Fetal Development
Fetal Development
 
Learn about your the fetus' development with this week-to-week guide of your growing baby.
 
 
Preganancy:  Weeks 1 to 6
 
The development of a baby in 40 weeks is an amazing thing. There's something new happening every week.
 
Week No. 1
 
You're not pregnant. This is the week of the start of your last period. As the uterus sheds its lining and causes bleeding, the hormones are preparing another egg for release.
 
Week No. 2
 
You're still not pregnant. The uterine lining is thickening and ovulation (the release of an egg) is getting ready to occur. You might feel a twinge of pain as ovulation occurs.
 
Week No. 3
 
This could be the week! When an egg is released into your fallopian tube, it meets up with your partner's sperm. (During the average ejaculation 350 million sperm have a chance at making the trip into the fallopian tube. The ones that make it will wait there for an egg for as long as four days.) When the sperm penetrates an egg, fertilization (conception) occurs. The fertilized egg is called a zygote. Immediately the egg begins dividing into identical cells as it floats down the fallopian tube to the uterus. Now you're pregnant.
 
Week No. 4
 
There's lots of action going on that you're completely unaware of. Your little zygote finds its nesting place in the uterus and is now called a blastocyst. When in the uterus, it divides in half: one-half sticks to the uterine wall and becomes the placenta (the life-support system that brings nourishment into your baby and takes waste out). The other half will become the baby. Transvaginal ultrasound (which is an ultrasound in which a small probe is placed into the vagina, giving a very close look inside the uterus) shows a gestational sac (a small circle that will become the baby's home). By the end of this week, you will have missed your period, giving you the first sign that something's up. Some women see a slight spotting called implantation bleeding that happens when the blastocyst nestles into the uterine wall.
 
Week No. 5
 
At this point a pregnancy test can confirm the pregnancy. Your baby is about the size of an apple seed and is now called an embryo and has a beating heart of its own. The placenta and the umbilical cord are in full operation. You would be shocked to see how rapidly your baby is developing:
  • The head and the tail folds are distinct.
  • The primary brain vesicles form.
  • The nervous system begins to develop.
  • Two heart tubes are fused in the midline and begin to contract.
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Week No. 6
 
Your little embryo is dropping its calling card. This is the week the physical sensations of pregnancy usually appear: nausea, sore breats, fatigue, and frequesnt urination are all signs of this incredible growth process. The embryo looks more like a tadpole than a human, but the body is growing rapidly:
  • The head, tail, and arm buds are easily recognizable.
  • The optic vesicles and lenses form.
  • Limb buds are present.
  • The earliest form of the liver, pancreas, lungs, thyroid gland, and heart appear.
  • Blood circulation is well established. The heart bulges from the body.
  • The cerebral brain hemispheres are enlarging.
  • In the stomach area, the primary intestinal loop is present.
In these few weeks in the first month of preganancy so much is happening. Just because you don't see your belly growing yet, don't be fooled into thinking there's nothing going on that needs medical attention. Don't let this first month go by without a visit to your health care provider.
 
 
Preganancy:  Weeks 7 to 10
 
The second month is a busy one for your little embryo; there's so much to do and so little time to do it.
 

Week Number 7

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  • The heart is completely formed.
  • The limb buds are present.
  • On the face, the nostrils have overhanging borders.
  • The eyes appear as dark spots.
  • The tongue is beginning to form.
  • Permanent kidneys begin to form.
  • The head is relatively larger.
  • The trunk of the main body is elongating and straightening.
  • The eyelid folds are forming.
  • In the middle area, the appendix and pancreas are present, and the midgut herniates into the umbilical cord.
  • The pituitary gland is forming in the middle of the brain.
  • Spleen and liver ducts are forming.
  • The intestines elongate.
  • The cerebral cortex (the part of the brain that directs motor activity and intellect) can be seen.
  • The stomach and esophagus begin to form.
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Week Number 8

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  • The embryo is half an inch long.
  • The leg buds have divided into thigh, leg, and foot units.
  • The arm buds have divided into hand, arm, and shoulder units.
  • The gonads, testes or ovaries, are forming.
  • Cartilage and bone are forming.
  • The hands are flexed over the wrists and meet over the heart bulge.
  • The feet are approaching the midline and might meet.
  • The eyelids have almost covered the eyes.
  • The external ear is well advanced in form.
  • The head is in a more erect position and the neck is more developed.
  • The limbs are longer.
  • Cartilage appears where bones will later develop.
  • The head is bent forward onto the chest.
  • The eyes develop pigment.
  • Nerve fibers connect the olfactory lobe of the brain, laying the groundwork for the sense of smell.
  • The brain stem is now recognizable.
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Week Number 9

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  • The abdomen and chest cavities become separated.
  • The eye muscles and upper lip develop.
  • Using ultrasound, the first fetal movements can be detected.
  • The nerve cells of the eyes' retinas form.
  • The semicircular canals of the ears form.
  • The nasal passages open to the outside.
  • The nerve connections from the retina to the brain are established.
  • A distinct neck connects the head to the body.
  • All fingers and toes are present.
  • The urinary and rectal passages are completely separate.
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Week Number 10

