At six weeks of pregnancy, the embryo measures roughly 5 to 9 millimeters, or about a quarter of an inch. That’s comparable to the size of a lentil. Growth happens quickly at this stage, with measurable changes day to day.
Actual Measurements at Six Weeks
Embryo size is measured by crown-rump length, which is the distance from the top of the head to the bottom of the torso (legs aren’t included because they’re curled tightly). At the start of week six, the crown-rump length sits around 5 mm. By the end of the week, it reaches about 9 mm. That range reflects how rapidly cells are dividing and organizing during this period.
To put those numbers in perspective, 5 mm is roughly the width of a pencil eraser, and 9 mm is close to the width of your pinky fingernail. The most common comparison you’ll see is a lentil, which captures both the size and the rounded, curved shape of the embryo at this point.
Why It’s Called an Embryo, Not a Fetus
At six weeks, the developing pregnancy is technically an embryo. The term “fetus” doesn’t apply until around the 11th week of pregnancy (ninth week after conception), when the major organ systems have formed and the focus shifts from building new structures to growing and maturing existing ones. Everything before that point is the embryonic stage, a period of intense, rapid development where the basic body plan takes shape.
What’s Developing at This Size
Despite being smaller than a pea, quite a lot is happening inside that quarter-inch structure. The neural tube, which becomes the brain and spinal cord, has already closed by the end of week five. At week six, the cluster of cells that will become the heart is pulsing, and a provider may be able to detect those pulses on a vaginal ultrasound. Tiny buds that will eventually become arms and legs are also starting to appear.
The embryo at this stage has a distinct curved shape, almost like a comma. There’s a visible head end and tail end, though it doesn’t yet look recognizably human. Facial features like the eyes, nostrils, and mouth are just beginning to form as small indentations and thickenings in the tissue.
How Fast Growth Happens in Early Pregnancy
The difference between the start and end of week six illustrates how dramatic early growth is. Going from 5 mm to 9 mm in a single week means the embryo nearly doubles in length over just seven days. This pace continues through the first trimester, with the embryo growing from barely visible to roughly 3 inches long by week 12.
Because growth is so rapid, even a few days can make a noticeable difference on an ultrasound. This is why early ultrasound dating is considered highly accurate. A measurement that’s off by just a millimeter or two can shift the estimated due date by several days.
What You Might See on an Ultrasound
If you have an ultrasound at six weeks, the embryo appears as a small bright spot inside the gestational sac. It won’t look like a baby yet. You’ll likely see the sac itself (a dark circle), a yolk sac (a smaller ring that provides nutrients), and the tiny embryo nearby. Depending on the exact day and the equipment used, your provider may also pick up cardiac activity, which shows up as a rapid flickering on the screen.
At this size, a transvaginal ultrasound is typically used rather than the standard abdominal approach, because the embryo is still too small to image clearly through the abdomen. Even with the closer view, details are limited. The main goals at a six-week scan are confirming the pregnancy’s location, checking for cardiac activity, and measuring the crown-rump length to establish a due date.