How your baby's growing

Week 4 – Month 1

This week marks the beginning of the embryonic period. From now until 10 weeks, all of your baby's organs will begin to develop and some will even begin to function. As a result, this is the time when she'll be most vulnerable to anything that might interfere with her development. Right now your baby is an embryo the size of a poppy seed.

4 weeks: Your baby is the size of a poppy seed

Structure A is the placenta.

The placenta is also developing. The placenta will be able to provide nutrients and oxygen to your growing baby when it starts to function at the end of this week.

Also present now are the amniotic sac, which will house your baby; the amniotic fluid, which will cushion her as she grows; and the yolk sac, which produces your baby's red blood cells and helps deliver nutrients to her until the placenta has developed and is ready to take over this duty.

How your baby's growing

Week 5 – Month 2

Deep in your uterus your embryo is growing at a furious pace. At this point, he's about the size of a sesame seed, and he looks more like a tiny tadpole than a human.

The neural tube — baby's brain, spinal cord, nerves, and backbone — is starting to develop.

His heart and circulatory system begin to form. (This week, in fact, his tiny heart begins to divide into chambers and beat and pump blood.) Your baby's muscles, cartilage, bone, and subcutaneous (under skin) tissue also begin to form.

5 weeks: Your baby is about the size of a sesame seed

Week 6 – Month 2

The nose, mouth, and ears are beginning to take shape. His heart is beating about 100 to 160 times a minute — almost twice as fast as yours — and blood is beginning to course through his body. His intestines are developing, and the bud of tissue that will give rise to his lungs has appeared.

6 weeks: Your baby is about the size of a lentil

Week 7 – Month 2

Your baby’s hands and feet are emerging from developing arms and legs. Your baby is still considered an embryo and has something of a small tail, which is an extension of her tailbone. The tail will disappear within a few weeks,

Both hemispheres of your baby's brain are growing, and her liver is churning out red blood cells.

Your baby has doubled in size since last week and now measures half an inch long, about the size of a blueberry.

7 weeks: Your baby is about the size of a blueberry

Week 8 – Month 2

Your baby's hands and feet are developing webbed fingers and toes, his eyelids almost cover his eyes, breathing tubes extend from his throat to the branches of his developing lungs, and his "tail" is just about gone. In his brain, nerve cells are branching out to connect with one another.

8 weeks: Your baby is about the size of a kidney bean

How your baby's growing

Week 9 – Month 3

Your baby's heart finishes dividing into four chambers, and the valves start to form — as do her tiny teeth. Your baby's organs, muscles, and nerves are kicking into gear. Her eyes are fully formed, but her eyelids are fused shut and won't open until 27 weeks. She has tiny earlobes, and her mouth, nose, and nostrils are more distinct.

Your baby is nearly an inch long — about the size of a grape — and weighs just a fraction of an ounce.

9 weeks: Your baby is about the size of a grape

Week 10 – Month 3

This is the beginning of the so-called fetal period, a time when the tissues and organs in his body rapidly grow and mature.

He's swallowing fluid and kicking up a storm. Vital organs — including his kidneys, intestines, brain, and liver — are in place and starting to function.

From crown to rump, he's about 1 1/4 inches long. In the coming weeks, your baby will again double in size — to nearly 3 inches.

10 weeks: Your baby is about the size of a kumquat

Week 11 – Month 3

Her hands will soon open and close into fists, tiny tooth buds are beginning to appear under her gums, and some of her bones are beginning to harden.

Your baby, just over 1 1/2 inches long and about the size of a fig, is now almost fully formed

11 weeks: Your baby is about the size of a fig

Week 12 – Month 3

The most dramatic development this week: reflexes. Your baby's fingers will soon begin to open and close, his toes will curl, his eye muscles will clench, and his mouth will make sucking movements. In fact, if you prod your abdomen, your baby will squirm in response, although you won't be able to feel it.

From crown to rump, your baby-to-be is just over 2 inches long (about the size of a lime) and weighs half an ounce.

12 weeks: Your baby is about the size of a lime

Week 13 – Month 3

Your baby's tiny fingertips have fingerprints, her veins and organs are clearly visible through her still-thin skin, and her body is starting to catch up with her head — which makes up just a third of her body size now. If you're having a girl, she now has more than 2 million eggs in her ovaries.

Your baby is almost 3 inches long about the size of a (pea pod) and weighs nearly an ounce.

13 weeks: Your baby is about the size of a peapod

Week 14 – Month 4

Your baby can now squint, frown, grimace, pee, and possibly suck his thumb! His kidneys are producing urine, which he releases into the amniotic fluid around him.

He's starting to develop an ultra-fine, downy covering of hair, called lanugo, all over his body. Your baby's liver starts making bile this week and his spleen starts helping in the production of red blood cells.

