At 13 weeks, a fetus is roughly the size of a plum, measuring about 2.5 inches from crown to rump and weighing around 2.5 ounces. It has a distinctly human shape, with a head, defined limbs, fingers, and toes all visible. This week marks the very end of the first trimester and the beginning of a period of rapid growth.
Overall Size and Proportions
Crown-rump length at 13 weeks ranges from about 67 to 80 millimeters (roughly 2.5 to 3 inches), depending on the exact day of the week. The head is still disproportionately large compared to the body, making up about a third of the total length. Over the coming weeks, the body will grow faster to even out these proportions, but at this stage, the fetus looks distinctly top-heavy.
The limbs have lengthened considerably. Arms and legs are now proportional enough that you can distinguish upper arms from forearms, and thighs from lower legs. Fingers and toes are fully separated, with no remaining webbing between them. On an ultrasound, you may even catch the fetus with a hand near its face.
Facial Features
The face is becoming more recognizably human. The eyes, which formed on the sides of the head earlier in development, have migrated closer together toward the front of the face. They’re still fused shut and won’t open for several more months, but the eyelids are fully formed. The ears have moved from the neck area to their final position on the sides of the head, though they’ll continue to develop internally.
A nose bridge, upper lip, and chin are now distinguishable in profile. The mouth can open and close. If you were to see a 3D ultrasound at this stage, the fetus would look like a very tiny, very thin newborn with closed eyes.
Skin and Bones
The skin at 13 weeks is thin and nearly transparent. Blood vessels are visible beneath the surface, giving the fetus a reddish appearance. The skin will thicken over the next several weeks, but right now it offers almost no barrier, which is why the network of veins and developing organs can sometimes be seen on detailed imaging.
Underneath that translucent skin, the skeleton is actively hardening. Bones in the skull, arms, and legs are transitioning from soft cartilage to calcified bone, a process called ossification that started weeks earlier but is now well underway. This hardening is one reason the fetus begins to show up more clearly on ultrasound around this time. Fingerprints haven’t formed yet (that starts around week 14), but the pads on the fingertips are developing.
Movement and Reflexes
A 13-week fetus is surprisingly active, though most pregnant people can’t feel it yet. The fetus can kick, stretch, flex its arms, and curl its fingers. These movements are largely reflexive rather than intentional, driven by the developing nervous system testing out new connections.
Swallowing is already happening. The fetus takes in small amounts of amniotic fluid, which helps the digestive system practice for life outside the womb. This swallowing reflex has been present since around week 11. The gag reflex is also developing, triggered when anything touches the back of the tongue or palate. Some ultrasounds at this stage catch the fetus with a thumb or fingers near its mouth, though true, coordinated thumb-sucking comes a bit later.
What You’d See on Ultrasound
On a standard 2D ultrasound at 13 weeks, the fetus appears as a bright white skeletal outline surrounded by a darker body. You can typically make out the skull, spine, and long bones of the arms and legs. The heartbeat is clearly visible as a rapid flicker in the chest, beating roughly twice as fast as yours.
If your provider uses a 3D ultrasound, you’d see surface-level detail: a small face in profile, tiny hands with individual fingers, and legs that may be tucked up or stretched out. Movement is common during scans at this stage. Don’t be surprised if the fetus changes position multiple times during the appointment. There’s still plenty of room in the amniotic sac, and the fetus takes full advantage of it.
What’s Happening Behind the Scenes
By 13 weeks, the placenta is fully formed and has taken over hormone production from the ovaries. It’s now the primary source of progesterone and estrogen, both of which sustain the pregnancy. This hormonal handoff is one reason many people notice their nausea and fatigue starting to ease around this time.
Internally, the fetus’s organs are largely in place but still maturing. The intestines, which temporarily developed in the umbilical cord due to limited abdominal space, have moved back into the abdomen. The kidneys are beginning to produce urine, which contributes to the amniotic fluid. Vocal cords are forming in the larynx, though they won’t produce sound until birth.