Fetal development by week: Your baby in the womb (2024)

Fetal development by week: Your baby in the womb (1)Fetal development by week: Your baby in the womb (2)

Medically reviewed by Layan Alrahmani, M.D., ob-gyn, MFM

Fetal development by week: Your baby in the womb (3)

Written by

Kate Marple | Feb 21, 2023

Nestled comfortably in the womb, your baby is constantly growing and changing. Use this timeline to learn what your baby's up to and how they're developing throughout pregnancy.

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

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 (called a zygote) 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.

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 releasing eggs.

Read about implantation.

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.

5 weeks

Your baby resembles a tadpole more than a human, but is growing fast. The circulatory system is beginning to form, and cells inthe 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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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.

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.

8 weeks

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

Your baby is the size of a kidney bean.

Read about your pregnancy at 8 weeks.

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.

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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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.

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.

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.

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

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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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.

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.

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.

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 theirnerves.

Your baby is the size of a bell pepper.

Read about your pregnancy at 18 weeks.

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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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.

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.

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.

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.

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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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.

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.

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.

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.

28 weeks

Your baby's eyesight is developing, which may enable them to sense light filtering in from the outside. They can blink, and their eyelashes have grown in.

Your baby is the size of a large eggplant.

Read about your pregnancy at 28 weeks.

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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.

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.

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.

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.

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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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.

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.

36 weeks

Your baby is gaining about an ounce a day. They're also losing most of their lanugo hair that covered their body, along with the vernix caseosa, a waxy substance that was protecting their skin until now.

Your baby is the size of a head of romaine lettuce.

Read about your pregnancy at 36 weeks.

37 weeks

Your due date is very close, and though your baby looks like a newborn, they're not consideredfull-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.

38 weeks

Are you curious about your baby's eye color? Their irises aren't fully pigmented at birth, so their eyes could change color up until they're about a year old.

Your baby is the size of a leek.

Read about your pregnancy at 38 weeks.

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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.

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.

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.

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.

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 our free childbirth class

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Fetal development by week: Your baby in the womb (4)

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  • How big is my baby? Week-by-week fruit and veggie comparisonsBy Kate Marple
  • Growth chart: Fetal length and weight, week by weekBy Karen Miles
  • Your first trimester pregnancy checklistBy Tahirah Blanding
  • Inside pregnancy: Weeks 1 to 9

BabyCenter's editorial team is committed to providing the most helpful and trustworthy pregnancy and parenting information in the world. When creating and updating content, we rely on credible sources: respected health organizations, professional groups of doctors and other experts, and published studies in peer-reviewed journals. We believe you should always know the source of the information you're seeing. Learn more about our editorial and medical review policies.

AAFP. 2020. Your baby's development: The first trimester. American Academy of Family Physicians.https://familydoctor.org/your-babys-development-the-first-trimester/Opens a new window[Accessed February 2023]

AAFP. 2021. Your baby's development: The second trimester. American Academy of Family Physicians.https://familydoctor.org/your-babys-development-the-second-trimester/Opens a new window[Accessed February 2023]

AAFP. 2020. Your baby's development: The third trimester. American Academy of Family Physicians.https://familydoctor.org/your-babys-development-the-third-trimester/Opens a new window[Accessed February 2023]

Mayo Clinic. 2022. Fetal development: The 2nd trimester.https://www.mayoclinic.org/healthy-lifestyle/pregnancy-week-by-week/in-depth/fetal-development/art-20046151Opens a new window[Accessed February 2023]

Mayo Clinic. 2022. Fetal development: The 1st trimester.https://www.mayoclinic.org/healthy-lifestyle/pregnancy-week-by-week/in-depth/prenatal-care/art-20045302Opens a new window[Accessed February 2023]

Mayo Clinic. 2022. Fetal development: The third trimester.https://www.mayoclinic.org/healthy-lifestyle/pregnancy-week-by-week/in-depth/fetal-development/art-20045997Opens a new window[Accessed February 2023]

MedlinePlus (ADAM). 2021. Fetal development.https://medlineplus.gov/ency/article/002398.htmOpens a new window[Accessed February 2023]

OWH. 2021. Stages of pregnancy. U.S. Office on Women's Health.https://www.womenshealth.gov/pregnancy/youre-pregnant-now-what/stages-pregnancyOpens a new window[Accessed February 2023]

Fetal development by week: Your baby in the womb (11)

Kate Marple

Kate Marple is a writer and editor who specializes in health, pregnancy, and parenting content. She's passionate about translating complicated medical information into helpful pregnancy and parenting advice that's easy to understand. She lives in San Francisco with her family.

