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Pregnancy is indeed a scientific marvel: A cluster of cells evolves into a
new life in just 280 days. But if you're an expectant parent, you're no doubt
wondering, "What's going on in there?" Find the answers below with a week-by-
week guide to your baby's fetal development. (Since doctors consider
pregnancy a 40-week affair starting with the first day of your last
period, your pregnancy really "begins" two weeks before you conceive.)
Week 1 | Week 2 | Week 3 | Week 4 | Week 5 | Week 6 | Week 7 | Week 8 | Week 9 | Week 10 | Week 11 |
Week 12 | Week 13 | Week 14 | Week 15 | Week 16 | Week 17 | Week 18 | Week 19 | Week 20 | Week 21 |
Week 22 | Week 23 | Week 24 | Weeks 25 through 29 | Weeks 30 through 34: | Weeks 35 through 40
Week 1:
You get your period.
Week 2:
While an egg ripens in one of your two fallopian tubes, the uterus starts to
form a lining of blood-rich tissue.
Week 3:
Conception.
Only one of the thousands of sperm that have traveled through the fallopian
tubes breaks through the egg's protective barrier to form a zygote. The cells start to
divide into a small cluster as they travel down the fallopian tube towards
the uterus.
Size: .006 inches
Week 4:
The tiny cluster of cells attaches itself to the uterine wall and then
divides in half. One half becomes the placenta and the other half the baby.
At the same time, different germ layers form. These are the building blocks
for the central nervous and skeletal systems.
Size: .04 inches
Week 5:
This is a week of crucial wiring when the placenta and umbilical cord hook
up to Mom. Your baby's heart -- about the size of a poppy seed -- starts beating.
Size: .05 inches
Week 6:
The cluster of cells takes on the shape of a baby with a prominent head and
dark eyespots. The vital organs including the brain, spinal cord, heart,
kidneys, liver, and stomach also start to form. With all this
activity, week 6 marks the beginning of a vital 4-week embryonic period (the
time when most malformations can happen.)
Size: .08 to .16 inches
Week 7:
With your child's biggest growth spurt until puberty, the embryo more than
doubles in size from .16 to .44 inches. The legs and arms extend and grow
from the torso and the heart divides into right and left chambers. The
forebrain also divides into two hemispheres.
Size: .44 to .52
Week 8:
Finite features including eyelids, ears, hair, fingers, and toes start to
take shape.
Size: .56 to .8 inches
Week 9:
The eyelids now cover the eyes and will remain sealed shut until week 27.
Pupils form and as the arms and legs grow longer, your future Olympian can
practice some early moves. But since the baby still has plenty of room in
the uterus, you won't feel these movements for several more weeks.
Size: 09 to 1.2 inches
Week 10:
About the size and shape of a medium shrimp, week ten marks the end of the
embryonic period and the embryo becomes a fetus with all vital organs and
weighs in at about .18 ounces.
Size: 1.25 to 1.68 inches
Week 11:
The baby's body isn't very proportionate and the head makes up half the
size. The neck develops and the head rises up from the chest. It may be
possible for curious parents to tell if they have a boy or girl.
Size: 1.75 to 2.4 inches
Weight: .3 ounces
Week 12:
The baby takes up more space and the uterus expands up from the pelvic floor
and pushes other organs out of the way. If you could peek into the womb, you
would see quit a busy little fellow: He's now squinting, opening and closing
his mouth, and moving his fingers and toes.The small intestine starts
functioning, relaxes and contracts to push substances out. Few structures
form after this point. Those already in place continue to grow and develop.
Size: 2.5 inches
Weight: .3 to .5 ounces
Week 13:
About the size of a peach, your baby's body starts to catch up to the
head. Eyes, which start out on the side of the face, move toward the center
and the ears move toward the side of the head. The genitalia is developed
enough to easily tell whether you have a boy or girl.
Size: 2.6 to 3.1 inches
Weight: .5-.7 ounces
Week 14:
The neck gets longer and pulls the head further up away from the chest.
