At 6 weeks of pregnancy, the embryo is roughly the size of a lentil or small pea, measuring between 1 and 7.5 millimeters depending on the exact day of the week. It has a distinct curved, C-shaped body that some people compare to a tadpole, complete with a small tail. It doesn’t look like a baby yet, but the basic building blocks for nearly every major organ are already forming beneath a thin layer of transparent skin.
Overall Shape and Size
The embryo at this stage is tiny. At the start of week 6, the crown-to-rump length (measured from the top of the head to the bottom of the torso) is just about 1 millimeter. By the end of the week, it reaches roughly 7 to 7.5 millimeters. That rapid growth over just seven days is one of the most dramatic size changes in the entire pregnancy.
The body is curled into a C-shape, with a large bulge at one end where the head is forming and a tapering tail at the other. The head already accounts for a disproportionately large share of the total body, a ratio that will gradually shift over the coming months but won’t fully even out until well after birth. The overall look is less “tiny human” and more “comma-shaped cluster of tissue,” which is completely normal for this stage.
What’s Forming on the Outside
Small bumps called limb buds appear along the sides of the body during week 6. These are the earliest beginnings of arms and legs, though they look more like tiny paddles or nubs than anything resembling fingers or toes. Those finer details won’t develop for several more weeks.
The face is just starting to take shape. Small pits mark where the nostrils will eventually be, and structures that will become the eyes, ears, and mouth are beginning to form. The eye development starts when small pouches push outward from the neural tube (the structure that becomes the brain and spinal cord), creating the earliest version of eye tissue. At this point, though, the face looks nothing like a recognizable human face. It’s a collection of thickenings and folds that will reorganize dramatically over the next month.
The Heart at 6 Weeks
One of the most notable developments at 6 weeks is the heart. It isn’t a fully formed four-chambered heart yet. Instead, it’s a tube-like structure that has begun to beat and pump blood. A visible bulge on the front of the embryo marks where this developing heart sits.
The heart rate at this stage typically falls around 100 beats per minute or slightly above. That’s slower than it will be in a few weeks, when it commonly reaches 150 to 170 bpm, but it’s already fast enough to be detected on a transvaginal ultrasound. Rates below 100 bpm before 6 weeks and 2 days, or below 120 bpm after that point, are considered below the normal range and may signal a higher risk of early pregnancy loss. Blood cells are also taking shape during this week, and the earliest circulation pathways are starting to function.
Brain and Nervous System
The neural tube, which closed during week 5, is now developing into three distinct sections that will become the forebrain, midbrain, and hindbrain. This tube runs the length of the embryo and will eventually form the entire brain and spinal cord. At 6 weeks, the brain end of the tube is growing much faster than the rest, which is part of why the head looks so oversized compared to the body.
Nerve cells are multiplying rapidly and beginning to connect with one another, though coordinated brain activity is still far off. The nervous system at this point is more of a rough scaffold than a functioning network.
Internal Organs Taking Shape
Week 6 falls in the middle of organogenesis, the period when all major organs begin to form. Inside the embryo, early versions of the digestive tract, lungs, and liver are developing. The kidneys are starting their first of three developmental stages (the embryo actually builds and discards two preliminary kidney structures before arriving at the final version). None of these organs are functional yet. They’re being laid out in position and will mature over the coming weeks and months.
What You’d See on an Ultrasound
If you have an ultrasound at 6 weeks, it will almost certainly be transvaginal rather than abdominal, because the embryo is too small to pick up reliably through the abdomen. Transvaginal ultrasound detects significantly more detail at this stage, successfully identifying the embryo in over 93% of normal pregnancies.
On the screen, you’d see a dark circle (the gestational sac filled with fluid), a smaller bright ring inside it (the yolk sac, which nourishes the embryo before the placenta takes over), and a tiny bright line or dot alongside the yolk sac. That line is the fetal pole, which is the embryo itself. It won’t look like a baby on screen. It looks more like a small grain of rice next to a circle. If the timing is right and the equipment is sensitive enough, the provider may also point out a flickering motion within that tiny structure, which is the heartbeat.
Don’t be alarmed if an ultrasound at exactly 6 weeks shows only a gestational sac and yolk sac without a clear fetal pole. At the very start of week 6, the embryo can be as small as 1 millimeter, which is right at the edge of what ultrasound can resolve. A follow-up scan a week later typically shows the embryo clearly.
How Week 6 Compares to What Comes Next
The embryo changes more between weeks 5 and 10 than during any other stretch of pregnancy. At 6 weeks, it’s a curved, tail-bearing shape with paddle-like limb buds and no recognizable face. By 8 weeks, the tail will have receded, the limbs will show distinct segments, and the facial features will be far more organized. By 10 weeks, it will look unmistakably human in outline, even though it will still be only about 3 centimeters long.
The rapid pace of development during week 6 is also why this period is especially sensitive to disruptions. The simultaneous formation of the heart, brain, limbs, and eyes all happening at once means that exposures to harmful substances carry higher risk during this window than they do later, when the organs are simply growing larger rather than being built from scratch.