From head to bottom, he measures 3 1/2 inches — about the size of a lemon — and he weighs 1 1/2 ounces.

14 weeks: Your baby is about the size of a

lemon

Week 15 – Month 4

She's busy moving amniotic fluid through her nose and upper respiratory tract, which helps the primitive air sacs in her lungs begin to develop. Although her eyelids are still fused shut, she can sense light. If you have an ultrasound this week, you may be able to find out whether your baby's a boy or a girl!

Your growing baby now measures about 4 inches long, crown to rump, and weighs in at about 2 1/2 ounces (about the size of an apple).

15 weeks: Your baby is about the size of an apple

Week 16 – Month 4

Your baby is about the size of an avocado: 4 1/2 inches long (head to rump) and 3 1/2 ounces. His heart is now pumping about 25 quarts of blood each day, and this amount will continue to increase as your baby continues to develop.

16 weeks: Your baby is about the size of an avocado

Week 17 – Month 4

Your baby's skeleton is changing from soft cartilage to bone. Your baby weighs 5 ounces now (about as much as a turnip), and she's around 5 inches long from head to bottom.

17 weeks: Your baby is about the size of a turnip

Week 18 – Month 5

A protective covering of myelin is beginning to form around his nerves, a process that will continue for a year after he's born.

Head to rump, your baby is about 5 1/2 inches long, and he weighs almost 7 ounces about the size of a (bell pepper).

18 weeks: Your baby is about the size of a bell pepper

Week 19 – Month 5

Your baby's sensory development is exploding! Her brain is designating specialized areas for smell, taste, hearing, vision, and touch. Some research suggests that she may be able to hear your voice now.

A waxy protective coating called the vernix caseosa is forming on her skin to prevent it from pickling in the amniotic fluid.

As for size, your baby weighs about 8 1/2 ounces and measures 6 inches, head to bottom — about the size of an heirloom tomato.

19 weeks: Your baby is about the size of an heirloom tomato

Week 20 – Month 5

He's swallowing more these days, which is good practice for his digestive system. He's also producing meconium, a black, sticky by-product of digestion. This gooey substance will accumulate in his bowels, and you'll see it in his first soiled diaper.

Your baby weighs about 10 1/2 ounces now. He's also around 6 1/2 inches long from head to bottom and about 10 inches from head to heel — about the size of a banana.

20 weeks: Your baby is about the size of a banana

Week 21 – Month 5

You may soon feel like she's practicing martial arts as her initial fluttering movements turn into full-fledged kicks and nudges.

Your baby now weighs about three-quarters of a pound and is approximately 10 1/2 inches long — about the size of a carrot.

21 weeks: Your baby is about the size of a carrot

Week 22 – Month 6

His lips, eyelids, and eyebrows are becoming more distinct, and he's even developing tiny tooth buds beneath his gums. His eyes have formed, but his irises (the colored part of the eye) still lack pigment.

At 11 inches and almost 1 pound (about the size of a spaghetti squash), your baby is starting to look like a miniature newborn.

22 weeks: Your baby is about the size of a spaghetti squash

Week 23 – Month 6

Blood vessels in her lungs are developing to prepare for breathing, and the sounds that your baby's increasingly keen ears pick up are preparing her for entry into the outside world.

She's more than 11 inches long and weighs just over a pound (about the size of a large mango)

23 weeks: Your baby is about the size of a large mango

Week 24 – Month 6

His brain is also growing quickly now, and his taste buds are continuing to develop. His lungs are developing "branches" of the respiratory "tree" as well as cells that produce

surfactant, a substance that will help his air sacs inflate once he hits the outside world.

He’s almost a foot long and almost 1 1/3 pounds.

24 weeks: Your baby is about the size of an ear of corn

Week 25 – Month 6

Your baby now measures about 13 1/2 inches and weights 1 1/2 pounds.

25 weeks: Your baby is about the size of a rutabaga

Week 26 – Month 6

The network of nerves in your baby's ears is better developed and more sensitive than before. He's inhaling and exhaling small amounts of amniotic fluid, which is essential for the development of his lungs. These so-called breathing movements are also good practice for when he's born and takes that first gulp of air.

He now weighs about 1 2/3 pounds and measures 14 inches (about the size of a scallion) from head to heel.

26 weeks: Your baby is about the size of a scallion

Week 27 – Month 7

She's sleeping and waking at regular intervals, opening and closing her eyes, and perhaps even sucking her fingers. With more brain tissue developing, your baby's brain is very active now. While her lungs are still immature, they would be capable of functioning — with a lot of medical help — if she were to be born now.

This week, your baby weighs almost 2 pounds (about the size of a head of cauliflower) and is about 14 1/2 inches long with her legs extended.

