At 8 weeks pregnant, the fetus measures roughly half an inch long, or about 11 to 14 millimeters from crown to rump. That’s comparable to a raspberry or a small kidney bean. Despite this tiny size, week 8 marks a burst of development, with facial features taking shape, limbs forming, and the brain building connections that will continue for months.
Exact Measurements at 8 Weeks
Fetal size is measured as “crown-rump length,” the distance from the top of the head to the bottom of the torso, since the legs are too small and curled to factor in. At the start of week 8, the average (50th percentile) length is about 14.6 mm. By the end of the week, that jumps to around 20.5 mm, nearly doubling over just a few days. This rapid growth means a fetus measured on Monday could look noticeably different by Friday on ultrasound.
There’s also a wide range of normal. At 8 weeks and 2 days, for example, international fetal size standards show that crown-rump length can fall anywhere from about 8 mm on the small end to 21 mm on the large end, and both are considered healthy. Your provider uses these ranges to estimate gestational age, not to flag problems.
Weight at this stage is almost negligible. The fetus weighs roughly 1 gram, or about 0.04 ounces. For perspective, that’s lighter than a single paperclip.
What’s Forming This Week
Week 8 is one of the most active periods for organ and limb development. The leg buds, which appeared a couple of weeks earlier, now take on a paddle-like shape. Fingers have started to form, though they’re still webbed together. Small swellings outline the future ears, and the eyes become clearly visible as dark spots on either side of the head. The upper lip and nose have taken shape, and the trunk and neck are beginning to straighten out from their earlier curved, C-shaped posture.
Inside the skull, the brain is rapidly organizing itself. The cerebellum and brainstem, which were established around week 6, are now more defined. The corpus callosum, the thick band of nerve fibers that will eventually connect the left and right halves of the brain, begins forming during week 8 and continues developing through week 20. At this point, the forebrain (telencephalon) is differentiating normally, which is one of the things visible on early ultrasound.
What an 8-Week Ultrasound Shows
If you have an ultrasound at 8 weeks, it’s typically done transvaginally because the fetus is still too small to see well through the abdomen. At this stage, your provider can usually see a flickering heartbeat, distinguish the head from the body, and measure crown-rump length to confirm or adjust your due date. The head and pelvic areas are beginning to look distinct from each other, though the fetus still looks more like a tiny blob with a large head than anything recognizably human.
Don’t be surprised if the image is hard to interpret on your own. At half an inch, the fetus is a small bright shape within the gestational sac, and even trained eyes rely on specific landmarks to assess what they’re seeing.
How Your Body Is Changing
Even though the fetus is the size of a berry, your uterus has already grown to roughly the size of a tennis ball, up from its usual walnut size. You likely can’t feel this from the outside yet, and most people aren’t visibly showing at 8 weeks. But the internal expansion explains why many people experience increased urinary frequency around this time: the growing uterus sits right against the bladder.
Nausea, fatigue, and breast tenderness are all common at this point, driven by rising hormone levels that are supporting the rapid construction happening inside. These symptoms tend to peak somewhere between weeks 8 and 12 before gradually easing for most people in the second trimester.
How Size Changes From Here
Growth during the first trimester is exponential. At 8 weeks, the fetus is about half an inch. By week 12, it will be closer to 2.5 inches and weigh about half an ounce. That’s a fivefold increase in length in just four weeks. Crown-rump length during this window is one of the most accurate ways to date a pregnancy, because growth follows a remarkably consistent pattern across different populations. After the first trimester, individual genetics and nutrition start to create more variation between fetuses, and other measurements (head circumference, femur length) become more useful for tracking growth.