At three months (about 12 weeks), a fetus is roughly 5.4 centimeters long from head to bottom, around the size of a plum. It has a recognizably human shape, with a distinct head, body, arms, and legs, though the head still makes up about a third of the total length. All major organs, limbs, bones, and muscles are present by this point, and the remaining months of pregnancy are devoted to growth and maturation rather than forming new structures.
Size and Proportions
The 5.4-centimeter measurement is crown-to-rump, meaning head to bottom, not including the legs. If you stretched the legs out, the total length would be a bit longer. The fetus weighs roughly 14 grams at this stage, about half an ounce. Its head is still disproportionately large compared to the body, giving it a top-heavy appearance that gradually evens out over the second trimester.
Facial Features
By 12 weeks, the face looks distinctly human. The eyes, which started forming around week six, have moved from the sides of the head toward the front and are now covered by fused eyelids that won’t open again until around week 28. The ears have taken on their external shape, sitting low on the sides of the head. A small nose, upper lip, and mouth are clearly defined. The facial features become noticeably more prominent between weeks 11 and 12, though the face is still very small and the forehead remains large relative to everything below it.
Arms, Legs, Fingers, and Toes
All four limbs are fully formed with individual fingers and toes that have separated from the webbed paddle shape of earlier weeks. Tiny fingernails and toenails have started to develop by around week 10. The arms and legs can move independently. The fetus can kick with enough force to shift its position, and its hands regularly touch its head, face, and sometimes its mouth. These movements are too small for you to feel yet, but they’re happening frequently throughout the day.
Skin and Early Hair
The skin at three months is extremely thin and nearly transparent. Blood vessels are visible beneath the surface, giving the fetus a reddish appearance. Around this time, the very first layer of fine body hair, called lanugo, begins to appear. This hair is soft, thin, and unpigmented. It covers much of the body and plays a role in protecting the skin as it develops over the coming months. The skin won’t begin to thicken and become opaque until well into the second trimester.
What’s Happening Inside
The heart has been beating since around week six and is now pumping at roughly 150 to 170 beats per minute, about twice the rate of an adult heart. This is the heartbeat you can hear on a Doppler device at a prenatal visit. Over the next couple of weeks, the rate gradually settles down to around 150 beats per minute as the heart’s electrical system matures.
The kidneys begin producing urine by about 10 weeks of gestation and are functional by 12 weeks. That urine flows into the amniotic fluid, and by 14 weeks the kidneys become a major contributor to the total volume of fluid surrounding the fetus. The liver, intestines, and other digestive organs are in place. The brain is developing rapidly, though it’s still in very early stages of forming the complex connections it will need later.
Movement and Reflexes
A 12-week fetus is surprisingly active. Movement starts as early as 7.5 weeks, and by 10 to 11 weeks the fetus can move its arms and legs independently, hiccup, and make small breathing-like motions. By 12 weeks, it regularly brings its hands to its face. A couple of weeks later, it begins rhythmically opening and closing its jaw and swallowing small amounts of amniotic fluid. None of these movements are strong enough to feel from the outside. Most people start noticing fetal movement between 16 and 22 weeks.
Sexual Development
External genitalia are in the process of differentiating at 12 weeks but are not fully formed. The basic structures that will become either male or female anatomy are present, and trained ultrasound specialists can sometimes distinguish between them after about 10 weeks by looking at the angle of a small tissue projection (sometimes called the “sagittal sign”). In practice, though, sex determination by ultrasound is much more reliable after 14 weeks, and most providers wait until the anatomy scan around 18 to 20 weeks to confirm.
What You See on an Ultrasound
A 12-week ultrasound shows a recognizable baby shape: you can see the head, body outline, and limbs. The fetus often appears to be moving, waving its arms or shifting position. The heartbeat is clearly visible as a flickering motion in the chest. Individual fingers and facial features are sometimes visible depending on the fetus’s position and the quality of the equipment, but the image is still quite small on the screen. This scan is typically when the first clear “baby-shaped” image appears, which is why many people find the 12-week ultrasound particularly striking compared to earlier scans that showed little more than a small blob with a heartbeat.