elements of its metabolic wastes. The
villi is a barrier that separates the babies bloodstream and
the mothers. It also allows small enough things to pass
through, like oxygen and various nutrients, but blood cells
are too large to get through.
o Umbilical Cord: Soft tube containing blood vessels that
connects the embryo to the placenta. How babys main
source of oxygen and food get to the embryo. Period of the Embryo:
Three layers of the embryonic disk: Outer layer (ectoderm)
becomes the nervous system, skin and hair. Middle layer
(mesoderm) becomes the muscles, bones and circulatory system.
Inner layer (endoderm) will become the internal organs.
Period of rapid development. 0.65 cm, Neural tube forms (early
spinal cord), heart begins to beat, nose, ears, mouth, eyes begin to
form, stubs where legs and arms will be appear. This is the fastest
period of development.
The second month: becomes more human in appearance. ~2.5 cm,
7 g. Primitive tail appears and then is sealed over. Eyes have
cornea and lenses, rudimentary skeleton is formed. Limbs are
developing from body outward (arms first, then legs). Rapid brain
development. Sexual development happens at eight weeks, genital
ridge called indifference gonad, which then changes depending on
the sex chromosomes. Circulatory system works on its own, liver
and spleen produce blood cells.
Period of the Fetus (last seven months)
~9.5 cm, 28 g. Period of rapid growth and refinement of all organ
systems. Fetus begins to move sense and behave, unique
characteristics develop (facial expressions)
Third month: Organs continue rapid growth, become
interconnected. Digestive and excretory systems being to work
together. Sexual differentiation processes rapidly (testes produce
testosterone, which then develop the penis and scrotum, in
absence of this, the female genitals form) By the end of the third
month, the sex of the baby can be determined
Second trimester (Fourth through Sixth months, ~35-38 cm, 0.9
o Week 16-25: Simple movements of the tongue, lips, pharynx
and larynx increase, fetus begins to suck, swallow, munch,
hiccup, cough, etc. Bone and cartilage increase. Distinctly
human appearance, although no possible chance of surviving
outside of the womb.
o 5-6 months: nails harden, skin thickens, eyebrows, eyelashes
and scalp hair appear.
o 20 weeks: Functioning sweat glands. Fetus now covered with
vernix (white cheesy substance that protects the fetus from
chapping from the amniotic fluid) and lanugo (fine hair
covering the fetus body that helps vernix stick to the skin.
o End of the sixth month: Visual and auditory systems are
clearly functional. MEG (magnetoencephalography) has
tested this through brain activity.
Third trimester (Seventh through 9 month, ~48-53 cm, 3.2-3.6kg):o All organs mature rapidly.
o Age of viability: a point between the 22and 28 prenatal
week when survival outside of the womb is possible.
o More predictable cycles of heart rate activity, motor activity
and sleep cycles. A layer of fat forms under the skin to
protect from temperature changes.
o Middle of ninth month: Fetal activity decreases and sleep
becomes more frequent.
External Influences on Prenatal Development
Teratogens: External agents such as viruses, drugs, chemicals and
radiation that can harm a developing embryo or fetus. Can cause
physical deformities, severely retarded growth, blindness, brain
damage and even death.
o The effects of teratogens on a body part or organ system are
worst during the period when that structure is forming and
growing most rapidly.
o Not all embryos or fetuses are equally affected by a
teratogen. Susceptibility to harm is influenced by the genetic
makeup and prenatal quality of the embryo, fetus, or
o The same effects can be caused by different teratogens.
o A variety of defects can be caused by one teratogen.
o Longer expos