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Fetal development week by week
Dr. Rasanjana Mendis
MOH- Aranayake

2 weeks: Fertilization
At the start of this week, you ovulate. Your egg is fertilized 12 to 24 hours later if a sperm
penetrates it – and this simple biological occurrence begins a series of increasingly complicated
processes that leads to a new human life, if all goes well. Over the next several days, the fertilized
egg will start dividing into multiple cells as it travels down the fallopian tube, enters your uterus,
and starts to burrow into the uterine lining.

3 weeks: Implantation
Now nestled in the nutrient-rich lining of your uterus is a microscopic ball of hundreds
of rapidly multiplying cells that will develop into your baby. This ball, called a
blastocyst, has begun to produce the pregnancy hormone hCG, which tells your
ovaries to stop releasing eggs.

4 weeks
Your ball of cells is now officially an embryo. You're now about 4 weeks from
the beginning of your last period. It's around this time – when your next
period would normally be due – that you might be able to get a positive result
on a home pregnancy test.

5 weeks
Your baby resembles a tadpole more than a human, but is growing fast. The
circulatory system is beginning to form, and the tiny heart will start to beat
this week.
Your baby is the size of a sesame seed.

6 weeks
Your baby's nose, mouth and ears are starting to take shape, and
the intestines and brain are beginning to develop.
Your baby is the size of a lentil.

7 weeks
Your baby has doubled in size since last week, but still has a tail, which will soon
disappear. Little hands and feet that look more like paddles are emerging from the
developing arms and legs.
Your baby is the size of a blueberry.

8 weeks
Your baby has started moving around, though you won't feel movement yet. Nerve
cells are branching out, forming primitive neural pathways. Breathing tubes now
extend from his throat to his developing lungs.
Your baby is the size of a kidney bean.

9 weeks
Your baby's basic physiology is in place (she even has tiny earlobes), but there's much
more to come. Her embryonic tail has disappeared. She weighs just a fraction of an
ounce but is about to start gaining weight fast.
Your baby is the size of a grape.

10 weeks
Your embryo has completed the most critical portion of development. His skin is still
translucent, but his tiny limbs can bend and fine details like nails are starting to form.
Your baby is the size of a kumquat

11 weeks
Your baby is almost fully formed. She's kicking, stretching, and even
hiccupping as her diaphragm develops, although you can't feel any activity
yet.
Your baby is the size of a fig.

12 weeks
This week your baby's reflexes kick in: His fingers will soon begin to open and close,
toes will curl, and his mouth will make sucking movements. He'll feel it if you gently
poke your tummy – though you won't feel his movements yet.
Your baby is the size of a lime.

13 weeks
This is the last week of your first trimester. Your baby's tiny fingers now have
fingerprints, and her veins and organs are clearly visible through her skin. If you're
having a girl, her ovaries contain more than 2 million eggs.
Your baby is the size of a pea pod.

14 weeks
Your baby's brain impulses have begun to fire and he's using his facial muscles. His
kidneys are working now, too. If you have an ultrasound, you may even see him
sucking his thumb.
Your baby is the size of a lemon.

15 weeks
Your baby's eyelids are still fused shut, but she can sense light. If you shine a flashlight
on your tummy, she'll move away from the beam. Ultrasounds done this week may
reveal your baby's sex.
Your baby is the size of an apple.

16 weeks
The patterning on your baby's scalp has begun, though the hair isn't visible
yet. His legs are more developed – find out when you're likely to feel your
baby kick! His head is more upright, and his ears are close to their final
position.
Your baby is the size of an avocado.

17 weeks
Your baby can move her joints, and her skeleton – formerly soft cartilage – is now
hardening to bone. The umbilical cord is growing stronger and thicker.
Your baby is the size of a turnip.

18 weeks
Your baby is flexing his arms and legs, and you may be able to feel those movements.
Internally, a protective coating of myelin is forming around his nerves.
Your baby is the size of a bell pepper.

19 weeks
Your baby's senses – smell, vision, touch, taste and hearing – are developing
and she may be able to hear your voice. Talk, sing or read out loud to her, if
you feel like it.
Your baby is the size of an heirloom tomato.

20 weeks
Your baby can swallow now and his digestive system is producing meconium,
the dark, sticky goo that he'll pass in his first poop – either in his diaper or in
the womb during delivery.
Your baby is the size of a banana.