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  • The embryo now weighs 1/3 ounce and is 11/4 inches from the top of the head to the rump.
  • The eyes have moved from the sides of the head closer to the front of the face.
  • Bones begin to replace the cartilage.
  • The palate and roof of the mouth form.
  • The stomach moves into its final position.
  • Taste buds begin to form.
  • Neck muscles are forming.
  • The clitoris appears in females and the testes begin to descend in males.
  • The two lobes of the lungs extend into many tiny tubes (bronchioles).
  • The diaphragm begins to separate the heart and lungs from the stomach.
In month number two your baby is rapidly growing and your hopes are high. Now is the time to balance the discomforts of the hormonal surge with the knowledge that it will all be worth it in the end when you hold your newborn in your arms.
 
 
Preganancy:  Weeks 11 to 14
 
As the third month ends your baby is officially a fetus, and it is completely formed.
 

Week Number 11

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  • The head is almost half the size of the fetus.
  • The eyelids have fused and will stay that way until week 24.
  • The external sex organs are developing.
  • The hair follicles of the skin are forming.
  • The ears are still abnormally low on the sides of the head.
  • Teeth are beginning to form.
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Week Number 12

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  • A skeleton of cartilage is forming.
  • The gallbladder secretes bile from the fetus.
  • The lungs are completely formed.
  • The thyroid gland and pancreas are now complete.
  • If the forehead is touched, the fetus will turn its head away.
  • The liver is functioning to make blood cells. This organ accounts for about 10 percent of the entire weight of the fetus.

 

Week Number 13

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  • The fetus is about three inches long (double its length at week seven).
  • The fingernails appear.
  • The beginnings of finger- and footprints begin to form.
  • Tooth buds appear for all 20 baby teeth.
  • The vocal cords begin to form.
  • The trachea, lungs, stomach, liver, pancreas, and intestines develop into their final functioning form.

 

Week Number 14

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  • The ears have moved from the neck onto the head.
  • The sex organs have fully differentiated into male or female.
  • The digestive glands are complete.
  • The taste buds are numerous and the salivary glands form.
  • The vocal cords are complete.
At the end of the third month, take some time to celebrate. Your baby is fully formed and secure in your womb, and you're starting to feel more like your old self with less fatigue and nausea.  As the first trimester ends, you can look back, breathe a sigh of relief, and pat yourself on the back for getting through a tough period with flying colors. Now you can look forward to the days ahead when your baby (and you!) will grow in size.
 
 
Preganancy:  Weeks 15 to 18
 
In the fourth month your baby's face looks more human. Her eyes, which started out on the sides of her head, have moved closer together. Her very thin skin is covered with hair (which usually disappears before she's born).
 
Week Number 15
  • Blood vessels can easily be seen through the skin, which is very thin and translucent.
  • The legs are longer than the arms.

 

Week Number 16

  • The fetus can make a fist, open its mouth, move its lips, and swallow.
  • The fetus might suck its thumb.
  • Fine lanugo hairs can be seen on the head.
  • The nails are well formed.
  • The heart is beating 117 to 157 times per minute.
  • The stomach is producing digestive juices.
  • The kidneys are producing urine.

 

Week Number 17

  • Brown fat begins to form.
  • The rapid growth rate begins to slow down.
  • White fatty material begins to enclose the nerve fibers of the spinal cord.
  • Between the 17th and 20th weeks hearing develops; the fetus can hear internal-organ and outside-world sounds.

 

Week Number 18

  • The skeleton can be clearly outlined on x-rays of the fetus.
  • Ears stand out from the head.
  • Lanugo covers the entire body.
 
 
Preganancy:  Weeks 19 to 22
 
In the fifth month, your baby's skeleton is mostly rubbery cartilage that will harden later. She might spend some time sucking her thumb.
 
Week Number 19
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  • Parts of the leg have now grown in proportion to the rest of the body.
  • You can feel the fetus kick, move its arms, and wiggle its fingers and toes.

 

Week Number 20

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  • A skin covering called vernix caseosa forms to protect the baby's skin from the amniotic fluid.
  • Delicate eyebrows are forming.

 

Week Number 21

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  • Body parts, tissues, and organs continue to mature.

 

Week Number 22

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  • The eyelids are very well developed.
  • Fingernails are completely developed.
The fifth month is a very busy one. Your body is changing in ways that can no longer be hidden and you're carrying around a baby who's ready to rock and roll. Fears and anxieties might keep you awake at night. But there's good news, too—you're more than halfway there.
 
 
Preganancy:  Weeks 23 to 26
 
In the sixth month, your baby's brain is growing rapidly and hearing is well developed. In fact, studies have found that unborn babies can communicate a preference for classical music over rock.

