FETAL DEVELOPMENT
WEEK – BY – WEEK
WEEK ONE
Week one of a 40 week
pregnancy actually starts
from the first day of the
mother’s last period.
Meaning, in the first one-two
weeks, the soon-to-be
mother, is not actually
pregnant.
During the first week of
pregnancy the uterus sheds
it’s lining and the hormones
are preparing to release
another egg.
WEEK TWO
During week two, the sperm
and egg unite in the fallopian
tube to form a zygote. The
zygote will then travel down
one of the fallopian tubes
toward the uterus. Within the
uterus, the zygote will divide
to form a cluster of cells.
The inner group of cells in
the cluster is the embryo.
The outer layer of cells acts
as membranes that nourish
and protect the embryo.
WEEK THREE
The developing embryo, also
called a blastocyst, is looking
for a spot to implant in the
uterus.
When the embryo reaches the
uterus, it will burrow into the
uterine wall for nourishment.
The placenta also starts to
form at this time.
Amniotic fluid also begins to
collect at this time around the
ball of cells in the cavity that
will become the amniotic sac.
WEEK FOUR
The fourth week marks
the beginning of the
embryonic period.
During this week, the
formation of the brain,
digestive system, spinal
cord, heart and other
organs begins.
At the end of this week
the embryo is approx.
10,000 times larger the
size at conception.
WEEK FIVE
During this week, the heart
begins to beat. The brain has
developed into 5 areas and
cranial nerves are visible.Arm
and leg buds are also visible,
as well as the formation of the
eyes, lips, nose and umbilical
cord.
At this point, the spinal cord is
growing more rapidly than the
rest of the body, and gives for
a tail-like appearance, which
eventually disappears as the
embryo grows.
During this week, it is the first
time the blood vessels complete
a circuit, meaning the circulatory
system is the first functional
organ system.
WEEK SIX
The formation of the
lungs have begun at
this time and further
development of the
brain. Arms and legs
have lengthened with
foot and hand areas
distinguishable.
The intestines begin to
grow outside the body,
within the umbilical
cord, nostrils start to
form as well as the
pancreas.
The embryo is about 1/8 of an inch long
at this point.
WEEK SEVEN
Embryo has developed
it’s own blood type and
elbows and toes are
visible. Also, all
essential organs have
at least begun to form.
The teeth have also
started to develop
under the gums.
WEEK EIGHT
During this week, rotation of
intestines occurs, facial
features continue to
develop, and eyelids are
more developed. The
external features of the ear
also begin to take their final
shape.
The tongue starts to form,
the intestines move into the
abdomen from the umbilical
cord and the fingers and
toes are still partially
webbed.
At the end of this week, the
embryo is about ½ an inch in
length. This week is also the end
of the embryonic period.
WEEK NINE
Movement takes place this week,
although the mother is not able to
feel it yet. The embryonic “tail” has
also disappeared by this week.
Most joints are formed by now and
finger prints are evident in the skin.
The head also consists of half of
the body’s size and mass.
The internal reproductive organs,
such as testes or ovaries, start to
develop. The eyelids have also
been fused shut, and won’t reopen
until the 26th-28th week.
Rapid growth will occur after
this week due to the fact that the
physical features have, or soon
will be, developed.
WEEK TEN
The embryo is now called a fetus. It is now in the fetal period.
Facial features, limbs, hands feet fingers and toes become apparent.
The fetus’s brain rapidly produces neurons, about 250,000 a minute.
If the fetus is a boy, the fetus will start producing testosterone this week.
The fetus is about an inch in length.
The fetus’s head is about half of it’s body length at this point, but the rest
of it’s body will catch up soon and will become proportionate to it’s brain.
WEEK ELEVEN
During this week, fingers
and toes have separated
and hair and nails have
started to develop.
All organ systems should
be functioning at this
point and the developing
bones should start to
harden.
The fetus should also be
making lots of
movement; kicking and
stretching.
Genitals begin to take the
characteristics of the proper gender.
The fetus should also be about 1 ½
inches in length by this point.
WEEK TWELVE
During this week, the
fetus’s vocal cords
begin to form. Their
eyes begin to move
closer together, ears
shift to the sides of their
head, and their
intestines move further
into their body.
The liver begins to
create bile, the kidney
begins to secrete urine
in the bladder and
pancreas begins to
produce insulin.
Now, the fetus is about 2 inches in
length and weighs about ½ an
ounce.
WEEK THIRTEEN
During this week, the fetus
starts making inhaling and
exhaling movements.
The fetus’s neck is getting
longer.
The placenta is providing
nourishment and removing
waste from the fetus.
The fetus’s hands are
becoming more functional,
they would either start
sucking their thumb at this
point or start playing with
their fist.
Fetus should be approx. 3
inches in length and nearly
weighs an ounce.