21 weeks
Your baby's movements have gone from flutters to full-on kicks and jabs against the
walls of your womb. You may start to notice patterns as you become more familiar
with her activity.
Your baby is the size of a carrot.

22 weeks
Your baby now looks almost like a miniature newborn. Features such as lips
and eyebrows are more distinct, but the pigment that will color his eyes isn't
present yet.
Your baby is the size of a spaghetti squash.

23 weeks
Your baby's ears are getting better at picking up sounds. After birth, she may recognize
some noises outside the womb that she's hearing inside now.
Your baby is the size of a large mango.

24 weeks
Your baby cuts a pretty long and lean figure, but chubbier times are coming. His skin is
still thin and translucent, but that will begin to change soon too.
Your baby is the size of an ear of corn.

25 weeks
Your baby's wrinkled skin is starting to fill out with baby fat, making her look more like
a newborn. Her hair is beginning to come in, and it has color and texture.
Your baby is now the same weight as an average rutabaga.

26 weeks
Your baby is now inhaling and exhaling amniotic fluid, which helps develop his lungs.
These breathing movements are good practice for that first breath of air at birth.
Your baby is the size of a bunch of scallions.

27 weeks
This is the last week of your second trimester. Your baby now sleeps and wakes on a
regular schedule, and her brain is very active. Her lungs aren't fully formed, but they
could function outside the womb with medical help.
Your baby is the size of a head of cauliflower.

28 weeks
Your baby's eyesight is developing, which may enable her to sense light filtering in
from the outside. She can blink, and her eyelashes have grown in.

29 weeks
Your baby's muscles and lungs are busy getting ready to function in the outside world,
and his head is growing to make room for his developing brain.
Your baby is the size of a butternut squash.

30 weeks
Your baby is surrounded by a pint and a half of amniotic fluid, although there will be
less of it as she grows and claims more space inside your uterus.
Your baby is the size of a large cabbage.

31 weeks
Your baby can now turn his head from side to side. A protective layer of fat is
accumulating under his skin, filling out his arms and legs.
Your baby is the size of a coconut.

32 weeks
You're probably gaining about a pound a week. Half of that goes straight to your baby,
who will gain one-third to half her birth weight in the next seven weeks in preparation
for life outside the womb.
Your baby is the size of a large jicama.

33 weeks
The bones in your baby's skull aren't fused yet. That allows them to shift as his head
squeezes through the birth canal. They won't fully fuse until adulthood.
Your baby is the size of a pineapple.

34 weeks
Your baby's central nervous system is maturing, as are her lungs. Babies born
between 34 and 37 weeks who have no other health problems usually do well
in the long run.
Your baby is the size of a cantaloupe.

35 weeks
It's getting snug inside your womb! Your baby's kidneys are fully developed, and his
liver can process some waste products.
Your baby is the size of a honeydew melon.

36 weeks
Your baby is gaining about an ounce a day. She's also losing most of the fine down that
covered her body, along with the vernix casosa, a waxy substance that was protecting
her skin until now.
Your baby is the size of a head of romaine lettuce.

37 weeks
Your due date is very close, but though your baby looks like a newborn, he isn't quite
ready for the outside world. Over the next two weeks his lungs and brain will fully
mature.
Your baby is the size of a bunch of Swiss chard.

38 weeks
Are you curious about your baby's eye color? Her irises are not fully pigmented, so if
she's born with blue eyes, they could change to a darker color up until she's about a
year old.
Your baby is the size of a leek.

39 weeks
Your baby's physical development is complete, but he's still busy putting on fat he'll
need to help regulate his body temperature in the outside world.
Your baby is the size of a mini watermelon.

40 weeks
If you're past your due date you may not be as late as you think, especially if you
calculated it solely based on the day of your last period. Sometimes women ovulate
later than expected.
But if you don't go into labor on your own by your due date, your healthcare provider
will probably do tests (such as a sonogram and a non-stress test) to make sure you can
safely continue your pregnancy
Your baby is the size of a small pumpkin.

41 weeks
Your baby is now considered late-term. Going more than two weeks past your due
date can put you and your baby at risk for complications, so your provider will
probably talk to you about inducing labor.

Labor and delivery
Meeting your baby for the first time is so exciting – but exactly what will lead up to
that moment is unpredictable, and it's natural to feel nervous. Here's some help as
you prepare for the big day. Find out how you'll know you're in labor and what to
expect from delivery, understand your childbirth choices, and more.

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1 fetal development week by week

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