At 6 weeks of pregnancy, the embryo is about the size of a lentil, measuring roughly 5 to 9 millimeters from top to bottom. It doesn’t look like a baby yet. It’s a curved, tadpole-shaped structure with a visible tail, tiny budding limbs, and a heart that has just started to beat.
Size at 6 Weeks
The standard measurement at this stage is called crown-rump length, which is the distance from the top of the head to the bottom of the torso. At exactly 6 weeks, that length is about 5 millimeters. By the end of the sixth week, it reaches closer to 9 millimeters. For reference, 5 millimeters is smaller than a grain of rice, and 9 millimeters is roughly the width of your pinky fingernail. The whole embryo, including the surrounding sac, is still small enough to sit on the tip of a pencil eraser.
Overall Shape and Appearance
The embryo at 6 weeks has a distinctly curved, C-shaped body. It looks nothing like a newborn. The head is oversized compared to the rest of the body because the brain is growing rapidly, and the face is barely beginning to form. Dark spots mark where the eyes will eventually develop, and shallow pits on the sides of the head indicate future ear placement. There’s no recognizable nose or mouth yet, just early tissue folds that will eventually shape those features.
One of the most striking details is a small tail-like structure extending from the bottom of the embryo. This is completely normal. It typically disappears by around 8 weeks as the lower spine continues to develop, eventually becoming the tailbone (coccyx). The overall impression at this stage is more aquatic creature than human baby.
Arms, Legs, and Limb Buds
Tiny buds appear on either side of the body during week 6. These are the very beginnings of arms and legs. At this point, they look like small rounded bumps, not like limbs at all. There are no fingers, toes, or joints. The arm buds tend to appear slightly before the leg buds. Over the next several weeks, these will elongate and develop distinct hand and foot plates, but at 6 weeks, they’re just barely visible nubs.
The Heartbeat
The heart is one of the first organs to function, and at 6 weeks it has already started beating. It’s not yet the four-chambered heart of a fully developed baby. At this stage, it’s a simple tube-like structure that pulses to move blood through the embryo’s tiny body. The heart rate at this point is relatively slow, typically between 90 and 110 beats per minute. Over the coming weeks, that rate will climb significantly before eventually settling into a more stable range.
A heartbeat can sometimes be detected on a transvaginal ultrasound once the embryo reaches about 5 to 7 millimeters in length, which usually happens around the sixth week. However, not detecting a heartbeat at exactly 6 weeks isn’t automatically a cause for concern, since the timing can vary by a few days depending on when ovulation and implantation actually occurred.
What’s Happening Inside
While the outside of the embryo looks simple, a tremendous amount of internal development is underway. Blood cells are forming and circulation is beginning. The neural tube, which will become the brain and spinal cord, is closing during this period. The front end of the neural tube is already dividing into sections that will eventually become different parts of the brain. Structures that will form the liver, lungs, and kidneys are in their earliest stages of development, though none of them are functional yet. At this point, the embryo gets all of its nutrients from a small pouch called the yolk sac, which sits next to it inside the gestational sac.
What You’d See on an Ultrasound
If you have an ultrasound at 6 weeks, it will almost certainly be a transvaginal scan rather than the standard abdominal type, because the embryo is still too small to see clearly through the abdomen. On the screen, you’ll see a dark circular area (the gestational sac), a smaller bright ring inside it (the yolk sac), and a tiny bright structure next to the yolk sac (the fetal pole, which is the embryo itself). It won’t look like a baby on the screen. It appears as a small white speck or smudge.
Your provider may point out a flickering motion within that speck, which is the heartbeat. At this size, the embryo is so small that detailed features aren’t visible on ultrasound. You won’t be able to make out limb buds or facial structures. The main purpose of a 6-week scan is to confirm the pregnancy is in the right location, check for a heartbeat, and verify the number of embryos.
How 6 Weeks Compares to Later Stages
The difference between 6 weeks and just a few weeks later is dramatic. By 8 weeks, the tail will be gone, the limb buds will have developed into recognizable arms and legs with webbed fingers, and the face will have more distinct features. By 12 weeks, the embryo (now called a fetus) will be about the size of a lime and will look unmistakably human, with formed fingers, toes, and facial features. At 6 weeks, though, you’re still in the very early stages where the basic body plan is being laid down. The embryo is establishing the foundation for every major organ system, even though it still looks like little more than a tiny curved bean with a tail.