Fetal Development and Growth during 41 weeks of pregnancy

From conception to labor, your baby is constantly growing and developing. Your baby moves through different stages, starting as a blastocyst, then maturing into an embryo, and then a fetus. Around the 5 week mark, cells in your baby’s future heart will begin to flicker. At 27 weeks they’ll have regular sleep and wake cycles, and at 39 weeks your baby is physically developed. Use this timeline to learn how big your baby is, plus how they’re developing throughout pregnancy.

fetal growth week by week

Follow your baby’s development week by week, from conception to labor, in these amazingly detailed, doctor-reviewed images.

an illustration of spermatozoa approaching the egg

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. 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.

Read about fertilization.

illustration of third week pregnancy implantantion

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 of cells, called a blastocyst, has begun to produce the pregnancy hormone hCG , which tells your ovaries to stop releasig eggs.

Read about implantation.

illustration of the early stage embryo

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.

Your baby is the size of a poppy seed.

Read about your pregnancy at 4 weeks.

illustration of an embryo

5 weeks

Your baby resembles a tadpole more than a human, but is growing fast. The circulatory system is beginning to form, and cells in the tiny “heart” will start to flicker this week.

Your baby is the size of a sesame seed.

Read about your pregnancy at 5 weeks.

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illustration of an embryo

6 weeks

Your baby’s nose, mouth and ears are starting to take shape, and their intestines and brain are beginning to develop.

Your baby is the size of a lentil.

Read about your pregnancy at 6 weeks.

embryo

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.

Read about your pregnancy at 7 weeks.

embryo

8 weeks

Your baby has started moving around, though you won’t feel your baby move yet. Nerve cells are branching out, forming primitive neural pathways. Breathing tubes now extend from their throat to their developing lungs.

Your baby is the size of a kidney bean.

Read about your pregnancy at 8 weeks.

embryo with his head down

9 weeks

Your baby’s basic anatomy is developing (they even have tiny earlobes now), but there’s much more to come. Their embryonic tail has disappeared and they weigh just a fraction of an ounce but are about to start gaining weight fast.

Your baby is the size of a grape.

Read about your pregnancy at 9 weeks.

illustration of a fetus in a womb

10 weeks

Your embryo has completed the most critical portion of development. Their skin is still translucent, but their tiny limbs can bend and fine details like nails are starting to form.

Your baby is the size of a kumquat

Read about your pregnancy at 10 weeks.

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illustration of a fetus in a womb

11 weeks

Your baby is almost fully formed. They’re kicking, stretching, and even hiccupping as their diaphragm develops, although you can’t feel any activity yet.

Your baby is the size of a fig.

Read about your pregnancy at 11 weeks.

illustration of a fetus in a womb

12 weeks

This week your baby’s reflexes kick in: Their fingers will soon begin to open and close, toes will curl, and their mouth will make sucking movements.

Your baby is the size of a lime.

Read about your pregnancy at 12 weeks.

illustration of a fetus in a womb

13 weeks

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

Your baby is the size of a pea pod.

Read about your pregnancy at 13 weeks.

illustration of woman's belly during 13th week of pregnancy

Entering the second trimester: What lies ahead

In this illustration, you can see how big – and yet, how tiny still – your baby is as you begin your second trimester.

After the first trimester, a miscarriage is much less likely. And for many moms-to-be, early pregnancy symptoms like morning sickness and fatigue have faded away. If you’re feeling more energetic now and haven’t been exercising, it’s a good time to start a regular pregnancy fitness routine.

Plus: See our ultimate pregnancy to-do list for the second trimester

illustration of a fetus in a womb

14 weeks

Your baby’s brain impulses have begun to fire and they’re using their facial muscles. Their kidneys are working now, too. If you have an ultrasound, you may even see them sucking their thumb.

Your baby is the size of a lemon.

Read about your pregnancy at 14 weeks.

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illustration of a fetus in a womb

15 weeks

Your baby’s eyelids are still fused shut, but they can sense light. If you shine a flashlight on your tummy, they’ll move away from the beam. Ultrasounds done this week may reveal your baby’s sex.

Your baby is the size of an apple.

Read about your pregnancy at 15 weeks.

illustration of a fetus in a womb

16 weeks

The patterning on your baby’s scalp has begun, though their hair isn’t visible yet. Their legs are more developed, their head is more upright, and their ears are close to their final position.

Your baby is the size of an avocado.

Read about your pregnancy at 16 weeks.

illustration of a fetus in a womb

17 weeks

Your baby can move their joints, and their 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.

Read about your pregnancy at 17 weeks.

illustration of a fetus in a womb

18 weeks

Your baby is flexing their arms and legs, and you may be able to feel those movements. Internally, a protective coating of myelin is forming around their nerves.

Your baby is the size of a bell pepper.

Read about your pregnancy at 18 weeks.

illustration of a fetus in a womb

19 weeks

Your baby’s senses – smell, vision, touch, taste and hearing – are developing and they may be able to hear your voice. Talk, sing or read out loud to them, if you feel like it.

Your baby is the size of an heirloom tomato.

Read about your pregnancy at 19 weeks.

