Chapter 4 - Prenatal Development
Prenatal development: Development that occurs between the moment of conception and the
beginning of the birth process.
From Conception to Birth
Prenatal development divided in to 3 major phases
1. Period of the zygote (germinal period) from conception through implantation; when
the zygote becomes firmly attached to the wall of the uterus. (10 to 14 days)
2. Period of the embryo from the beginning of the third week through the end of the eight.
Time when all the major organs are formed and the heart begins to beat.
3. Period of the fetus from the ninth week until the child is born. All major organs begin to
The Period of the Zygote
Zygote moves down fallopian tube towards uterus, divides by mitosis into two cells.
Those 2 cells and all other cells continue to divide and form a ball-shaped structure called
blastocyst. (Contains 60 to 80 cells within 4 days of conception)
The inner layer of the blastocyst or embryonic disk becomes the embryo, whereas the outer
layer of cells will develop into tissues that protect and nourish the embryo.
- Blastocyst approaches the uterus 6 to 10 days after conception; small tendrils emerge from its
outer surface. When reaching the uterine wall, tendrils burrow inward, tapping the mother’s
blood supply implantation (burrowing of blastocyst into lining of uterus)
- Specific time where blastocyst must communicate with uterine wall to position and attach
- Process takes about 48 hrs. And occurs 7 to 10 days after ovulation. Entire process competed
about 10 to 14 days after ovulation.
- After being implanted, the blastocyst’s outer layer forms four major support structures to
protect and nourish the organism.
- Amnion: Watertight sac that fills with fluid from the mother’s tissues. Amniotic fluid
o Cushion the developing organism against blows, regulates its temperature, provides
weightless environment to make it easier to move.
o In watertight sac, there is another sac that produces blood cells until the embryo is capable
of producing its own.
- That yolk sac attaches to chorion: surrounds amnion and becomes the lining of the placenta.
Gathers nourishment for embryo
o Placenta: multipurpose organ. Provides for respiration and nourishment and elimination
of metabolic wastes.
- Fourth membrane: allantois: forms embryo’s umbilical cord: soft tube containing blood
vessels that connects the embryo to the placenta.
- Fed by blood vessels from the mother and the embryo.
- Prevents these two bloodstreams from mixing together
- Oxygen, carbon dioxide, salts, sugars, proteins and fats can pass through.
- Maternal blood delivers oxygen and nutrients to embryo through umbilical cord. Umbilical
cord also transports carbon dioxide and metabolic wastes.
- Wastes products cross-placental barrier, enter the mother’s bloodstream and are excreted
along with her own wastes.
See Figure 4.1 for Period of the Zygote
Period of the Embryo
- From implantation (3 week) through 8 week.
- By 3 week, embryonic disk is differentiating into 3 cell layers
o Outer layer - ectoderm will become nervous system, skin and hair
o Middle layer - mesoderm will become the muscles, bones and circulatory system.
o Inner layer - endoderm will become the digestive system, lungs, urinary tract,
- 3 after conception, ectoderm folds into neural tube becomes brain and spinal cord.
- End of 4 week, heart starts beating - eyes, ears, nose and mouth are also forming/buds that
will become arms and legs also appear.
The Second Month
- Primitive tail appears but soon enclosed becomes the tip of the backbone and the coccyx.
- Ears are formed, rudimentary skeleton.
- Arms appear; upper arm first, followed by fore arms, hands and then fingers.
- Leth folthw a few days later.
- 7 or 8 week, genital ridge appears called the indifferent gonad.
- Circulatory system now functions on its own, liver and spleen can now produce their own
Period of the Fetus
- Rapid growth and refinement of all organ systems
- Fetus begins to move, sense and behave.
The Third Month
- Organs become interconnected.
o Ex. Coordination between nervous and muscular systems allow fetus to kick its legs, make
fists, twist body.
o Digestive and excretory system are also working together
The Fourth through Sixth Month (Second Trimester - 13 to 24 )
- 15 or 16 weeks to 24 or 25 weeks movement tongue, lips, pharynx, larynx, suck, swallow,
cough, and snort…
- 5 to 6 month nails harden, skin thickens, eyebrows, eyelashes, scalp hair appear
2 - 20 weeks sweat glands are functioning
- Fetus is now covered in white, cheesy substance called vernix, and a fine layer of body hair
o Vernix protects fetal skin against chapping during long exposure to amniotic fluid
o Lanugo helpthvernix stick to the skin.
o End of 6 month Fetus’s visual and auditory senses are functioning
Seventh to Ninth Month (Third Trimester)
- Finishing phase
- Between 22 and 28 weeks, fetus reaches age of viability: the point at which survival outside
the uterus is possible.
- Middle of ninth month fetal activity slows and sleep increases
See Table 4.1 for Summary
Environmental Influences on Prenatal Development
Any disease, drug, or other environmental agent that can harm a developing embryo or fetus
by causing physical deformities, severely retarded growth, blindness, brain damage, and even
- Effects worst during period when structure is forming and growing most rapidly.
- Not all embryos or fetuses are affected the same way
- Same effect can be caused by different teratogens
- Variety of defects can result from one teratogens
- The longer a structure is exposed to a certain teratogen, the worst the effect is going to be.
- Embryos and fetuses can be affected by both mother and father’s exposure to teratogens
- A long-term effect of teratogens depends on the quality of postnatal environment.
- Some can cause sleeper effects - that may not be apparent until later in life.
Each part of our body has a sensitive period when it is most susceptible to teratogen agents.
Teratogens can have a subtle effect on babies’ behavior that are not obvious at birth but influence
their psychological development. (Ex. a mother that consumer 30ml a day of alcohol; the child
will not have any physical deformities but it is more likely to be slower at processing information
and have a lower IQ.
Rubella: A disease that has little effect on a mother but may cause a number of serious birth
defects in unborn children who are exposed in the first three to four months of pregnancy.
- Blindness, deafness, cardiac abnormalities, mental retardation.
- Most dangerous during first trimester
Toxoplasmosis: Disease caused by a parasite found in raw meat and cat feces; can cause bi