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Fetal development by week: Your baby in the womb (2024)

FAQs

What is the most important week of fetal development? ›

In general, major birth defects of the body and internal organs are more likely to happen between 3 to 12 embryonic/fetal weeks. This is the same as 5 to 14 gestational weeks (weeks since the first day of your last period). This is also referred to as the first trimester.

How many weeks until a fetus is fully formed? ›

Almost all organs are completely formed by about 10 weeks after fertilization (which equals 12 weeks of pregnancy). The exceptions are the brain and spinal cord, which continue to form and develop throughout pregnancy. Most malformations (birth defects) occur during the period when organs are forming.

How much does a fetus grow each week? ›

Here are some highlights, based on estimations: Up until 16 weeks, a fetus grows an average of about 19 grams per week, gradually increasing from 7 grams per week at 8 weeks to 15 grams per week at 12 weeks and 29 grams per week at 16 weeks. By 20 weeks, a fetus is gaining about 59 grams per week (just over 2 ounces).

How do I know if my fetus is growing enough? ›

How Is IUGR Diagnosed?
  1. fetal monitoring to track the baby's heart rate and movements.
  2. screening the mother for infections that could affect the baby.
  3. amniocentesis to look for genetic causes of IUGR (and sometimes to help them see how mature the baby's lungs are)

Which weeks of pregnancy are most critical? ›

The fetus is most vulnerable during the first 12 weeks. During this period of time, all of the major organs and body systems are forming and can be damaged if the fetus is exposed to drugs, infectious agents, radiation, certain medications, tobacco and toxic substances.

What are the important stages of fetal development? ›

Important fetal development milestones at each stage of prenatal development include: Germinal stage: The division of cells and implantation of the blastocyst. Embryonic stage: The development of the neural tube and organs. Fetal stage: Continued growth of organs and physical development in preparation for birth.

What is the last organ to develop in a fetus? ›

The lungs are the last major organ to finish developing. When fully mature, they produce a chemical that affects the hormones in your body.

What is the first organ to develop in a fetus? ›

The first organ system to develop during organogenesis is the cardiovascular system. The heart has established its four chambers by four weeks of development, whereas week six involves cardiac outflow separation and descent of the heart (and lungs) into the thorax.

How to know baby is healthy in womb without ultrasound? ›

Top Signs of Healthy Pregnancy
  • Gaining Healthy Weight: During pregnancy, gaining weight is a natural and essential part of the process as the body undergoes significant changes to support the growth and development of the baby. ...
  • Sore and Enlarged Breasts: ...
  • Increased Vaginal Discharge: ...
  • Right growth of fetus and belly:
Jul 25, 2023

How am I 4 weeks pregnant if I conceived 2 weeks ago? ›

Week 4 of pregnancy

For example, a fertilised egg may have implanted in your womb just 2 weeks ago, but if the first day of your last period was 4 weeks ago, this means you're officially four weeks pregnant! Pregnancy normally lasts from 37 weeks to 42 weeks from the first day of your last period.

Can ultrasound be wrong about weeks? ›

As pregnancy progresses, the accuracy of an ultrasound for predicting due dates decreases. Between 18 and 28 weeks of gestation, the margin of error increases to plus or minus two weeks. After 28 weeks, the ultrasound may be off by three weeks or more in predicting a due date.

What causes your baby not to grow fast enough in womb? ›

Your baby may be small for lots of reasons, such as: your placenta isn't giving them what they need to grow. they have a health condition which is stopping them from growing. your diet's lacking in healthy food.

What happens if the baby is not growing during pregnancy? ›

But if your baby is not growing at all or has other problems, your doctor may decide that an early delivery could help. In this case, your doctor may want to induce labor. Your baby's heart rate and movements will be closely watched to help you and your doctor make this decision.

Why is 37 weeks important? ›

Your baby's lungs, liver, and brain go through a crucial period of growth between 37 weeks and 39 weeks of pregnancy. Waiting until 39 weeks, now called "full term," gives your baby the best possible chance for a healthy start in life.

Why is week 20 important? ›

Key Takeaways at 20 Weeks Pregnant

Yes, you can find out baby's sex at this appointment (if you want). More importantly, an ultrasound tech will look at baby's organs, take measurements and assess that baby is developing appropriately. Your doctor or midwife will probably start measuring your fundal height soon.

What is the most important predictor of fetal well being? ›

FHR baseline variability, defined as variation of successive beats in the FHR, is central to the assessment of fetal well-being. It is an important index of cardiovascular function and is regulated by the fetal autonomic nervous system.

Why is 24 weeks so important? ›

By the time you're 24 weeks pregnant, the baby has a chance of survival if they are born. Most babies born before this time cannot live because their lungs and other vital organs are not developed enough. The care that can now be given in baby (neonatal) units means more and more babies born early do survive.

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