At 8 weeks pregnant, the embryo measures roughly 16 to 22 millimeters from head to bottom, about the size of a kidney bean. Despite being tiny enough to sit on your fingernail, it already has a recognizable shape: a large head, a curved body, and small limb buds that are rapidly forming fingers and toes. This is the final week your developing baby is technically called an embryo. Starting at week 9, it’s reclassified as a fetus.
Overall Size and Shape
Crown-rump length, the standard measurement from the top of the head to the bottom of the tailbone, ranges from about 16 mm at 8 weeks and 1 day to 22 mm by 8 weeks and 6 days. That’s roughly half an inch to just under an inch. The head is disproportionately large compared to the rest of the body, making up nearly half the total length. The once-prominent embryonic tail, visible in earlier weeks, disappears by the end of week 8.
Face and Head
The face is starting to look more human, though it’s still far from what you’d picture. The upper lip and the borders of the nostrils have formed as facial tissue fused together during week 7 and continued shaping through week 8. Tiny dark spots mark where the eyes are developing, and thin folds of skin are beginning to form what will eventually become eyelids. The outer ears are taking shape as small folds on either side of the head, sitting lower than they will at birth.
Arms, Legs, Fingers, and Toes
The arms and legs have grown noticeably by this point, bending at the elbows and knees for the first time. Fingers and toes exist but are still webbed, connected by thin membranes of tissue. Over the next two weeks, that webbing will disappear and the digits will lengthen and separate fully. At 8 weeks, the hands look a bit like tiny paddles with ridges (called digital rays) where each finger is forming.
Developing Organs
All of the major organs and body systems are developing by week 8. The heart has been beating since around week 5, when it started pulsing at about 110 times per minute. By week 8 the heart rate is faster, and the heart itself has divided into its four chambers. The lungs, liver, kidneys, and digestive tract are all forming, though none of them are functional yet. They’ll continue maturing throughout pregnancy.
Brain and Nervous System
The brain is growing rapidly but is still in its earliest stages. At 8 weeks, the most developed part is the brainstem, the region that will eventually control basic functions like breathing and heart rate. It contains large neurons that are beginning to form the long communication pathways of the adult brain. The cerebral cortex, the outer layer responsible for thinking, memory, and consciousness, is barely there. It consists mostly of cells that are still dividing and multiplying rather than functioning as nerve cells. The cerebellum, which will later coordinate movement and balance, is also just a tiny bud at this stage. The brain has an enormous amount of building left to do: the adult brain requires roughly 86 billion neurons, and most of them haven’t been produced yet.
What You’d See on Ultrasound
If you have an ultrasound at 8 weeks, it’s typically a transvaginal scan, which provides a clearer image this early. On screen, you’ll see a small, bright shape (the embryo) inside a dark fluid-filled sac. Next to or near the embryo, you may notice the yolk sac, a small circular structure that has been providing nutrients and will gradually shrink over the next couple of weeks. The embryo itself appears as a curved, bean-like form, sometimes called the fetal pole. You can usually see a flickering motion where the heart is beating, though details like fingers or facial features aren’t visible on a standard 2D ultrasound at this size.
Embryo vs. Fetus
Week 8 marks a transition point. According to the American College of Obstetricians and Gynecologists, the developing baby is called an embryo for the first 8 weeks after fertilization and a fetus from week 9 until birth. The shift isn’t arbitrary. By the end of week 8, all major organ systems have at least begun to form, and the basic body plan is in place. The fetal period that follows is focused on growth and maturation of structures that already exist rather than building new ones from scratch.