27 weeks: Your baby is about the size of a head of cauliflower

Week 28 – Month 7

She can blink her eyes, which now sport lashes. With her eyesight developing, she may be able to see the light that filters in through your womb. She's also developing billions of neurons in her brain and adding more body fat in preparation for life in the outside world.

By this week, your baby weighs 2 1/4 pounds (about the size of a large eggplant) and measures 14.8 inches from the top of her head to her heels.

28 weeks: Your baby is about the size of a large eggplant

Week 29 – Month 7

His muscles and lungs are continuing to mature, and his head is growing bigger to make room for his developing brain.

Your baby now weighs about 2 1/2 pounds (about the size of a butternut squash) and is a tad over 15 inches long from head to heel.

29 weeks: Your baby is about the size of a butternut squash

Week 30 – Month 7

Her eyesight continues to develop, though it's not very keen; even after she's born, she'll keep her eyes closed for a good part of the day. When she does open them, she'll respond to changes in light but will have 20/400 vision — which means she can only make out objects a few inches from her face. (Normal adult vision is 20/20.)

Your baby's about 15.7 inches long now and weighs almost 3 pounds (about the size of a large cabbage).

30 weeks: Your baby is about the size of a large cabbage

Week 31 – Month 8

He can turn his head from side to side, and his arms, legs, and body are beginning to plump out as needed fat accumulates underneath his skin.

Your baby measures over 16 inches long and weighs about 3 1/3 pounds.

31 weeks: Your baby is about the size of a coconut

Week 32 – Month 8

She'll gain a third to half of her birth weight during the next 7 weeks as she fattens up for survival outside the womb. She now has toenails, fingernails, and real hair.

Your baby weighs 3 3/4 pounds (about the size of a large jicama) and is about 16.7 inches long

32 weeks: Your baby is about the size of a jicama

Week 33 – Month 8

The bones in his skull aren't fused together, which allows them to move and slightly overlap, thus making it easier for him to fit through the birth canal.

This week your baby weighs a little over 4 pounds and has passed the 17-inch mark (about the size of a pineapple).

33 weeks: Your baby is about the size of a pineapple

Week 34 – Month 8

Her fat layers — which will help regulate her body temperature once she's born — are filling her out, making her rounder. Her skin is also smoother than ever. Her central nervous system is maturing and her lungs are continuing to mature as well. If you've been nervous about preterm labor, you'll be happy to know that babies born between 34 and 37 weeks who have no other health problems generally do fine. They may need a short stay in the neonatal nursery and may have a few short-term health issues, but in the long run, they usually do as well as full-term babies.

Your baby now weighs about 4 3/4 pounds (about the size of a cantaloupe) and is almost 18 inches long.

34 weeks: Your baby is about the size of cantaloupe

Week 35 – Month 8

His kidneys are fully developed now, and his liver can process some waste products. Most of his basic physical development is now complete — he'll spend the next few weeks putting on weight.

Your baby doesn't have much room to maneuver now that he's over 18 inches long and tips the scales at 5 1/4 pounds (about the size of a honeydew melon).

35 weeks: Your baby is about the size of a honeydew melon

Week 36 – Month 9

Your baby is still packing on the pounds — at the rate of about an ounce a day. She's shedding most of the downy covering of hair that covered her body as well as the vernix caseosa, the waxy substance that covered and protected her skin during her nine-month amniotic bath. Your baby swallows both of these substances, along with other secretions, resulting in a blackish mixture, called meconium, will form the contents of her first bowel movement

She now weighs almost 6 pounds and is more than 18 1/2 inches long (about the size of a head of romaine lettuce).

36 weeks: Your baby is about the size of a head of romaine lettuce

Week 37 – Month 9

Your due date is very close now, but doctors don't consider your baby "full term" until 39 weeks. Spending the next two weeks in the womb allows your baby's brain and lungs to fully mature.

At this point your baby weighs 6 1/3 pounds and measures a bit over 19 inches, head to heel (about the size of a bunch of Swiss chard).

37 weeks: Your baby is about the size of a bunch of Swiss chard

Week 38 – Month 9

Your baby has really plumped up. She has a firm grasp, which you'll soon be able to test when you hold her hand for the first time! Her organs have matured and are ready for life outside the womb.

She weighs about 6.8 pounds and she is over 19 1/2 inches long (about the size of a leek).

38 weeks: Your baby is about the size of a leek

Week 39 – Month 9

Your baby's full term this week and waiting to greet the world! He continues to build a layer of fat to help control his body temperature after birth.

It’s likely he already measures about 20 inches and weighs a bit over 7 pounds, about the size of a mini-watermelon.

39 weeks: Your baby is about the size of a mini-watermelon

Week 40 – Month 9

The average newborn weighs about 7 1/2 pounds (about the size of a small pumpkin) and is about 20 inches long.

40 weeks: Your baby is about the size of a small pumpkin