A baby in the womb changes dramatically from week to week, starting as a tiny cluster of cells smaller than a grain of rice and ending as a plump, fully formed newborn. In the earliest weeks, the embryo looks nothing like a baby at all. By the end of pregnancy, it looks remarkably like the infant you’ll hold at birth. Here’s what’s happening at each stage.
Weeks 6 to 8: The Tadpole Stage
At six weeks, the embryo is only a few millimeters long and has small buds where arms will eventually grow. By week seven, it has a prominent tail and an oversized head, giving it the appearance of a tiny tadpole or seahorse. Lower limb buds appear, the earliest traces of nostrils become visible, and the retinas of the eyes begin forming. The arm buds take on a paddle-like shape.
Week eight marks a visible leap. The leg buds also become paddle-shaped, fingers start to form, and the eyes become noticeable. Small swellings outline what will become the outer ears, and the upper lip and nose take shape. The hands and feet still have a webbed look at this point, with skin connecting the tiny digits. Despite all this progress, the embryo is still only about the size of a raspberry.
Weeks 9 to 12: Starting to Look Human
This stretch is when the embryo begins resembling something recognizably human. At nine weeks, arms have grown longer and elbows appear. Toes become visible, and eyelids start to form. By week ten, the head has become rounder, the elbows can bend, and the webbing between fingers and toes disappears as they lengthen individually.
At eleven weeks, the face is broad with widely spaced eyes, fused eyelids, and ears set low on the sides of the head. Tooth buds appear beneath the gums, and the external genitals begin developing, though they’re not yet distinct enough to identify on an ultrasound. By week twelve, fingernails are sprouting, the face has a more defined profile, and all the organs, limbs, bones, and muscles are present. They’ll spend the rest of pregnancy growing and maturing, but the basic blueprint is complete.
The Skeleton: From Cartilage to Bone
Early on, the skeleton is made entirely of soft cartilage, which is why the embryo appears so flexible and translucent in early imaging. Bone formation begins between the sixth and seventh weeks, but it’s a slow, gradual replacement. Throughout pregnancy, cartilage is steadily converted into harder bone tissue. By birth, the majority of cartilage has been replaced, though the process continues into a person’s mid-twenties. This is why a newborn’s skull still has soft spots: those areas haven’t fully hardened yet, which allows the head to compress slightly during delivery.
Eyelids: Sealed Shut for Months
One of the more surprising details of fetal development is what happens with the eyes. At the start of week eight, there’s still a gap between the developing upper and lower eyelid folds, and the cornea is exposed. Specialized skin cells then migrate inward from both eyelid rims and fuse together, sealing the eyes shut by the end of week eight.
The eyelids stay fused for months. Separation begins somewhere between weeks 16 and 20, with most sources placing it around week 20. This means that for roughly half of pregnancy, the baby’s eyes are sealed closed while the visual structures behind them continue to develop.
Weeks 16 to 24: Skin, Hair, and Fingerprints
Between weeks 16 and 20, the fetus develops a fine, downy hair called lanugo that covers most of the body. This hair serves a specific purpose: it holds a waxy, white coating called vernix in place against the skin. Without this system, the amniotic fluid surrounding the baby would irritate and damage its extremely sensitive skin. The vernix acts as a waterproof barrier and insulator, and the lanugo keeps it from washing away.
During this same window, fingerprints are permanently configured. By week 20, the unique ridge patterns on each fingertip are set for life. The skin itself is still thin and somewhat translucent at this stage, with blood vessels visible beneath the surface. Hair follicles on the scalp are developing and beginning to produce pigment, though the color and amount of hair at birth varies enormously from baby to baby.
The Third Trimester: Filling Out
The third trimester is when a fetus transforms from a lean, wrinkled figure into the round, smooth baby most people picture. Fat accumulation accelerates early in the third trimester, around 28 to 30 weeks, with soft tissue volume increasing rapidly. This is especially noticeable in the limbs, which start to look chubby as fat pads build up beneath the skin.
By 36 weeks, the skin has become smooth as the fat layer thickens, and the wrinkled appearance of earlier months fades. The limbs are visibly plump. At 39 weeks, fat is still being added across the body. This isn’t just cosmetic: that fat layer is critical for temperature regulation after birth, since newborns lose heat quickly and need insulation to stay warm outside the womb.
Most of the lanugo hair sheds during the final weeks, though some babies are born with patches of it still visible on their shoulders, back, or ears. The vernix coating also thins, though babies born earlier tend to have more of it on their skin at delivery.
What Ultrasounds Actually Show
If you’re picturing all of this because you’ve seen (or are about to see) an ultrasound, it helps to know what you’re looking at. Early ultrasounds, around 8 to 10 weeks, show a small, bean-shaped form with a flickering heartbeat. The head is disproportionately large compared to the body, and limbs are tiny stubs. By the anatomy scan at around 20 weeks, the profile is clearly human: you can see the nose, lips, fingers, toes, and often the baby opening and closing its mouth or bringing a hand to its face.
Third-trimester 3D ultrasounds give the most recognizable images, showing rounded cheeks, a button nose, and sometimes even facial expressions. These images are the closest representation of what your baby actually looks like in the womb, though the golden-brown coloring in 3D ultrasounds is an artificial rendering. In reality, the baby is surrounded by clear amniotic fluid in a dark environment, and its skin color at birth is determined by genetics and blood flow rather than anything visible on a scan.