At 5 weeks of pregnancy, the developing embryo is roughly the size of a sesame seed, about 2 millimeters long. It doesn’t look like a baby yet. It’s a tiny, curved structure with a distinct C-shape, and most of its major organs are just beginning to form from three foundational layers of cells.
Technically, at this stage it’s called an embryo, not a fetus. The term “fetus” applies starting around week 10 of pregnancy. But regardless of terminology, week 5 is when development shifts from a cluster of cells into something with recognizable body systems taking shape.
What the Embryo Looks Like at 5 Weeks
The embryo at this point resembles a tiny tadpole more than anything human. It has a rounded head end, a narrow tail-like structure, and a pronounced C-shaped curve along its length. There are no arms, legs, fingers, or facial features yet. The outer surface is smooth, and the entire structure is enclosed within a fluid-filled sac inside the uterus.
Three layers of cells, established in the weeks prior, are now actively building different parts of the body. The outer layer is forming what will become the brain, spinal cord, skin, hair, and nails. The middle layer is developing into the heart, muscles, bones, and connective tissue. The innermost layer is giving rise to the lining of the digestive tract, liver, pancreas, and lungs. At 5 weeks, none of these organs are functional yet, but the cellular groundwork is being laid quickly.
The Heart and Neural Tube
Two of the biggest milestones of week 5 are the formation of the neural tube and the first heartbeat. The neural tube is the structure that will become the brain and spinal cord. It’s one of the earliest recognizable features of the embryo, running along its back like a ridge that gradually closes into a tube.
The heart at this stage isn’t a four-chambered organ. It’s a simple tube that begins to pulse around 3 weeks and 1 day after fertilization (which corresponds to roughly the start of week 5 in pregnancy dating). By the end of week 5, this tiny heart tube pulses about 110 times per minute. The sound is far too faint to hear with a stethoscope or even a standard doppler device. It’s detectable only on sensitive ultrasound equipment, and often not until a week or two later.
The Placenta and Umbilical Cord
The embryo isn’t sustaining itself. By week 5, a primitive network of blood vessels is forming, and some of these vessels are already bundling together into what will become the umbilical cord. The placenta is also developing at this point. Once fully formed, it will deliver oxygen and nutrients to the embryo and carry waste products away. At 5 weeks, this system is still being built, but it’s already beginning to function in a basic way.
What You’d See on an Ultrasound
If you have a transvaginal ultrasound at 5 weeks, don’t expect to see much. The embryo itself is often too small to visualize clearly. What your provider is looking for is the gestational sac, a small, dark, fluid-filled circle inside the uterus that confirms the pregnancy is in the right location. Inside or near that sac, a yolk sac may also be visible. The yolk sac is a small round structure that provides early nourishment before the placenta takes over.
At 5 weeks, it’s common not to see a heartbeat on ultrasound. This doesn’t necessarily mean something is wrong. The embryo is simply very small, and cardiac activity often isn’t detectable until closer to 6 or 7 weeks. Many providers will schedule a follow-up scan a week or two later to confirm a heartbeat rather than drawing conclusions from a single early scan.
What’s Happening in Your Body
At 5 weeks, your body is producing a hormone called hCG (the same hormone pregnancy tests detect). Typical hCG levels at this stage range from 200 to 7,000 µ/L, a wide range that reflects how quickly levels rise in early pregnancy. This hormone is part of what causes early pregnancy symptoms like nausea, breast tenderness, and fatigue, though many people don’t feel much of anything yet at 5 weeks.
A missed period is often the first clue that brings people to this stage. Home pregnancy tests are reliably positive by now, and blood tests can confirm rising hCG levels if there’s any uncertainty. The wide variation in hCG is normal. A single number matters less than whether levels are rising appropriately over time.
How Big Is 5 Weeks, Really
It helps to put this in perspective. The entire embryo at 5 weeks is about the size of a sesame seed or the tip of a pen. It weighs almost nothing. The gestational sac surrounding it is roughly the size of a small blueberry. Everything happening, the neural tube forming, the heart beginning to beat, blood vessels branching out, is occurring on a scale that’s nearly invisible to the naked eye. The rapid pace of development is remarkable: by next week, the embryo will roughly double in size, and early limb buds will start to appear.