Fetal development week by week
Follow your baby’s development week by week, from conception to labor, in these amazingly detailed, doctor-reviewed images.
Illustrations by 3 Dart LLC
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 he presses on nerves in your hips and back.
The so-called honeymoon phase of pregnancy is over and parenthood is visible on the horizon has begun. 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.
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.
Your baby is the size of a large eggplant.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Visit babycenter.com for advice and support on pregnancy and parenting