At 8 weeks of pregnancy, the developing baby measures roughly 16 to 22 millimeters from head to bottom, which is about 0.6 to 0.9 inches. That’s approximately the size of a raspberry. It weighs around 1 gram, or about 0.04 ounces, lighter than a single paperclip.
How Size Is Measured at 8 Weeks
At this stage, doctors measure what’s called crown-rump length, the distance from the top of the head to the bottom of the torso. Legs are too small and curled to factor into length measurements this early. On an ultrasound, a crown-rump length of 16 mm corresponds to about 8 weeks and 1 day, while 22 mm lines up with 8 weeks and 6 days. So even within a single week of pregnancy, the embryo nearly doubles its measurement, growing roughly a millimeter per day.
Your doctor uses this measurement to estimate or confirm your due date, especially during a first-trimester ultrasound. Crown-rump length is the most accurate way to date a pregnancy in the early weeks because growth follows a very predictable pattern at this stage. Individual variation is minimal, so a measurement that’s a couple of millimeters off from expected can shift your estimated due date by several days.
What the Embryo Looks Like
At 8 weeks, the developing baby still looks more like a tiny tadpole than a miniature person, but recognizable features are taking shape. Small arm and leg buds have appeared, and fingers and toes are beginning to form, though they’re still webbed. The head is disproportionately large compared to the rest of the body, making up nearly half the total length. Facial features like the upper lip, nose, and eyelids are starting to develop.
Internally, all the major organ systems have begun forming. The heart has been beating since around week 5 or 6 and now has four distinct chambers. The brain is developing rapidly, and the neural tube (which becomes the brain and spinal cord) has closed. Bones are starting to harden, though most of the skeleton is still soft cartilage.
Embryo vs. Fetus: The Transition Point
Week 8 marks the very end of the embryonic period. After the 8th week, healthcare providers officially refer to the developing baby as a fetus rather than an embryo. The distinction isn’t just a name change. The embryonic stage is when all the major structures and organs first form from scratch. The fetal stage, which begins around week 9 and lasts until birth, is when those structures grow, mature, and become functional. By the time the embryonic period wraps up at 8 weeks, the basic blueprint of the body is in place.
How Much Growth Happens Next
To put the 8-week size in perspective, consider how quickly things change from here. By week 12, the baby will measure about 2.5 inches and weigh around half an ounce. By week 20, roughly 6.5 inches and 10 ounces. The jump from raspberry-sized to roughly the length of a banana happens in just 12 weeks.
Growth in the first trimester is focused heavily on forming structures rather than gaining weight, which is why the embryo is so tiny yet so complex at 8 weeks. Weight gain accelerates significantly in the second and third trimesters, when the organs that already exist spend months developing and the baby adds layers of fat.
What You Might See on an Ultrasound
If you have an ultrasound around 8 weeks, the embryo will appear as a small, bright shape inside the gestational sac. You’ll likely see a flickering heartbeat, which at this stage runs between 150 and 170 beats per minute, roughly twice the rate of an adult heart. The yolk sac, a small circular structure that provides nutrients before the placenta takes over, is usually visible nearby.
Don’t be surprised if the embryo looks smaller or larger than you expected based on your last period. Menstrual dates and actual conception dates often differ by a week or more, so what you think is 8 weeks might measure as 7 weeks and 3 days or 8 weeks and 4 days on the screen. Small discrepancies like this are completely normal and simply help your provider pin down a more accurate due date.