 

Week Number 23

  • Eyebrows and eyelashes are forming.
  • Lips are becoming more distinct.
  • Eyes are becoming more developed, but they still lack color.
  • he first signs of teeth appear beneath the gum line.

 

Week Number 24

  • Blood vessels in the lungs are developing.
  • The head is still large compared to body size.
  • The baby can swallow.

 

Week Number 25

  • The body is beginning to become plump.
  • The skin becomes wrinkled.
  • If the baby were born now, it would attempt to breathe.
  • Taste buds are forming.

 

Week Number 26

  • The face and the body generally assume the appearance of an infant at birth.
  • Baby begins to make breathing movements although there is no air in the lungs.
  • Will respond to touch.

As your second trimester ends, take some time to sit back and enjoy it. Very soon your pregnancy will be over and all the days you dedicated to creating a new human being will be only a vague memory. Cherish them now.

 
Preganancy:  Weeks 27 to 30
 
In the seventh month, almost all the parts are in place. Your baby is very busy making faces, hiccuping, crying, kicking, and elbowing you.

 

Week Number 27

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  • The network of nerves to the ear is completed.
  • The fetus takes some breaths and although breathing in water, not air (in utero, babies breathe in amniotic fluid through the nose and it passes through the lungs and exits back into the amniotic sac), it is good practice for their lungs.
  • Because the retinas of the eyes are not finished forming yet, an eye problem called retinopathy of prematurity can occur in babies born at this time.

 

Week Number 28

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  • Brain tissue continues to develop.
  • The baby begins to dream.
  • The eyes open and close.
  • The baby sleeps and wakes at regular intervals.
  • Although the lungs are still immature, they are capable of sustaining life in the event of a premature birth, often with the help of a respirator.

 

Week Number 29

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  • Your baby can turn his head to find the source of a bright light.
  • The fat layers continue to form.
  • Fingernails are budding.

 

Week Number 30

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  • In male babies, the testicles move from near the kidneys through the groin en route to the scrotum.
  • In female babies, the clitoris appears large and exposed because it's not yet covered by the folds of skin called the labia.
  • The baby's head is getting bigger.
The seventh month is both busy and long. There's so much to do, but there's still so long to wait. Your baby seems anxious to join the world, too, as her kicking and punching become stronger and more frequent.
 
 
Preganancy:  Weeks 31 to 34
 
This month you feel and probably look huge! Your baby is putting on more fat layers and weight every day—about half a pound a week! In fact, she'll gain more than half her birth weight in the seven weeks before delivery.

 

Week Number 31

  • Lungs and digestive track are nearly mature.
  • The baby can see in the womb. (If you shine a bright light on your belly, he might move his head to follow the light or reach out to touch it.)
  • Eyebrows and eyelashes are complete.

 

Week Number 32

  • Layers of fat are being deposited beneath the skin.
  • Arms and legs are fully proportioned in relation to the size of the head.
  • The baby is passing water from his bladder.
  • The baby's movements might be less frequent and less forceful because he is running out of room to move around.

 

Week Number 33

  • The baby is practicing breathing by inhaling amniotic fluid to exercise his lungs.
  • Some babies have a full head of hair by now; others have only a bit of fuzz.
  • Boys' testicles are descending into the scrotum.

 

Week Number 34

  • The skeleton hardens, although the head remains pliable to maneuver through the birth canal.
  • The skin becomes less red and wrinkled.
  • The toenails are present and the fingernails have reached the tips of the fingers.
In the eigth month you might feel this pregnancy idea is getting old. It's time to move on to motherhood.  Be patient and give your baby just a little more time to plump up before he makes his grand entrance.
 
 
Preganancy:  Weeks 35 to 40
 
The last weeks of your pregnancy give your baby time to grow in length and weight.

 

Week Number 35

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  • Babies born at Week 35 have a 99 percent chance of survival. The lungs are fully developed and the respiratory problems that used to kill these preemies are now easily treated.

 

Weeks Number 36 and 37

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  • More fat layers are added to help control body temperature after birth.

 

Week Number 38

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  • Your baby is now "full term." This means that if he's born today he'll be considered a full-grown baby. Babies born before 38 weeks are called premature babies.

 

Week Number 39

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  • The baby sheds her fuzzy layer of lanugo hair and the vernix caseosa, which is the skin coating that covers a fetus to protect the skin. The baby swallows this waste and will discharge it as his first bowel movement in a blackish mixture called meconium.
  • All organs are ready to function outside your womb.
  • The lungs are the last organs to mature. They should be fully developed by this time.

 

Week Number 40

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  • Bingo! This is the due-date week when your baby just might make her entrance. But then again, maybe she did that last week or will wait until next week. Remember that due dates are rarely exact. (Only 5 percent of babies are born on their due dates.) Even carrying a baby to Week 42 is considered normal. After that, the baby is post-term.

What a month! The anticipation can drive even the calmest expectant mom crazy. But you can take your mind off this pregnancy that seems never-ending by shopping for all those cute baby thing that your new baby will be using very soon.