Fetal Development Week by Week

If you’ve just found out you’re pregnant, congratulations! There's a lot to look forward to from the moment of conception until the day you meet your baby. In this article, we cover the highlights of your little one’s prenatal development, trimester by trimester and week by week.

If you’re in the early stages of pregnancy and not sure about your due date or how far along you currently are, talk to your doctor. You’ll just need to know either the date you conceived or the date of the first day of your last menstrual period.

Stages of Fetal Development Week by Week

Exciting times are ahead during the early days and weeks of your pregnancy. Whether you’ve been anticipating becoming pregnant or it’s a sweet surprise, keep in mind that it may be several weeks after you conceive before you even suspect you may be pregnant. Take our quiz to find out how much you already know about the first signs of pregnancy.

First Trimester

This is when your exciting pregnancy journey begins. Below you'll find a basic fetal development timeline starting with the first trimester. During this phase, your baby will grow from a microscopic cluster of cells to a plum-sized fetus with early facial features that might just be starting to look a little like yours.

Weeks 1-3

• Your pregnancy "starts" on the last day of your menstrual period.
• Ovulation takes place at about the midpoint of your cycle.
• The sperm meets the egg and fertilization takes place.
• The placenta begins to form, and will help nourish your baby until birth.

Week 4

• The fertilized egg implants in the uterus and begins to develop into an embryo.
• The umbilical cord begins to form from the placenta.
• Your little one is about the size of a poppy seed.

Week 5

• Big steps are under way as the neural tube (which eventually forms the brain, backbone, and spinal cord) develops.
• The lungs and heart begin to form, and the heart may start to beat. You might get to hear it via ultrasound sometime between week 6 and 12.
• The amniotic sac has formed and is filling with amniotic fluid, which will help protect your baby throughout your pregnancy.
• This might be when you suspect you’re pregnant — time to take a home pregnancy test and schedule a visit with your healthcare provider.
• The embryo is about the size of an orange seed.

Week 6

• The embryo is now huddled up in a curved C-shape.
• Little buds are appearing that will become arms and legs.
• The embryo is about the size of a pomegranate seed.

Week 7

• The brain and face are growing.
• Eyelids and bones start to form.
• Nostrils and retinas begin to develop.
• The embryo is about the size of a blueberry.

Week 8

• All major organs have formed and are growing — the building blocks are in place.
• Tubes connect the throat with the lungs to allow for future breathing.
• By the end of this week, your little one is no longer an embryo and is instead called a fetus, medically speaking, that is.
• The fetus is now about the size of a raspberry.

Week 9

• Little facial features form, including a more prominent nose.
• The head is large in proportion to the body, but your little one now has more of a mini-baby shape.
• The fetus is about the size of a cherry.

Week 10

• The head is rounding out.
• Eyelids and ears continue to develop — someone’s getting ready to see and hear you.
• Your little one is about the size of a strawberry.

Week 11

• Muscle formation begins.
• The backbone is still soft and flexible, but other bones begin to harden.
• Red blood cell production begins in the liver.
• The head still makes up half of your baby’s body length.
• The fetus is about the size of a Brussels sprout.

Week 12

• Your little one is moving, but you won’t feel it quite yet.
• Hands and arms are more developed than the lower limbs.
• Facial features are becoming more defined. Your little one might just be growing into your nose or your partner’s chin.
• If you have a Doppler exam this week, you might hear a heartbeat.
• The fetus is about as big as a passion fruit.

Week 13

• The sex hormones, testosterone and estrogen, are being produced.
• Your little one can now produce urine after swallowing amniotic fluid.
• All organs are fully formed and growing steadily.
• The average fetus is the size of a large plum.

Second Trimester

The second trimester is full of exciting prenatal development milestones, one being feeling your baby move for the first time. You might also find out your baby’s gender this trimester, if you want to. If you do find out whether you’re expecting a little girl or boy, and you want to share the good news with family and friends, you might want to have a gender reveal party to let the cat out of the bag.

Week 14

• Your little one can now move hands to mouth.
• The senses of taste and smell are starting to develop.
• The skin is beginning to thicken.
• The neck and lower limbs are becoming more defined.
• The fetus is now about the size of a nectarine.

Week 15

• The fetus can now roll and flip.
• That tiny heart pumps up to 100 pints of blood every day.
• Hair follicles have started to develop.
• The fetus is now about the size of a small grapefruit.

Week 16

• The digestive system starts working.
• The fetus can now hear some sounds, so keep talking and singing.
• Your little one could be the size of an apple.

Week 17

• The glands may start to produce vernix, a greasy substance that protects the skin in the womb.
• The eyes are now able to move slowly.
• Toenails are starting to grow.
• The fetus is now roughly the size of a pear.

Week 18

• Fine hair known as lanugo begins to cover that little body.
• The fetus may wake from sleep in response to your movements.
• Your little one is about the size of a sweet potato.

