At 5 weeks of pregnancy, your baby is roughly the size of a sesame seed, measuring about 2 millimeters long. It doesn’t look like a baby yet. The embryo is a tiny, curved structure shaped like the letter C, with a small tail and the very first beginnings of a head, heart, and limb buds.
What the Embryo Actually Looks Like
At this stage, the embryo has a distinct C-shaped curvature with a relatively large tail bud that tapers gradually to a rounded end. This tail is a normal part of human development. It contains early spinal tissue, and it will continue to grow for another week or two before it begins to shrink. By around week 8, it disappears completely.
The head end is already the largest part of the body, curling forward over what will become the chest. The embryo is translucent and would be barely visible to the naked eye. There are no recognizable facial features yet, but the sites where the eyes and ears will eventually form are marked by tiny thickenings in the tissue called placodes. By the end of week 5 (around day 35 after fertilization), the earliest traces of pigment appear in the developing retina, the first hint that eyes are on their way.
What’s Forming Inside
Despite its tiny size, week 5 is one of the most active periods of organ development. The embryo has three layers of cells, and each one is building something different.
The heart is the star of this week. It began beating just a few days ago, around 22 days after fertilization, making it the first functioning organ. At 5 weeks, the heart is a simple tube that has started to fold and divide into separate chambers. It’s pumping blood through a basic circulatory system, though the heartbeat is still too faint to hear with any device. On an early ultrasound, a technician may be able to see the flicker of motion, but many providers won’t attempt to detect cardiac activity this early because it’s simply too soon to pick up reliably.
The lungs are just appearing as two tiny buds branching off a central tube. The liver is already enlarging and developing its network of blood vessels. The spleen appears as a small bulge in surrounding tissue. Even the pituitary gland, a pea-sized structure in the brain that will eventually regulate hormones, is beginning to take shape and connect to the developing brain.
Upper and lower limb buds are visible by around day 32. These look like small, paddle-shaped bumps on the sides of the body. They don’t resemble arms or legs yet, but over the next few weeks they’ll elongate and develop fingers and toes.
What You’d See on an Ultrasound
If you have an ultrasound at 5 weeks, don’t expect to see anything that looks like a baby. A transvaginal ultrasound at this stage typically shows a gestational sac, which is a small, dark circle of fluid inside the uterus. Inside that sac, a smaller bright ring called the yolk sac may be visible. The yolk sac provides nutrients to the embryo during these early weeks while the placenta is still forming.
The embryo itself may or may not be visible at 5 weeks. It’s so small that it can be easy to miss, and many providers prefer to wait until 6 or 7 weeks for a more informative scan. Seeing only a gestational sac and yolk sac at 5 weeks is completely normal and doesn’t indicate a problem.
How the Embryo Gets Nutrients
The placenta is already under construction but won’t be fully functional for several more weeks. Right now, cells from the embryo are burrowing into the uterine wall and beginning to remodel your blood vessels to create a blood supply for the placenta. During this early phase, the placenta operates in a relatively low-oxygen environment. The yolk sac fills the gap, delivering essential nutrients to the embryo until the placental connection matures, which happens gradually through the first trimester.
Putting the Size in Perspective
It helps to remember that “5 weeks pregnant” is measured from the first day of your last period, not from conception. The embryo itself is only about 3 weeks old at this point. That three-week-old cluster of cells has already built a beating heart, started laying the groundwork for every major organ, and grown limb buds that will become arms and legs. Over the next week alone, the embryo will roughly double in size, and facial features will begin to take a more recognizable shape. Week 5 is early, but the pace of change from here is rapid.