WEEK FOURTEEN
During this week, the thyroid gland has matured and is now producing
hormones.
In boys, prostate gland develops and in girls, the ovaries move from the
abdomen to the pelvis.
The fetus becomes covered in fine hair called lanugo, which protects the
fetus’s skin from the water surrounding it. Eyebrows also start to come in
and hair on top of the fetus’s head starts to sprout.
At this point, the fetus is about 3 ½ inches in length and approx. weighs 1
½ ounces.
WEEK FIFTEEN
Legs are now longer than
the arms, nails are fully
developed and all joints and
limbs are able to move.
With the help of an
ultrasound, the genitals of
the fetus may now be
apparent enough to
determine of which gender
it is.
The fetus has also started
developing taste buds.
Sweat glands also start to
appear on the fetus.
The fetus is about 4 inches in
length and weighs about 2 ½
ounces.
WEEK SIXTEEN
Fat is starting to develop
underneath the fetus’s skin acting
as insulation.
The fetus’s heart is pumping about
6 gallons of blood per day. Also, in
girls millions of eggs are forming
in the ovaries.
The fetus has now developed
reflexes such as sucking,
swallowing, hiccupping and
blinking.
The fetus has learned to breathe !
Though, it is not breathing air at
the moment, but amniotic fluid.
The fetus is about 4 ½ inches
in length now, and weighs
approx. 3 ½ ounces.
WEEK SEVENTEEN
The fetus has more human-like
posture, with head erect and
away from chest and the body
and limbs more proportionate to
the head. The umbilical cord is
also being stronger and thicker.
Merconium starts to develop; a
black gooey substance which is
will become the fetus’s first
feces.
The fetus’s skeleton, which is
still rubbery cartilage at this
point, is starting to harden into
bone.
The fetus is a bit over 5 inches in
length and approx. weighs 5
ounces.
WEEK EIGHTEEN
If the fetus is a girl, her uterus
and fallopian tubes are formed
and in place. If the fetus is a boy,
his genitals are visible.
Ventricles and chambers within
the fetus’s heart are visible
through an ultrasound.
The ears have shifted to it’s
proper location, and the nerves
from the brain have “hooked up”
to the ears, giving the fetus the
ability to hear now.
Pads are also developing on the
fingers and toes.
The fetus is about 6 inches in
length and nearly weighs 7
ounces.
WEEK NINTEEN
During this week, the fetus has
recognizable sleep patterns, those of
a newborn.
Myelin, a fatty substance, starts to
insulate the nerves so impulses can
flow smoothly.
Vernix, a pasty white coating,on top of
Lanugo, helps protect the fetus from
the water surrounding it.
Sensory development reaches a peak.
The brain has developed regions for
the fetus’s senses – taste, vision,
hearing, touch and smell.
WEEK TWENTY
HALF WAY THERE !
The fetus’s heart is growing stronger,
their legs are reaching their relative
size and immune cells are being
transferred from the mother to the
baby.
The rapid growth is starting to slow
down.
With the help of vernix, the fetus’s
skin is thickening and developing
layers.
“Quickening” usually occurs during
this week; the mother being able to
feel the fetus moving.
The fetus is about 6 ½ inches in
length and weighs approx. 9 –11
ounces.
WEEK TWENTY-ONE
White blood cells are starting to
produce.
The fetus’s skin has changed
from translucent to opaque, and
the it’s tongue has fully formed.
If the fetus is a girl, the womb and
vagina are formed.
The fetus starts to swallow more,
it takes in amniotic fluid that
helps with their digestive system.
From the amniotic fluid, the fetus
receives sugar.
The fetus is about 11 inches
in length and weighs approx.
13 ounces.
WEEK TWENTY-TWO
At this point, fetuses will be
able to hear you if you sing
or read to them.
Eyelids and eyebrows are
fully formed and fingernails
now cover the fetus’s
fingertips.
If the fetus is a male, his
testes begin to descend to
the scrotum.
The fetus will be approx.
weigh a pound at this point.
WEEK TWENTY-THREE
The fetus’s eyes are
formed, but the iris still
lacks pigment.
If the fetus were to be born
now, there would be a a
slim chance of survival.
Blood vessels within the
lungs of the fetus develop
to help prepare the baby
for breathing once the
fetus is born.
WEEK TWENTY-FOUR
The fetus will gain close to 6
ounces this week in muscle,
bone and organs.
This week the fetus is
considered to be viable.
The fetus is about 12 inches in
length and close to 1 ½ lbs.
Footprints start to develop, as
well as aveoli (air sacs) in the
lungs.
The fetus is beginning to go
through rapid brain
development at this time.
WEEK TWENTY-FIVE
The spine begins to form
joints, ligaments and rings
which serves protection for
the fetus’s spinal cord.
The fetus’s nostrils begin to
open.
