At 5 weeks pregnant, the embryo is roughly the size of a sesame seed, measuring about 2 millimeters long. It doesn’t look like a baby yet. What you’d see under magnification is a tiny, curved shape resembling a tadpole, with a distinct head end, a tapering tail-like structure, and the very earliest bumps where arms and legs will eventually grow.
Size and General Shape
The embryo at this stage is essentially a small, C-shaped curve. The head end is noticeably larger than the rest of the body because the brain is already one of the fastest-developing structures. Tiny arm and leg buds are just beginning to push outward, though they look more like small paddles than anything resembling limbs. A tail-like extension at the bottom of the spine is clearly visible and won’t disappear for several more weeks.
Three layers of cells are actively specializing into different body systems. The outer layer is forming what will become skin, the nervous system, eyes, and inner ears. The middle layer is building the foundation for bones, muscles, kidneys, and the circulatory system. The innermost layer will eventually develop into the lungs and intestines. None of these organs are functional yet, but the blueprint is being laid down at remarkable speed.
The Heart Is Already Beating
One of the most dramatic developments at 5 weeks is that cardiac tissue has started to pulse. The heart isn’t a four-chambered organ yet. It’s actually two tubes that have fused together in the middle, forming a trunk with four smaller tubes branching off. This primitive structure contracts rhythmically, and those contractions can sometimes be detected on a transvaginal ultrasound. It’s not quite a heartbeat in the way most people imagine, but it is the very first sign of a circulatory system at work.
Brain and Nervous System
The neural tube, a narrow channel running along the embryo’s back, has already folded and closed during weeks three and four. By week 5, the upper portion of that tube is beginning to develop into three distinct sections that will eventually become the brain. The lower portion will become the spinal cord. This is a critical window for neural development, which is why folic acid intake in early pregnancy matters so much for preventing neural tube defects.
What You’d See on an Ultrasound
If you have a transvaginal ultrasound at 5 weeks, you won’t see anything that looks like a baby. What the technician looks for is a gestational sac, which appears as a small, dark circle of fluid inside the uterus. Inside that sac, a yolk sac may be visible. The yolk sac is a small, round structure that provides nutrients to the embryo before the placenta takes over.
A fetal pole, the earliest visible form of the embryo itself, may also appear next to the yolk sac. It looks like a tiny thickening along the edge of the sac. At this stage, though, it’s common for only the gestational sac to be clearly visible. Seeing less than expected on a 5-week ultrasound is normal and doesn’t necessarily indicate a problem. Dating can be off by several days, and even a few days makes a big difference at this scale.
How the Embryo Gets Nutrients
The placenta is still in its early stages of construction. Finger-like projections called villi are extending from the outer layer of cells surrounding the embryo into the uterine wall. At 5 weeks, these villi are in their secondary stage: a layer of supportive tissue is growing into them, and they cover the entire surface of the sac surrounding the embryo. Blood vessels within these structures won’t fully form until the following week. For now, nutrients pass through direct contact between the embryo’s outer cells and the uterine lining, supplemented by the yolk sac.
Interestingly, during these early weeks, cells from the developing placenta actually block maternal blood from flowing freely into the space around the embryo. This creates a low-oxygen environment that appears to help regulate how the placenta grows and attaches. Full maternal blood flow to the placenta won’t establish itself until later in the first trimester.
Pregnancy Hormone Levels at 5 Weeks
The hormone that pregnancy tests detect, hCG, is rising rapidly. At 5 weeks, typical levels range from 200 to 7,000 ยต/L. That’s a wide range because hCG roughly doubles every 48 to 72 hours in early pregnancy, so whether you’re at the start or end of week 5 makes a significant difference. A single hCG number matters less than the trend over multiple blood draws, which is why doctors sometimes order repeat testing rather than relying on one result.
This rapid rise in hCG is also what’s driving early pregnancy symptoms like nausea, breast tenderness, and fatigue. If your symptoms feel intense at 5 weeks, that’s your body responding to hormone levels that are climbing steeply every day.