FETAL DEVELOPMENT

WEEK – BY – WEEK

WEEK ONEWeek 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 TWODuring 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 THREEThe 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 FOURThe 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 FIVEDuring 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 SIXThe 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 EIGHTDuring 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 NINEMovement 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 ELEVENDuring 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 TWELVEDuring 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

Fetus should be approx. 3 inches in length and nearly

weighs an ounce.

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.

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

The fetus is about 4 inches in length and weighs about 2 ½

ounces.

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.

WEEK SIXTEEN

The fetus is about 4 ½ inches in length now, and weighs

approx. 3 ½ ounces.

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.

WEEK SEVENTEENThe 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

The fetus is about 6 inches in length and nearly weighs 7

ounces.

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.

WEEK NINTEENDuring 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 TWENTYHALF 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-TWOAt 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-FOURThe 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-FIVEThe 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-SIXDue 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-SEVENThe 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 FORTYBABY 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.