At 5 weeks of pregnancy, the embryo is roughly 2 to 4 millimeters long, about the size of a sesame seed. It doesn’t look like a baby yet. What you’d see, if you could look closely enough, is a tiny curved shape with a distinct head end and tail end, resembling a small tadpole more than anything human.
Size and Overall Shape
The embryo at this stage measures around 2.5 millimeters from crown to rump, based on ultrasound measurements of healthy pregnancies. That’s smaller than a grain of rice. The body is curved into a C-shape, with a rounded head region that’s disproportionately large compared to the rest. A small tail-like structure extends from the bottom, which is completely normal and will disappear over the coming weeks as the lower spine develops.
The embryo is surrounded by a fluid-filled gestational sac, which at this point is about 5 to 6 millimeters across. The entire structure, sac included, would fit on the tip of a pencil eraser.
What You’d See on an Ultrasound
If you have an early ultrasound at 5 weeks, don’t expect to see much. The gestational sac is usually visible as a small dark circle within the uterus, and the yolk sac (a tiny round structure that nourishes the embryo before the placenta takes over) may also be visible. The yolk sac is often the first thing a sonographer can identify inside the gestational sac, sometimes appearing before the embryo itself is detectable.
The embryo, called a “fetal pole” at this stage, may or may not be visible yet. It shows up as a small thickening next to the yolk sac. At many 5-week scans, it’s simply too small to distinguish clearly, which is why doctors often recommend waiting until 6 or 7 weeks for a more informative ultrasound. If the embryo is visible, a flickering motion representing early heart activity might be detected, though its absence at 5 weeks is not a cause for concern.
The Heart Is Already Forming
One of the most remarkable things happening at 5 weeks is that the heart has already begun to beat. The embryonic heart starts rhythmic contractions around 21 to 23 days after fertilization, which falls right within the fifth week of pregnancy. At this point, the heart isn’t the four-chambered organ you’d recognize. It’s a simple tube that has begun looping into a C-shape, pushing plasma in one direction through the embryo’s tiny body.
Early heart rates vary quite a bit. Studies measuring heart activity at this stage have recorded rates anywhere from 65 to 130 beats per minute, depending on the exact day of development. The rate accelerates rapidly over the next few weeks.
Three Cell Layers Building the Body
The embryo doesn’t have recognizable organs yet, but three distinct layers of cells established during the third week are now busy building them. Each layer is responsible for different parts of the body:
- The outer layer is forming the nervous system, skin, and parts of the sensory organs like the eyes and ears.
- The middle layer is building the circulatory system, bones, muscles, and kidneys.
- The inner layer is developing into the digestive and respiratory systems, as well as the liver, which begins its first appearance around week 3 and grows rapidly from weeks 5 through 10.
The tissue that will become the lungs is also starting to take shape, with the membranes surrounding the future lung cavities forming during weeks 5 through 7.
The Neural Tube Is Closing
Perhaps the most critical development at 5 weeks involves the nervous system. A narrow channel called the neural tube folds and closes during the third and fourth weeks of pregnancy. The upper portion of this tube becomes the brain and skull, while the lower portion becomes the spinal cord and the bones of the back. By the end of week 5, this closure is typically complete.
This is why folic acid is so important in early pregnancy. Neural tube defects occur when this structure doesn’t close properly, and the closure happens so early that it’s often finished before many people even know they’re pregnant. Between weeks 5 and 8, the closed neural tube begins developing into distinct brain structures, with small vesicles (fluid-filled pockets) forming that will eventually become different regions of the brain.
Limb Buds Are Just Starting
At 5 weeks, there are no arms or legs yet. What’s happening instead is that cells beneath the skin are beginning to accumulate in specific spots along the embryo’s sides, creating tiny rounded bumps called limb buds. These are clusters of cells that signal to each other through chemical messengers, triggering the growth that will eventually produce arms and legs. At this stage, the buds look like small paddles or bumps, with no fingers, toes, or joints. Upper limb buds typically appear slightly before lower limb buds.
What’s Happening With hCG
The hormone your body produces during pregnancy, hCG, is rising rapidly at 5 weeks. Typical levels range from about 200 to 7,000 units per liter, which is a wide range because hCG roughly doubles every 48 to 72 hours during early pregnancy. This is the hormone that home pregnancy tests detect, and by 5 weeks most tests will show a clear positive result. The rapid rise in hCG is also what triggers early pregnancy symptoms like nausea, breast tenderness, and fatigue that many people start noticing around this time.