Size: 3.2 to 4.1 inches
Weight: 1 ounce
Week 15:
The skeletal system is getting harder as your baby takes in nutrients from
Mom, namely calcium. If you did a x-ray or even looked through the baby's
very thin skin, you would see the outline of a skeleton. Fine hairs called
lanugo also cover the body.
Size: 4.1 to 4.5 inches
Weight: 1.75 ounces
Week 16:
The baby continues to grow and take up more space in the uterus while
showing a decided flair for movement. Less space means an elbow here, a foot
there. You, therefore, may experience movements for the first time, called
fluttering. (This can take place anytime through week 20. Notify your doctor
if you don't notice them by then.)
Size: 4.3 to 4.6 inches
Weight: 2.8 ounces
Week 17:
While most of your baby's organs are in place, he's still very thin and
lanky. During week 17 your baby starts to form fat. This is vital for the
baby's heat production and metabolism.
Size: 4.4 to 4.8 inches
Weight: 3.5 ounces
Week 18:
The rapid growth rate of previous weeks slows down. Finite features of the
heart including ventricles and chambers should be visible during an
utltrasound and some abnormalities can be detected.
Size: 5 to 5.6 inches
Weight: 5.25 ounces
Week 19:
The sebaceous glands start producing vernix caseosa, a covering that
protects the delicate fetal skin from the amniotic fluid. Myelination of the
spinal cord also takes place.
Size: 5.2 to 6 inches
Weight: 7 ounces
Week 20:
As they say in the baby business: You've reached the halfway point. By now
the fetus is so large, you notice most movements. The skin
also forms two layers: the epidermis and the dermis.
Size: 5.6 to 6.4 inches
Weight: 9 ounces
Week 21:
The different organ systems developed during the crucial embryonic phase
start to perform their duties, prepping the baby for survival outside the
womb. During this week, your baby may start swallowing small amounts of
amniotic fluid, absorbing most of the water, and passing the rest through to
the large bowel.
Size: 7.2 inches
Weight: 10.5 ounces
Week 22:
Eyelids, eyebrows, and even fingernails have developed. By now your baby's
liver, with some help from Mom, is starting to break down bilirubin, a
substance produced by red blood cells. If your baby can not sufficiently
break down this material by birth, he might be slightly jaundiced and need
to be placed under lights.
Size: 7.6 inches
Weight: 12.25 ounces
Week 23:
Your baby starts practicing breathing by taking in and expelling small
amounts of amniotic fluid. The pancreas also produces insulin, important for
the breakdown of sugars. By now your baby resembles a small doll with all
the facial features of an infant at birth.
Size: 8 inches
Weight: Almost 1 pound
Week 24:
With all of the rapid organ development, and in-womb practice, your baby may
be able to survive outside the womb. (Week 25 remains the official viability
week.) The baby's once wrinkled skin has filled out with weight gain, and
there is little space left inside the womb.
Size: 8.4 inches
Weight: 1.2 pounds
Weeks 25 through 29:
By this time your baby has a pretty good chance of surviving outside the
womb. The eyes open and the retina forms different layers which can sense
light levels and send messages to the brain. Also, the once smooth surface
of the brain starts to form grooves and indentations.
Size: 8.8 inches to 10.4 inches
Weight: 1.5 pounds to 2.7 pounds
Weeks 30 through 34:
While continuing to plump up, the baby is sensitive to the world around him, responding to light, sounds, and making faces.
Size: 11.2 inches to 12.8 inches
Weight: 3.5 pounds to 5 pounds
Weeks 35 through 40:
In anticipation of delivery, the baby positions itself with the head down
towards the pelvis and sheds lanugo hairs. At any point between week 35 and
40, your little one will let you know he's ready to make a grand entrance
and give you your first glimpse at 40 weeks of amazing development.
Size: ?
Weight: ?
*Sizes are approximations, and are from the top of the head
to the rump, and don't include the legs.
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