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illustration of a fetus in a womb

20 weeks

Your baby can swallow now and their digestive system is producing meconium, the dark, sticky goo that they’ll pass in their first poop – either in their diaper or in the womb during delivery.

Your baby is the size of a banana.

Read about your pregnancy at 20 weeks.

illustration of a fetus in a womb

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 their activity.

Your baby is the size of a carrot.

Read about your pregnancy at 21 weeks.

illustration of a fetus in a womb

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 their eyes isn’t present yet.

Your baby is the size of a spaghetti squash.

Read about your pregnancy at 22 weeks.

illustration of a fetus in a womb

23 weeks

Your baby’s ears are getting better at picking up sounds. After birth, they may recognize some noises outside the womb that they’re hearing inside now.

Your baby is the size of a large mango.

Read about your pregnancy at 23 weeks.

illustration of a fetus in a womb

24 weeks

Your baby cuts a pretty long and lean figure, but chubbier times are coming. Their skin is still thin and translucent, but that will begin to change soon too.

Your baby is the size of an ear of corn.

Read about your pregnancy at 24 weeks.

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illustration of a fetus in a womb

25 weeks

Your baby’s wrinkled skin is starting to fill out with baby fat, making them look more like a newborn. Their hair is beginning to come in, and it has color and texture.

Your baby is now the same weight as an average rutabaga.

Read about your pregnancy at 25 weeks.

illustration of a fetus in a womb

26 weeks

Your baby is now inhaling and exhaling amniotic fluid, which helps develop their 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.

Read about your pregnancy at 26 weeks.

illustration of a fetus in a womb

27 weeks

This is the last week of your second trimester. Your baby now sleeps and wakes on a regular schedule, and their brain is very active. Their 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.

Read about your pregnancy at 27 weeks.

illustration of woman's belly during 27th week of pregnancy

Entering the third trimester: What lies ahead

In this illustration, you’ll notice that your growing baby takes up quite a bit of room these days. In the third trimester, you might be peeing more often or have leg cramps as they press on nerves in your hips and back.

Now’s the time to do things like sign up for a childbirth class, choose a doctor for your baby, and create a baby registry, if you haven’t done so already.

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illustration of a fetus in a womb

29 weeks

Your baby’s muscles and lungs are busy getting ready to function in the outside world, and their head is growing to make room for their developing brain.

Your baby is the size of a butternut squash.

Read about your pregnancy at 29 weeks.

illustration of a fetus in a womb

30 weeks

Your baby is surrounded by a pint and a half of amniotic fluid, although there will be less of it as they grow and claim more space inside your uterus.

Your baby is the size of a large cabbage.

Read about your pregnancy at 30 weeks.

illustration of a fetus in a womb

31 weeks

Your baby can now turn their head from side to side. A protective layer of fat is accumulating under their skin, filling out their arms and legs.

Your baby is the size of a coconut.

Read about your pregnancy at 31 weeks.

illustration of a fetus in a womb

32 weeks

You’re probably gaining about a pound a week now. Half of that goes straight to your baby, who will gain one-third to half their birth weight in the next seven weeks in preparation for life outside the womb.

Your baby is the size of a large jicama.

Read about your pregnancy at 32 weeks.

illustration of a fetus in a womb

33 weeks

The bones in your baby’s skull aren’t fused yet. That allows them to shift as their head squeezes through the birth canal. They won’t fully fuse until adulthood.

Your baby is the size of a pineapple.

Read about your pregnancy at 33 weeks.

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illustration of a baby in the womb

34 weeks

Your baby’s central nervous system is maturing, as are their 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.

Read about your pregnancy at 34 weeks.

illustration of a baby in the womb

35 weeks

It’s getting snug inside your womb – but you should still feel your baby moving as much as ever. Your baby’s kidneys are fully developed, and their liver can process some waste products.

Your baby is the size of a honeydew melon.

Read about your pregnancy at 35 weeks.

illustration of a baby in the womb

37 weeks

Your due date is very close, and though your baby looks like a newborn, they’re not considered full-term until 39 weeks. Over the next two weeks, their lungs and brain will continue to mature.

Your baby is the size of a bunch of Swiss chard.

Read about your pregnancy at 37 weeks.

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illustration of a baby in mothers belly

Turning full term

At 39 weeks, your baby will be considered full-term. In the illustration, you can see the mucus plug sealing your uterus and how squished your intestines are now.

illustration of a baby in the womb

39 weeks

Your baby’s physical development is complete, but they’re still busy putting on fat and growing bigger.

Your baby is the size of a mini watermelon.

Read about your pregnancy at 39 weeks.

illustration of a baby in the womb

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.

Your provider will continuously assess your pregnancy to make sure you can safely continue your pregnancy.

Your baby is the size of a small pumpkin.

Read about your pregnancy at 40 weeks.

illustration of a baby in the womb

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. They may perform a non-stress test (NST) to monitor your baby’s fetal heart rate and your contractions to make sure your baby isn’t in any distress.

Read about your pregnancy at 41 weeks.

illustration of a woman giving birth

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:

Learn the signs of labor and stages of labor

Read when to go to the hospital for labor

Take childbirth class

Source: babycenter.com

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