This document summarizes the stages of prenatal human development from conception through the first trimester. It describes the key biological processes during this period, including fertilization, implantation, formation of the blastocyst and initial development of organs and body systems. The summary highlights that during the first trimester the main task is formation of the embryo and fetus, and that miscarriage can occur if development does not proceed properly.

2. Life begins in the reproductive
system of the female, set inside
the pelvic cavity.
The term “natal”
means “birth”…
so pre-natal
means before
birth.

3. Ampulla
Fimbria
.
Ovum is
ripening inside
follicle
Endometrium
(uterine wall)
Maturing
follicles
Cervix… a mucous
plug forms in this
area during
pregnancy
Vagina
Fallopian
Tube
Ovary
Uterus
(womb)

4. Egg maturation and ovulation is stimulated by two hormones secreted by the
pituitary gland- follicle stimulating hormone (FSH) and luteinizing hormone (LH).
Approximately every 28 days, at the start of a cycle, these hormones cause
several follicles to start growing. Of these, only one matures to form a large fluid-
filled structure that contains a ripening egg. When the egg matures it is released
from the follicle when the follicle ruptures…in a process called ovulation. This
occurs on about the 14th day of the cycle.
The egg contains 23 chromosomes…the mother’s genetic material.

5. The egg, or ovum, slowly travels through
the fallopian tube, and if sperm are present
at that location, one may unite with the
ovum in a process called fertilization. Only
one sperm can enter the egg’s outer
membrane. The moment the sperm
penetrates the egg is called conception,
creating a new and separate cell called the
Zygote.
The penetrating sperm loses it’s tail, the
head dissolves, and out spills it’s genetic
material…23 chromosomes.

6. The fertilized egg now has 23
PAIR, or 46 chromosomes.
Immediately the cell begins to
duplicate, taking approximately
30 hours to complete the first cell
division.
At an increasingly faster rate, new
cells are added until they form a
hollow, fluid-filled ball, called a
blastocyst (about 4 or five days
after conception).
Approximately 60 to 70 cells form
the blastocyst. Those on the inside
(called the embryonic disk) will
become the new organism whereas
those on the outside will provide
the protective covering.

7. Multiple births can occur two different ways. Sometimes two (or more)
different eggs are released, and both are fertilized by two different sperm.
The result is fraternal twins. They may be both boys or both girls or one of
each; they may or may not look similar. In other cases, a single egg fertilized
by a single sperm will split completely in half during that very first cell
division. This results in identical twins. They will both be of the same sex and
will look very similar if not the same.

8. Identical twins have
identical DNA, but
different fingerprints!

9. Around the seventh or ninth day after
conception, the blastocyst burrows into the
uterine lining in a process called implantation.
The protective covering quickly develops into
the amnion, surrounding the organism in
amniotic fluid. A yolk sac also develops,
producing blood cells until the liver, spleen,
and bone marrow is mature. The woman’s
ovaries produce massive doses of the
hormone progesterone, signaling the body
that a pregnancy is in progress.
All the time an egg has been
ripening and preparing for
ovulation, the endometrium of
the uterus has been forming a
nutrient-rich lining. It is
preparing to receive a fertilized
egg. (If no egg is fertilized, this
lining will leave the body during
a process called menstruation.)

11. Nerve growth begins when a sheet of cells on the
back of the embryo folds in the middle to form a tube,
which will become the future spinal cord. At one end
tube enlarges to form the brain's major sections. The
amniotic fluid that cushions the fetus begins to form.

12. 5 weeks - At this time the embryo is
still the size of a raisin. Low on the
sides of the head are two folds of
tissue that will become the ears.
Although not completely developed,
all the major body organs and systems
are formed. The neural tube enlarges
into three parts, that will soon to
develop to become a very complex
brain. Also the placenta begins
functioning, including the chorionic
villi and the umbilical cord, through
which the baby will receive
nourishment and oxygen. The spine
and spinal cord grows faster than the
rest of the body at this stage and will
give the appearance of a tail. This
disappears as the baby grows.
6 weeks - The embryo's tiny heart begins to beat. The embryo itself is about a 1/4
inch long. Other major organs, including the kidneys and liver, have begun to
develop, and the neural tube, which connects the brain and spinal cord, closes.
The physical sensations of pregnancy start--nausea, fatigue, sore breasts and
frequent urination.
The “tail” in this “tadpole”
stage is found in many
organisms, so was used as
evidence in Darwin’s theory of
evolution.

13. There are two patterns of development occurring throughout
development of the being…
1. Cephalo-caudal, meaning head to tail (foot), top to bottom
2. Proximal-distal, (proximodistal) meaning inward to outward
(the head before the heart; the internal organs before the limbs)

14. 7 weeks - The embryo's hands and feet are shaped like paddles, but the fingers
are beginning to take shape. The embryonic tail has almost disappeared. The
pituitary gland is also forming and the fetus is beginning to grow muscle fibers.
The heart has divided into the right and left chambers and is beating about 150
beats a minute which is about twice the rate of an adult. The baby's facial
features are visible, including a mouth and tongue. The eyes have a retina and
lens. The major muscle system is developed, and the baby starts to practice
moving. The baby has its own blood type and the blood cells are produced by
the liver now instead of the yolk sac.

