At 8 weeks of pregnancy, a fetus measures roughly half an inch long, or about 15 to 16 millimeters from head to rump. That’s close to the size of a kidney bean. Despite being tiny enough to sit on your fingertip, this is a period of rapid change, with visible limbs, a beating heart, and the beginnings of a recognizable human shape.
Exact Measurements at 8 Weeks
Fetal size at this stage is measured using crown-rump length (CRL), which is the distance from the top of the head to the bottom of the torso, since the legs are too small and curled to include. At exactly 8 weeks, the median CRL is about 15 mm. By the end of the eighth week (8 weeks and 6 days), that grows to around 20.5 mm. So over the course of just a few days, the fetus nearly doubles in length from what it was at the start of the week.
Weight at 8 weeks is approximately 1.6 grams, or about 0.04 ounces. For context, that’s lighter than a paperclip. Most of the fetus’s mass at this point is water and developing tissue rather than fat or muscle.
What an 8-Week Fetus Looks Like
On ultrasound, an 8-week fetus has a distinct body curvature that technicians sometimes describe as resembling a gummy bear. The trunk and neck are beginning to straighten out, but there’s still a pronounced C-shaped curve to the body. The head is disproportionately large compared to the rest, which is normal at this stage.
Several features are now visible or taking shape:
- Limbs: Leg buds have taken on a paddle-like shape, and fingers are beginning to form, though the hands and feet still look webbed.
- Face: The upper lip and nose have formed. Eyes are becoming noticeable, and small swellings mark where the outer ears will develop into their shell-like shape.
- Heart: The heart is beating at 145 to 170 beats per minute, which is significantly faster than an adult heart rate and close to its peak speed for the entire pregnancy. It will slow slightly after 9 weeks.
If you have an ultrasound at 8 weeks, you’ll likely see the flickering of the heartbeat and the general outline of the body, though fine details like individual fingers won’t be clear yet.
How Growth Varies at This Stage
First-trimester measurements are remarkably consistent across healthy pregnancies, which is why doctors use crown-rump length to date pregnancies so precisely. A single CRL measurement at this stage can estimate gestational age to within about 5 days. If three separate measurements are taken during the same scan, that accuracy tightens to within about 3 days.
That said, some variation does occur. Factors like maternal age, smoking, ethnicity, body mass index, and vaginal bleeding can all influence early fetal growth slightly. Chromosomal differences in the fetus itself can also affect size. A measurement that falls a few millimeters above or below the median is usually nothing to worry about, but your provider may adjust your due date if the CRL doesn’t match your expected gestational age.
Changes in Your Body at 8 Weeks
While the fetus is barely half an inch, your uterus has already grown to about the size of a tennis ball, roughly double its pre-pregnancy size. You won’t have a visible bump yet, since the uterus is still tucked behind the pubic bone, but you may notice your waistband feeling tighter or bloating that comes and goes. The size difference between the tiny fetus and the expanding uterus reflects how much of the space is taken up by the amniotic sac, fluid, and the developing placenta.
What Happens Next
The end of week 8 marks a significant transition. Up until this point, the developing baby is technically called an embryo. Starting at week 9, it’s reclassified as a fetus, which signals that the foundational body structures are in place and the focus shifts from forming new organs to growing and refining them. Over the next four weeks, the fetus will roughly triple in length, reaching about 2 to 3 inches by the end of the first trimester. Weight gain accelerates even more dramatically, jumping from under 2 grams now to around 23 grams by week 12.