At 8 weeks of pregnancy, the developing baby is roughly 15 to 20 millimeters long, about the size of a kidney bean. Despite being small enough to sit on your fingertip, it already has a recognizable head, body, and the beginnings of arms and legs. This week also marks an important medical milestone: the transition from “embryo” to “fetus,” meaning all the major organ systems have started to form.
Overall Size and Proportions
The standard measurement at this stage is crown-rump length, which is the distance from the top of the head to the bottom of the torso (legs aren’t included because they’re still curled up). At 8 weeks, the average crown-rump length is about 14 to 17 millimeters, growing to roughly 20 mm by the end of the week. Weight is negligible, around 1 gram or less.
The head makes up a large proportion of the total body, nearly half. The trunk and neck are just beginning to straighten out from the curved, C-shaped posture of earlier weeks. On an ultrasound, you’ll see a small bright shape inside a dark, fluid-filled gestational sac, with a smaller bubble-like yolk sac visible nearby.
Facial Features Taking Shape
The face is changing rapidly during week 8. The eyes, which started on the sides of the head, are migrating toward a more forward-facing position. Eyelids have begun forming from small folds of skin that will eventually grow over the eyes and fuse shut (they won’t reopen until much later in pregnancy, around 26 to 28 weeks). The nose is flat with a low bridge, and the upper lip has formed. Small swellings on either side of the head outline the future outer ears, which have already started to take on a recognizable shape during week 7.
At this point, the face still looks very different from a newborn’s. The jaw is small, the forehead is prominent and flat, and the eyes appear wide-set. These proportions will shift gradually over the coming months.
Arms, Legs, Fingers, and Toes
Limb development is one of the most visible changes during this period. By 8 weeks, the arms are growing and fingers have begun to form, though they’re still webbed together with thin tissue. The legs are slightly behind the arms in development, with leg buds now shaped like small paddles. Toes are not yet visible at 8 weeks but become apparent the following week.
Elbows start to appear around this time, though they can’t bend yet. That ability develops over the next couple of weeks, along with the separation of individual fingers and toes as the webbing between them gradually breaks down. By 10 weeks, the fingers and toes are fully distinct and noticeably longer.
Organ Development
Week 8 is significant because all major organ systems are now in some stage of formation. The heart has been beating since around week 6, and at 8 weeks the heartbeat is fast, typically well above 110 beats per minute. (Heart rates below that threshold at this stage are considered a warning sign.) On an ultrasound, the heartbeat is often the most dramatic thing you’ll see or hear at this appointment.
The lungs are forming the branching tubes that will eventually carry air in and out. They won’t be functional for months, but the basic architecture is being laid down now. The liver has begun to develop and will soon take over the job of producing blood cells, a role handled by the yolk sac in earlier weeks. The intestines are growing rapidly, and a portion of them temporarily extends into the umbilical cord because the abdomen is still too small to contain them.
Early Movement
Perhaps surprisingly, the baby is already moving at 8 weeks. These are spontaneous, reflexive movements as the muscles and nervous system begin to connect. The motions are too tiny for you to feel, and they won’t become noticeable for another 8 to 12 weeks. But on a detailed ultrasound, small twitches and shifts can sometimes be observed.
What You’ll See on an Ultrasound
If you have an ultrasound at 8 weeks, the technician will look for several key markers. The gestational sac appears as a dark circle filled with fluid. Inside it, the fetal pole (the early form of the baby) is visible as a small, bright shape. The yolk sac shows up as a small round bubble near the fetus. The crown-rump length will be measured to confirm how far along you are, and the heartbeat will be checked to make sure it falls in a healthy range.
Don’t expect the image to look like a baby yet. At this stage, the shape on the screen is small and somewhat abstract. You’ll be able to distinguish the head from the body, and you may see flickering in the chest area, which is the heartbeat. The limbs are usually too small to see clearly on a standard ultrasound, though they may be visible with higher-resolution equipment. By 12 weeks, the proportions will look much more recognizably human.