15. 8 weeks- The unborn baby is now
called a fetus. The fetus is protected
by the amniotic sac and filled with
fluid. Inside the child swims and
moves gracefully. The fetus is now
about 3/4 inches long. The arms and
legs have lengthened. During this
time of development, the baby's head
appears much larger than the body
because the brain is growing very
rapidly. Brain waves can now be
measured. The teeth and the palate
are beginning to form and the larynx
is developing. Through its parchment-
thin skin, the baby's veins are clearly
visible. By the eighth week the ears
begin to take shape.
9 weeks - Though the fetus is
constantly moving, you won't be able
to actually feel fetal movement for
several more weeks. All of the
organs, muscles, and nerves are in
place and beginning to function. As
the hands and feet develop fingers
and toes, they lose their paddle-like
look, and the touch pads on the
fingers form.

16. 10 weeks - The fetus
weighs about 1/3 of
an ounce. The heart
is almost completely
developed and very
much resembles
that of a newborn
baby. The eyelids
have fused shut.
The wrists and
ankles have formed
and the fingers and
toes are clearly
visible. Genitals
have begun to form,
but it is a bit early to
tell the sex of the
fetus.
11 weeks - The vital organs--the liver, kidneys, intestines, brain and lungs--are
fully formed and may begin to function, while the the head is almost half the
length of the entire body. The fetus is only about 2 inches long and weighs
less than a half ounce but it is busy moving and kicking.

17. Vocal chords are complete, and
the baby can and does sometimes
cry silently. The brain is fully
formed. The fetus may even suck
his thumb. The eyelids now cover
the eyes, and will remain shut until
the seventh month to protect the
delicate optical nerve fibers. The
hair is on the head and the fingers
and toes have developed soft
nails. The kidneys are developed.
The skull is not solid bone, but
has formed in sections. These
sections can overlap during the
birth process. There are even a
couple of soft spots where bone
has not formed at all called
fontenals.
THIS IS THE END OF THE FIRST
TRIMESTER, WHEN THE MAIN TASK
WAS FORMATION!
During this first trimester period of
“formation”, many things can go wrong.
Sometimes the woman’s body will self-
abort. This is called a miscarriage.

18. Umbilical cord
Placenta
Amniotic sac
filled with
amniotic fluid
Caution!... besides oxygen and nutrients, the cord delivers nicotine, alcohol,
drug compounds, and other things ingested, inhaled, or absorbed through the
skin of the mother.
Mucous
Plug
Chorionic Villi

19. Nutrition is extremely important during pregnancy. Eating a well-balanced
diet can prevent birth defects in the fetus, low-birth weight/mortality of the
infant, a difficult labor/delivery, and mortality of the mother during
childbirth. The fetus is creating new bones, blood cells, tissue, etc. If it
does not receive the nutrients it needs through mother’s diet…it will get it
by taking it out of mother’s own system, i.e. robbing calcium from the
mother’s bones to build it’s own bones. If deficiencies exist, problems
may show up right away or years later.
Common health problems developing during pregnancy
Preeclampsia and Eclampsia (high blood pressure)
Gestational Diabetes
Infections (toxemia)
Clotting Disorders
Common disorders of pregnancy
Placental Problems
Preterm Labor
Preterm Premature Rupture of the Membranes
Fetal complications
Congenital Abnormalities
Growth Abnormalities
Remember exercise and the
emotional health of the
mother as well!

20. By now, the fetus has grown large enough that
the movements can sometimes be felt by the
mother. Often felt like a flutter or a "flip-flop",
these first movements that can be felt by the
mother are called quickening.

21. At some point later, a white waxy, cheese-like substance also forms over the
entire surface of the skin. This is called vernix caseosa. The vernix is a perfect
skin lotion, which along with the lanugo, protects he skin from the constant
exposure to amniotic fluid. It is also believed that it serves as a sort of skin
cleanser and antibacterial substance. The lanugo and vernix caseosa diminishes
in amount during the last weeks of the gestation period and usually rubs off
during delivery. The presence of a great deal of vernix or lanugo at the time of
birth, therefore, is often a sign of prematurity.
At the end of the fourth month, white, downy hair
called lanugo (lu- noo- go) covers the entire body
of the fetus. It provides warmth, since the fetus
has no layer of fat beneath the skin yet.

22. During the 5th month, at 22
weeks, the fetus weighs a little
over 1 pound, and is about 1
foot in length. At this time, the
movements can clearly be felt
by the mother and by others
who place their hands over the
mother's abdomen.
The fetus also shows a sensitivity to light
and can be stimulated and irritated.
However, it still has a long way to go
before it is mature enough to survive
outside of the womb.
Although there are a few cases of infants
being born and surviving at this time, the
chance of survival (and without later
complications), is very slim.

23. During the 4th-6th months,
most of the organs that were
formed during the first
trimester begin to function. If
born at this time, the fetus has
a chance survival. The point
in which it can first survive is
referred to as the age of being
viable and occurs sometime
between 22 and 26 weeks.
THIS IS THE END OF THE
SECOND TRIMESTER,
WHEN THE MAIN TASK
WAS FUNCTIONING!

24. At only about 3 pounds, the 7 month old fetus has a few pounds to go
before reaching the average 7.5 pounds.
During this time, the brain continues to develop at an increasingly fast
rate. By 7 months, the fetus clearly responds to sounds outside of the
womb, and develops a preference for the tone and rhythm of its
mother's voice.
By the eight month, the fetus
has little room for large
movements. During this month,
a layer of fat is added that will
assist with temperature
regulation.
The lungs however, still remain
immature. If born at this time,
the infant will likely require
some help with breathing. It is
usually not until the ninth
month that the lungs are
mature enough to regulate
breathing without assistance.

25. During this last month of the gestation
period, baby continues to grow… at a
rate of ½ to 1 pound per week. Baby
will turn around in the womb, to a head-
down position. The lungs are maturing,
preparing for that first breath.
THIS IS THE END OF THE THIRD TRIMESTER, WHEN
THE MAIN TASK WAS GROWTH!