At 8 weeks of pregnancy, a fetus measures roughly 15 to 17 millimeters from crown to rump, about the size of a raspberry. That’s just over half an inch long. Despite being tiny enough to sit on the tip of your finger, a remarkable amount of development is already underway.
Gestational Age vs. Actual Age
When doctors say “8 weeks pregnant,” they’re counting from the first day of your last menstrual period, not from conception. The actual age of the embryo is typically about two weeks less than the gestational age, meaning the developing baby has really been growing for roughly six weeks. This distinction matters because pregnancy apps, ultrasound reports, and your OB will all use the gestational age number. So an “8-week fetus” by standard medical dating is about 6 weeks post-conception.
Crown-to-Rump Length at 8 Weeks
Fetal size at this stage is measured as crown-rump length, the distance from the top of the head to the bottom of the torso (legs aren’t included because they’re curled up tight). At 8 weeks and 1 day, crown-rump length averages about 16 mm. By 8 weeks and 2 days, it’s around 17 mm. That’s roughly the length of a kidney bean. Weight is essentially negligible at this point, registering at about 1 gram.
Your uterus, meanwhile, has already expanded to about the size of a tennis ball to accommodate the gestational sac, yolk sac, and growing embryo, even though none of this is visible from the outside yet.
What Has Formed by 8 Weeks
Week 8 marks the tail end of the embryonic period, the phase when all major organ systems are being laid down. The brain and spine have already begun forming. By the end of this week, the embryo officially transitions to being called a fetus, a milestone that reflects the fact that the basic structural blueprint of the body is now in place. From this point forward, development shifts from building new organs to growing and refining the ones that already exist.
At this stage, fingers and toes are lengthening and starting to separate from each other. The upper limbs are longer and beginning to bend at the elbow. Hands and feet have turned slightly inward. Facial features are becoming more distinct: the nose, eyes, eyelids, and external ears are taking shape, and the head has become more rounded. The head is still almost the same size as the rest of the body, giving the embryo its characteristic top-heavy appearance.
What You See on an 8-Week Ultrasound
If you have an ultrasound at 8 weeks, the image will show a small figure that looks something like an oblong bean. It won’t look much like a baby yet. The technician will identify the gestational sac (the fluid-filled space surrounding the embryo), the yolk sac (a small bubble-like structure that provides early nourishment), and the fetal pole, which is the embryo itself.
The two most important things your provider looks for at this scan are the crown-rump length, which confirms how far along the pregnancy is, and the heartbeat. A fluttering cardiac motion is typically visible on ultrasound by this point, and it’s one of the key signs that the pregnancy is progressing normally. The heart has been beating since around week 5 or 6, and by week 8 it’s pumping at a rapid pace, often between 150 and 170 beats per minute.
How Size Changes From Here
Growth accelerates quickly after week 8. To put the raspberry-sized starting point in perspective: by week 12, the fetus will be about the size of a lime (roughly 5 to 6 cm). By week 20, it will measure around 25 cm from head to heel. The period between weeks 8 and 12 is when the fetus roughly triples in length, making it one of the fastest growth stretches of the entire pregnancy.
Because the embryo is so small at 8 weeks, even a difference of a few millimeters in crown-rump length can shift the estimated due date by several days. This is why early ultrasounds are considered the most accurate way to date a pregnancy. After the first trimester, size becomes more variable from one baby to the next, making later measurements less precise for dating purposes.