At 6 weeks of pregnancy, the embryo measures about 5 millimeters long, or roughly 1/8 of an inch. That’s about the size of a grain of rice. It weighs less than 0.04 ounces, making it far too small to feel but just large enough to detect on a transvaginal ultrasound.
How Size Is Measured at 6 Weeks
At this stage, doctors measure what’s called the crown-rump length, which is the distance from the top of the head to the bottom of the torso. There are no measurable legs yet, so this is the standard way to estimate size and confirm gestational age in early pregnancy. At exactly 6 weeks and 0 days, the crown-rump length is typically 5 millimeters.
Keep in mind that “6 weeks pregnant” is counted from the first day of your last menstrual period, not from conception. Fertilization usually happens about 2 weeks after that date. So a 6-week pregnancy actually represents about 4 weeks of embryonic development. This is why pregnancy timelines can feel confusing, but it’s the standard used by virtually all doctors and pregnancy apps.
What’s Developing at This Size
Despite being tiny, a lot is happening inside those 5 millimeters. The neural tube along the back is closing during week 6, and this tube will become the brain and spinal cord. The heart and other major organs are beginning to form. Structures needed to eventually create the eyes and ears are developing, and small buds are appearing where the arms will grow. The body has a distinct C-shaped curve at this point.
Technically, what’s growing at 6 weeks is still called an embryo, not a fetus. That terminology shift doesn’t happen until about 11 weeks of pregnancy (9 weeks after fertilization). The distinction matters because the embryonic period is when all the major organ systems are first laid down, while the fetal period is when those structures grow and mature.
What You’d See on an Ultrasound
If you have a transvaginal ultrasound at 6 weeks, the embryo appears as a small bright structure called the fetal pole. It sits next to a small pouch called the yolk sac, which provides nutrients at this stage. Both are enclosed inside a larger fluid-filled circle called the gestational sac.
A heartbeat is often detectable once the fetal pole reaches 5 to 7 millimeters, which lines up with around 6 weeks. Not everyone will see a heartbeat at exactly 6 weeks, though. Being off by even a few days in your cycle can mean the embryo is slightly smaller than expected, and your provider may recommend a follow-up scan a week or two later. This is common and doesn’t necessarily signal a problem.
How Quickly Size Changes
Growth at this stage is rapid. The embryo roughly triples in size during the sixth week alone. By week 7, it will be closer to 10 millimeters. By week 8, it can reach about 16 millimeters, roughly the size of a kidney bean. This exponential growth is why even a few days can make a noticeable difference on an ultrasound. If your dating scan puts you a few days ahead or behind where you expected, that’s normal variation and is one reason early ultrasounds are used to fine-tune your due date.