Week 19

• Kicks and movements may be getting stronger.
• Ovaries have developed in girls and testes have formed in boys.
• The fetus may be able to (adorably) suck a thumb.
• Your little one is about the size of a mango.

Week 20

• Some facial features and external genitals may be visible via ultrasound.
• Your baby is producing meconium, which is the waste that will become his first poop after birth.
• His fingernails are growing.
• The average baby is about the size of a bell pepper.

Week 21

• Fingers and toes have completed their development, ready to be counted at birth.
• The rhythmic movements you feel may be the baby hiccupping.
• She may have a favorite sleeping position.
• Your baby is about the size of a banana.

Week 22

• The eyes have been able to move for a few weeks, but eyelids are still fused shut.
• Your baby’s hearing becomes more acute; he may move suddenly when he hears loud noises.
• His eyebrows are growing.
• Your baby is now about the size of a papaya.

Week 23

• Your baby might now respond to the sound of your voice by moving.
• She spends much of her time in the rapid eye movement (REM) phase of sleep.
• Your baby is approximately the size of an eggplant.

Week 24

• Fetal lung development is under way. The lungs have formed but are not functional just yet.
• The skin is thin and wrinkled, and blood vessels are visible.
• The average baby is as big as an ear of corn.

Week 25

• Your baby’s nervous system is developing quickly.
• She's gaining weight and adding fat, filling out those chubby cheeks you’ll soon get to squeeze.
• Your baby could be about the size of an acorn squash this week.

Week 26

• Your baby starts to produce melanin, the pigment that gives skin its color.
• Fetal lung development continues as the lungs start producing surfactant to help them inflate properly.
• Your little one is about the size of a zucchini.

Week 27

• Your little one can now make grasping motions.
• Hearing your voice is calming for her; her heart rate may decrease when you talk or sing to her.
• Your baby may kick and stretch more often.
• Your baby is now about the size of a head of cauliflower.

Third Trimester

Time flies, and you and your little one have made it to the third trimester. It will be over before you know it, so get your hospital bag packed with all the essentials you, your birth partner, and baby will need. Here's what's happening with your baby’s development this trimester.

Week 28

• His eyes can now open and close.
• Breathing is becoming more rhythmic.
• He may start to grow eyelashes this week.
• The average baby is the size of a head of lettuce.

Week 29

• Kicks and movements may be more noticeable.
• Download our fetal movement tracker to help make counting kicks easier.
• Your baby might be as big as a butternut squash.

Week 30

• Fine body hair called lanugo may begin to shed.
• Your baby may have hair on his head.
• Bone marrow starts to produce red blood cells, which help transport oxygen.
• Your baby is now about the size of a head of cabbage.

Week 31

• Your baby is now able to regulate her own body temperature.
• Bones begin to harden.
• The skull is still soft and pliable, and won’t harden completely until your baby is around 2 years old.
• Your baby is now about the size of a coconut.

Week 32

• Toenails may be visible this week — it might be time for a little pedicure shortly after birth.
• Your baby is adding fat, and his skin is now less transparent.
• He’s approximately the size of a Napa cabbage.

Week 33

• Your baby will likely gain about a half a pound each week from now until birth.
• Brain development continues, and all five senses are working.
• Your baby is now about the size of a pineapple.

Week 34

• Your baby might shift to a head-down position as he prepares for birth.
• Fingernails continue to grow.
• Your baby continues to plump up, adding fat.
• He’s now about the size of a cantaloupe.

Week 35

• Fetal brain development continues; other parts of the nervous system and the lungs are also still developing.
• As your baby puts on fat, her skin becomes more pinkish, and less transparent.
• The musculoskeletal system has fully developed to enable movement after birth.
• Your baby could be as big as a honeydew melon.

Week 36

• He’s taking up more space in the amniotic sac.
• Fetal kicks, rolls, and other movements continue.
• Your little one is about the size of a head of romaine lettuce.

Week 37

• Your baby is still growing and is considered to be early term.
• Body hair called lanugo has almost been entirely shed.
• Your baby has added nearly enough fat to keep her warm after birth.
• The average baby is as big as a bunch of Swiss chard.

Week 38

• Liver and lung development are nearly complete.
• The head and abdomen may have about the same circumference.
• He may start to drop further into your pelvis.
• Your baby is about the size of a bunch of rhubarb.

Week 39

• Your baby is now officially full term.
• Fetal brain development continues, and her brain will keep growing well into early childhood.
• Your baby is about the size of a watermelon.

Week 40

• In terms of development, your baby is ready to be born.
• His length will likely be between 18 and 20 inches.
• The average newborn weighs between six and nine pounds, about as big as a pumpkin.

Forty weeks may seem like a long time to wait to meet your little one, but that day will be here before you know it. As you can see, your baby will be very busy in the weeks and months before the big debut. In the meantime, you’ll have plenty to do to get ready for the big day.