At five weeks of pregnancy, the embryo is roughly the size of a sesame seed, about 2 millimeters long, and looks nothing like a baby. It’s a tiny, curved structure shaped like the letter C, with a small tail-like extension at one end and a slightly bulging head region at the other. To the naked eye, it would appear as a barely visible speck. On an ultrasound, you may not even see the embryo itself yet.
Gestational Age vs. Actual Embryo Age
One important detail: “five weeks pregnant” means five weeks since your last menstrual period, not five weeks since conception. The embryo itself is actually only about three weeks old at this point. Pregnancy dating starts from the first day of your last period, which is roughly two weeks before ovulation and fertilization. So when your pregnancy app says “5 weeks,” the embryo has been developing for closer to 21 days. This matters because descriptions of embryonic development sometimes use one system and sometimes the other, which can get confusing fast.
What It Looks Like Up Close
At this stage, the embryo has a distinct C-shaped curve and a small tail-like structure that will eventually disappear as development continues. There’s no face, no fingers, no recognizable human features. What you would see, if you could look closely enough, are the very earliest building blocks of a body just starting to take shape.
The head end is starting to develop pharyngeal arches, which are rounded ridges of tissue that will eventually form the jaw, ears, and neck structures. Tiny thickenings called placodes mark the future locations of the eyes and inner ears, though they look nothing like eyes at this point. The heart area is visible as a small bulge on the front of the embryo, and the liver region creates another subtle bump. By the end of the fifth week, small limb buds, the earliest precursors to arms and legs, begin to emerge as tiny paddle-shaped projections from the body.
The embryo is also developing somites, which are small blocks of tissue running along its back. These are the foundation for the vertebrae, ribs, and muscles. At five weeks, there are roughly 30 or more pairs of these segments lined up in a row, giving the embryo a faintly segmented appearance along its length.
What’s Happening Inside
Despite its minuscule size, the embryo at five weeks is undergoing rapid internal development. It’s organized into three foundational cell layers that will give rise to every organ and tissue in the body. The outer layer (ectoderm) is forming the nervous system, skin, hair, and sensory organs. The middle layer (mesoderm) is building the skeleton, muscles, heart, blood vessels, and kidneys. The inner layer (endoderm) is creating the lining of the digestive tract, liver, pancreas, and lungs.
The neural tube, which becomes the brain and spinal cord, has typically closed completely by around day 28. This is one of the most critical milestones in early pregnancy, since incomplete closure can lead to conditions like spina bifida. By the end of week five, the brain is already beginning to divide into distinct regions, though it’s still a hollow tube of cells rather than anything resembling a functioning brain.
The primitive heart has already started beating. Cardiac cells begin contracting around 22 to 23 days after conception, making the heart one of the first organs to function. At this stage, it’s a simple tube that loops and pulses rather than a four-chambered heart. It won’t be detectable on most ultrasounds yet, but the rhythmic contractions are already moving blood through the embryo’s tiny developing circulatory system.
What You’d See on an Ultrasound
If you have an ultrasound at five weeks, don’t expect to see much. A transvaginal ultrasound (the internal kind, which provides better detail this early) will typically show a gestational sac, a small dark circle of fluid inside the uterus. Inside that sac, a yolk sac may be visible as a tiny round structure. The yolk sac provides nutrients to the embryo before the placenta takes over and is one of the first things a provider looks for to confirm a viable pregnancy.
The embryo itself, sometimes called the fetal pole at this stage, may or may not be visible next to the yolk sac. At five weeks, it can be too small to detect, and many providers won’t even attempt an ultrasound this early because the results can be ambiguous and cause unnecessary worry. Seeing just a gestational sac without a visible embryo at five weeks is completely normal. A follow-up scan a week or two later will typically show much more.
How Big It Actually Is
The entire embryo at five weeks measures roughly 1.5 to 2.5 millimeters from top to bottom, depending on the exact day of development. That’s smaller than a grain of rice. The gestational sac surrounding it is larger, usually around 6 to 12 millimeters, which is why the sac is visible on ultrasound before the embryo is. The size difference between the embryo and its surrounding structures is dramatic at this stage. The yolk sac, for instance, is nearly as large as the embryo itself.
What Changes in the Days Ahead
Week five sits right at the beginning of the most intense period of embryonic development. Over the next three to four weeks, the embryo will grow from a tiny curved speck into something that starts to look vaguely human, with visible eyes, defined limb buds that split into fingers and toes, and a straightening body. By week eight, it will be about the size of a raspberry and will have all its major organ systems at least partially formed. But at five weeks, it’s still very much in the early construction phase, with the basic blueprint being laid down but very little that’s recognizable to the human eye.