Optic nerves are functioning;
if a light was placed on the
mother’s stomach, the fetus
would turn it’s head away
from the light.
Bones are continuing to
become harder and stronger.
WEEK TWENTY-SIX
Lungs begin to secrete surfactant, to help prevent the fetus’s lungs from
sticking together after the fetus is born.
The fetus’s eyes may start to open and blink.
Veins are visible through the fetus’s skin.
The fetus may be able to cry.
The fetus will be approx. 14 inches in length and approx. weigh 2 lbs.
WEEK TWENTY-SEVEN
The fetus’s response to sound continues to grow.
The fetus’s lungs, liver, brain and immune system are maturing.
It is believed fetuses start to have dreams around this week.
Eyelids become open more frequently; the fetus can distinguish between
light and dark.
The fetus will weigh around 2 lbs. at this point and will approx. be 14 ½
inches in length.
WEEK TWENTY-EIGHT
The fetus is becoming more
filled out due to the increase
in muscle tissue and bone.
In girls, the clitoris is
relatively prominent because
it's not yet covered by the
still-small labia.
The fetus will now weigh just
over 2 lbs. and be almost 15
inches in length.
WEEK TWENTY-NINE
Fat is being developed underneath the fetus’s skin.
The fetus’s brain is now able to control breathing and body temperature.
The fetus’s eyes can move in their sockets. Also, it is sensitive to changes;
light, sound, taste and smell.
The fetus is starting to store iron, calcium and phosphorus.
The fetus’s movements might be a bit more aggressive and frequent.
WEEK THIRTY
Early lanugo is starting to disappear.
Toenails are almost completely formed.
Bone marrow is now in charge of red cell production.
The fetus is able to follow a light source around.
The fetus would approx. weigh around 3 lbs. And would be about 15 ½
inches in length.
WEEK THIRTY-ONE
The fetus goes through another rapid brain growth where billions of nerve
cells are formed.
Lungs are the only major organ that isn’t completely developed. The fetus
would need assistance for breathing if they were born now.
Arms, legs and body are filling out and are now finally proportionate to the
fetus’s head.
WEEK THIRTY-TWO
The fetus is not moving as
much now due to the lack of
room inside the uterus.
Toenails are completely
formed.
Lanugo is starting to shed
and fall off of the baby.
The fetus weighs just under
4 lbs. and is nearly 17
inches in length.
WEEK THIRTY-THREE
Fetus’s head has increased in size by approx. 3/8th of an inch due to the
rapid brain development.
Neurons and synapses are also increasing in huge numbers.
Fetus’s skull is developed, but still pliable and not completely joined, this
helps ease the fetus out of the narrow birth canal.
The fetus’s skin is becoming less red and wrinkled.
WEEK THIRTY-FOUR
The fetus is turning into the
head-down position in
preparation for birth.
Hearing is fully developed.
The fetus will continue to fill
out and become rounder
due to fat layers developing
underneath the fetus’s skin.
The fetus urinates approx. a
pint a day.
The fetus weighs nearly 5 lbs.
and is approx 18 inches in
length.
WEEK THIRTY-FIVE
Kidneys have fully developed and liver can now produce some waste
products.
The fetus will survive if born now.
Organs systems are all present, and are just going through some finishing
touches before the fetus’s birth.
Most of the physical development is complete, from now until birth the
fetus will put on weight.
WEEK THIRTY-SIX
Due to the recent fat deposit,
fat is dimpling the fetus’s
elbows, knees and is creating
creases in the fetus’s neck and
wrists.
The fetus continues to drop
further into the pelvis of the
mother.
The lanugo and vernix has
almost been completely shed
off of the fetus.
The baby should weigh around
6 lbs. and be about 19 inches
in length.
WEEK THIRTY-SEVEN
The fetus is now cradled in the
pelvic cavity of the mother and is
protected by her pelvic bones.
At this time, many fetuses have a
full head of hair, where others do
not.
The fetus continues to become
rounder as more fat layers are
being developed.
The fetus is now considered full
term, meaning the fetus will be
able to survive if born now.
WEEK THIRTY-EIGHT
All organ systems are still developing and fine tuning.
More meconium is developing.
The fetus is nearly 7 lbs. and is about 20 inches in length.
WEEK THIRTY-NINE
Outer layers of the fetus’s skin if shedding off while new skin developing
underneath.
Placenta provides fetus with antibodies to help it to survive during the first
six months after birth.
Some fetuses are already born by this week.
WEEK FORTY
BABY DUE DATE !
If fetus is not born during
this week, it is normal.
If the baby is born, there
might still be some vernix
present on the baby, the
baby will be approx. 15% fat
and the baby’s skull bones
would still be partially
separated.
The average newborn is
approx. a bit over 7 lbs. and
20 inches in length.