At 5 weeks of pregnancy, the embryo is roughly the size of a sesame seed, measuring about 2 millimeters from top to bottom. It doesn’t look like a baby yet. On an ultrasound, you’d see a small fluid-filled sac rather than anything resembling a human form. But inside that tiny structure, development is moving fast.
What You’d See on an Ultrasound
If you have a transvaginal ultrasound at 5 weeks, don’t expect to see much. The main visible structure is the gestational sac, a dark, round pocket of fluid inside the uterus that surrounds the embryo. Inside that sac, a smaller circle called the yolk sac becomes visible around this time. The yolk sac provides nutrients to the embryo before the placenta takes over.
The embryo itself is often too small to distinguish clearly at this point. Many women who get early ultrasounds at 5 weeks leave feeling uncertain because there’s so little to see. A follow-up scan a week or two later typically shows much more, including a visible embryo and, shortly after, a heartbeat.
What the Embryo Actually Looks Like
Without magnification, the embryo at 5 weeks is a tiny curved speck, roughly shaped like a tadpole or the letter C. It has no recognizable face, limbs, or fingers. The “head” end is slightly larger than the “tail” end, but both are barely distinguishable. The entire structure is translucent and would be difficult to see with the naked eye.
Under a microscope, though, the picture gets more interesting. The embryo has organized itself into three distinct cell layers that will eventually build every organ and tissue in the body. The outer layer gives rise to the skin, hair, and nervous system. The middle layer becomes the heart, muscles, bones, and blood. The inner layer forms the lining of the digestive tract, lungs, liver, and pancreas. At 5 weeks, these layers are actively folding and rearranging into the earliest versions of organs.
The Heart Is Starting to Form
One of the most dramatic developments at this stage is the heart. It doesn’t yet look like a heart. It’s a simple tube of cells that has begun to loop and fold into the shape that will eventually become four chambers. This primitive heart tube is already starting to generate electrical impulses that cause rhythmic contractions, though a heartbeat isn’t reliably detectable on ultrasound until around week 6 or 7.
Blood vessels are also forming rapidly, connecting the embryo to the yolk sac and to the early placenta. A primitive version of the umbilical cord is taking shape between weeks 4 and 8, as the tissues connecting the embryo to the uterine wall get bundled together and wrapped by the expanding membrane around the embryo.
Brain and Spinal Cord Development
The neural tube, which becomes the brain and spinal cord, is one of the first major structures to develop. It forms as a flat plate of cells that folds inward and zips shut, like a closing zipper, during weeks 3 and 4. By week 5, this tube is typically closed. The upper portion is already beginning to bulge into three distinct sections that will become the forebrain, midbrain, and hindbrain. The lower portion extends down as the earliest spinal cord.
This is why folic acid is so important in the weeks before and after conception. If the neural tube doesn’t close completely, it can lead to neural tube defects like spina bifida or anencephaly. By week 5, that critical window of closure is largely finished, often before many women even realize they’re pregnant.
How Pregnancy Hormones Confirm What’s Happening
Even though the embryo is barely visible, your body is already responding to its presence. The hormone hCG, which pregnancy tests detect, is climbing rapidly. At 5 weeks, typical hCG levels range from about 200 to 7,000 units per liter. That’s a wide range because hCG roughly doubles every 48 to 72 hours during early pregnancy, so levels can vary significantly depending on exactly when in the week they’re measured.
This rising hCG is also what’s behind many early pregnancy symptoms. Nausea, breast tenderness, fatigue, and heightened sense of smell are all common around this time, driven by the hormonal shifts sustaining the pregnancy. Some women feel these symptoms intensely by week 5, while others notice very little yet.
What’s Developing but Not Yet Visible
Though the embryo looks like little more than a dot, the groundwork for major organs is already being laid. Small buds that will become arms and legs haven’t appeared yet, but the regions where they’ll sprout are being chemically signaled. The digestive tract is forming as a simple tube running through the embryo’s center. Tiny clusters of cells are organizing into what will become the kidneys, liver, and lungs over the coming weeks.
The placenta is also developing alongside the embryo. At 5 weeks, it’s beginning to establish the network of blood vessels that will eventually exchange oxygen, nutrients, and waste between you and the growing embryo. For now, the yolk sac is still handling much of the nutritional support, but the placenta is rapidly building the infrastructure to take over that role by the end of the first trimester.