A fetus changes dramatically from week to week, starting as a tiny curved shape no bigger than a grain of rice and gradually developing into a recognizable human form. The transformation from a featureless cluster of cells to a baby with fingernails, eyelashes, and chubby limbs happens across roughly 40 weeks, with the most striking visual changes occurring in the first trimester.
Weeks 5 Through 8: The Embryo Stage
Before week 9, the developing organism is technically called an embryo, not a fetus. At week 5, it’s about the size of a sesame seed and looks nothing like a baby. By week 7, many people think it resembles a small tadpole or seahorse because of its prominent tail and oversized head. That tail recedes over the following weeks.
Around week 6, tiny limb buds appear where arms and legs will eventually grow, and the earliest structures for ears, eyes, and mouth start to take form. By week 8, the embryo has grown to roughly the size of a raspberry. The eyes become visible and ears begin to form, but the hands and feet still look webbed, with no distinct fingers or toes yet. A heartbeat is typically detectable on a transvaginal ultrasound by six completed weeks of pregnancy.
Weeks 9 Through 12: Taking Human Shape
Week 9 marks the official transition from embryo to fetus, and this is when the appearance starts shifting fast. By week 10, the arms, hands, fingers, feet, and toes are fully formed, with the webbing between digits completely gone. The fetus is roughly the size of a strawberry.
By week 11, facial features become more prominent. The head is still disproportionately large compared to the body, making up nearly half the total length. By the end of the first trimester at week 12, the fetus has distinct facial features, including a nose, chin, and forehead. It measures about 2 to 3 inches long and weighs around half an ounce. On a standard 2D ultrasound (which produces a flat, cross-sectional image), you can see the profile of the head, the curve of the spine, and the movement of tiny limbs.
External sex organs also begin developing during this period. The male phallus can be visualized on ultrasound as early as 10 to 11 weeks, though most providers wait until a later anatomy scan for confirmation.
Weeks 13 Through 27: The Second Trimester
The second trimester is when the fetus starts looking unmistakably human. The body catches up to the head in proportion, and the face fills in with finer details. Around week 15, the female labia become visible on imaging. By week 22, eyebrows and hair are visible. Around week 23, ridges form on the palms and soles of the feet, creating the foundation for unique fingerprints and footprints.
Between weeks 16 and 20, a soft, feathery layer of hair called lanugo develops across the skin. It looks like a thin coat of delicate fur and serves a practical purpose: it helps a waxy, cheese-like coating (called vernix) stick to the skin, which protects the fetus from the amniotic fluid. Together, the lanugo and vernix give the fetus a somewhat greasy, fuzzy appearance during this stage.
By week 26, eyebrows and eyelashes have formed, and the eyes are fully developed, though they typically don’t open for another couple of weeks. The skin at this point is still thin, wrinkled, and somewhat translucent, with blood vessels visible beneath the surface. The fetus weighs roughly 1.5 to 2 pounds and is around 14 inches long.
What You See on Ultrasound
Most pregnant people get a detailed anatomy scan around weeks 18 to 20. A standard 2D ultrasound creates a flat, black-and-white cross-sectional view. It shows outlines and structures clearly but doesn’t look like a photograph. A 3D ultrasound stitches together many 2D images from different angles to create a picture that looks much more like a typical photo, showing surface details like the nose, lips, and closed eyelids. A 4D ultrasound adds movement to the 3D image, so you can see the fetus kicking, yawning, or opening and closing its eyes in real time.
Weeks 28 Through 40: Filling Out
The third trimester is less about forming new structures and more about gaining weight and looking increasingly like a newborn. At the start of this period, the fetus still appears lean and wrinkled because there isn’t much fat beneath the skin. Over the next several weeks, fat deposits build up steadily across the body.
By week 36, the skin becomes noticeably smoother as more fat is added underneath it, and the limbs start to look chubby. The lanugo hair mostly sheds during this time, though some babies are born with patches of it still clinging to their shoulders or back. The vernix coating also begins to thin out.
At week 39, fat continues to accumulate all over the body. This layer serves a critical function after birth: keeping the baby warm. By this point, the fetus typically weighs 6 to 9 pounds, measures 19 to 21 inches long, and looks like the round, plump newborn most people picture. The skin has shifted from translucent and reddish to its near-final tone, the fingernails extend to the tips of the fingers, and the eyes can open and close freely.
Size Comparisons Week by Week
It can be hard to grasp how small a fetus is, especially in early pregnancy. Here are some common size comparisons that help put things in perspective:
- Week 5: sesame seed (about 2 mm)
- Week 8: raspberry (about half an inch)
- Week 12: lime (about 2 to 3 inches)
- Week 16: avocado (about 4.5 inches)
- Week 20: banana (about 6.5 inches, head to rump)
- Week 28: large eggplant (about 14 to 15 inches total)
- Week 36: head of romaine lettuce (about 18 inches)
- Week 40: small watermelon (about 19 to 21 inches)
These are averages, and individual variation is completely normal. The growth curve is steepest in the second and third trimesters, when the fetus gains several inches and multiple pounds per month.