At 6 weeks of pregnancy, the embryo measures about 5 millimeters long, roughly the size of a small lentil or a pencil eraser. That’s just under a quarter of an inch from the top of the head to the bottom of the torso, a measurement doctors call “crown-rump length.” Despite being tiny enough to sit on your fingertip, the embryo is in one of its most active phases of development.
Exact Measurements at Week 6
Crown-rump length at the start of week 6 averages about 5 mm. By the end of the week, it reaches roughly 9 mm. That means the embryo nearly doubles in size over just seven days, growing about half a millimeter to a full millimeter each day. Standardized measurement charts used in clinical ultrasound put the 50th percentile at 5 mm at exactly 6 weeks and 0 days, climbing to 9 mm by 6 weeks and 6 days.
If your ultrasound shows a measurement slightly above or below that range, it doesn’t necessarily signal a problem. Embryos at this stage are so small that even a fraction of a millimeter difference in how the ultrasound image is captured can shift the reading. Your provider uses this measurement primarily to confirm or adjust your estimated due date.
What the Embryo Looks Like
At 6 weeks, the embryo doesn’t look like a baby yet. It’s a curved, comma-shaped structure with a visible bulge where the head is forming and a small tail-like extension at the bottom that will disappear in coming weeks. Dark spots mark where the eyes will eventually develop, and tiny paddle-shaped bumps are appearing where the arms and legs will grow.
The body is translucent. On ultrasound, it often appears as a small bright spot within the gestational sac, sometimes described as looking like a grain of rice. Most of the visible structure at this point is the developing head and trunk, with limbs still too small to distinguish clearly on a standard scan.
What’s Developing Inside
Week 6 is one of the most critical periods of organ formation. The neural tube, which runs along the embryo’s back, is closing. This tube becomes the brain and spinal cord. Folic acid intake during this window (400 micrograms daily, ideally started before conception) helps protect this process and reduces the risk of conditions like spina bifida.
The heart is one of the first organs to function. By 6 weeks, it has started beating at around 90 to 110 beats per minute. That rate will climb steadily, peaking at 140 to 170 bpm around weeks 9 to 10. On a transvaginal ultrasound, your provider may be able to detect this heartbeat as a flickering motion, though it’s not always visible at exactly 6 weeks. If it’s not detected yet, a follow-up scan a week later typically picks it up.
The liver, the beginnings of the digestive system, and early muscle and bone tissue are also forming. The embryo’s circulatory system is establishing its first blood vessels. All of this is happening in a structure smaller than a pea.
How Size Is Measured This Early
At 6 weeks, abdominal ultrasounds often can’t produce a clear enough image to measure the embryo reliably. Most providers use a transvaginal ultrasound instead, where the probe is placed closer to the uterus and can capture more detail at this tiny scale. Crown-rump length is the standard measurement because the embryo is curled up, making a head-to-toe measurement impossible. The measurement runs from the top of the head to the base of the spine.
This single number is surprisingly useful. In early pregnancy, embryos grow at a remarkably consistent rate regardless of genetics or the parent’s size, so crown-rump length is the most accurate way to date a pregnancy. A measurement taken between 6 and 13 weeks can estimate gestational age to within about five days.
How Quickly Things Change From Here
The growth rate during the first trimester is rapid relative to the embryo’s size. At 5 weeks, the embryo is barely 2 mm. By the end of week 6, it’s around 9 mm. By week 8, it will reach roughly 16 mm (about the size of a kidney bean), and by week 12, it’ll measure around 5 to 6 centimeters, large enough to see clearly on an abdominal ultrasound.
The jump from week 6 to week 7 is particularly dramatic. Facial features begin taking shape, the limb buds lengthen into recognizable arm and leg structures, and the brain starts dividing into distinct sections. What looks like a featureless dot on an early ultrasound transforms quickly into something with identifiable anatomy within just two to three weeks.