At 5 weeks pregnant, the embryo is tiny, roughly the size of a sesame seed, and has a distinct C-shaped curve. It doesn’t yet look like a baby. Most people describe its shape as resembling a small tadpole, with a rounded head end, a tapering tail-like structure, and no recognizable facial features or limbs.
How Big the Embryo Actually Is
At this stage, the embryo measures roughly 1 to 2 millimeters long, though by the end of the fifth week it can reach up to half an inch (11 to 14 millimeters) from the top of the head to the bottom of the tailbone. That rapid growth is one reason size estimates vary so much depending on where in the week you’re measuring. The body is curled into a C-shape, with a proportionally large head that makes up a significant portion of the total length. A small tail-like extension protrudes from the bottom, which will gradually recede over the coming weeks.
A Note on Dating
When doctors say “5 weeks pregnant,” they’re counting from the first day of your last menstrual period, not from conception. Because ovulation typically happens around day 14 of a 28-day cycle, the embryo at 5 weeks gestational age is actually only about 3 weeks old. This distinction matters because many pregnancy resources use gestational age, and the developmental details you’ll find can shift depending on which counting method a source uses. Everything in this article follows the standard gestational age convention.
What’s Forming Inside
Week 5 is the beginning of a critical window called organogenesis, when the major organ systems start taking shape. The most significant development is the neural tube, a flat strip of cells that folds and closes to become the brain and spinal cord. This process begins around day 22 to 23 after conception and is usually complete by the end of this period. Adequate folate intake before and during these early weeks supports proper neural tube closure.
The heart is also developing rapidly. A primitive heart tube forms and begins contracting around 22 to 23 days after conception, making it one of the first organs to function. At this point it’s not a four-chambered heart; it’s a simple tube that pulses to move fluid through the embryo’s developing circulatory system. If detected on ultrasound between weeks 5 and 7, a heart rate below 100 beats per minute may prompt follow-up imaging, though it’s not uncommon for the heartbeat to be undetectable this early.
Tiny limb buds, the earliest precursors to arms and legs, begin to appear by the end of week 5 as small bumps on either side of the body. They won’t look like arms or legs for several more weeks.
What You’d See on an Ultrasound
If you have a transvaginal ultrasound at 5 weeks, the embryo itself is often too small to see clearly. What your provider is looking for is the gestational sac, a dark, fluid-filled cavity inside the uterus. Inside that sac, a smaller circular structure called the yolk sac may be visible. The yolk sac provides nutrients to the embryo before the placenta takes over and is one of the first reassuring signs of a developing pregnancy.
In some cases, the ultrasound may also pick up a fetal pole, which is the earliest visible form of the embryo itself, appearing as a tiny thickening next to the yolk sac. Whether or not the fetal pole is visible depends on the exact timing, the quality of the ultrasound equipment, and the position of the gestational sac. Not seeing one at 5 weeks is common and doesn’t necessarily indicate a problem. Providers often schedule a follow-up scan a week or two later if the findings are inconclusive.
A heartbeat flicker may or may not be detectable at this stage. Many providers wait until closer to 6 or 7 weeks to confirm cardiac activity, since checking too early can cause unnecessary worry.
What’s Happening With hCG
At 5 weeks, the pregnancy hormone hCG is rising quickly. Typical blood levels range from about 200 to 7,000 units per liter, a wide range that reflects how rapidly levels double (roughly every 48 to 72 hours in a healthy early pregnancy). This is the hormone that makes a home pregnancy test turn positive, and it’s also responsible for many early symptoms like nausea and breast tenderness. Your provider may check hCG levels through blood draws spaced a few days apart to confirm that levels are rising appropriately, especially if ultrasound findings are too early to be definitive.
Why It Doesn’t Look Like a Baby Yet
At 5 weeks, there are no eyes, no fingers, no recognizable face. The embryo is essentially a layered disc of cells that has just begun folding into a three-dimensional shape. The outer layer will become skin and the nervous system. The middle layer develops into the heart, muscles, and bones. The inner layer forms the digestive and respiratory systems. All of this is happening simultaneously, but the structures are microscopic and undifferentiated. The tadpole-like appearance persists through about week 7, when the tail recedes and the head, face, and limbs begin taking on a more recognizable form.