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’s developing inside you right now more closely resembles a tiny curved tadpole, with a head end, a tail end, and the very earliest foundations of a heart, brain, and spinal cord taking shape.
What You’d See on an Ultrasound
If you had a transvaginal ultrasound at 5 weeks, you wouldn’t see much that looks like a baby. The most visible structure is the gestational sac, a small dark circle filled with fluid inside your uterus. Within that sac, a round structure called the yolk sac is often the first thing that shows up on screen, sometimes before the embryo itself is even visible. The yolk sac is tiny, about 5 to 6 millimeters across, and it provides nutrients to the embryo until the placenta takes over that job in later weeks.
The embryo itself may or may not be detectable at this point, depending on the exact day and the quality of the equipment. Many women who get an early ultrasound at 5 weeks see only the gestational sac and are asked to come back a week or two later, when more is visible. This is completely normal and doesn’t indicate a problem.
The Embryo’s Shape and Size
At this stage, the embryo is a flat, layered disc that’s beginning to curl into a C-shape. It has three distinct cell layers that are already assigned to build different parts of the body. The outer layer will eventually form skin, hair, and parts of the nervous system. The middle layer is destined to become muscle, bone, the heart, and the circulatory system. The innermost layer will develop into the lining of the digestive tract, lungs, liver, and pancreas.
There are no arms, legs, fingers, or recognizable facial features yet. What exists is a foundation: a blueprint of cells organizing themselves into the structures that will become organs over the coming weeks.
The Heart Is Just Starting
One of the most remarkable things happening at 5 weeks is the very beginning of heart activity. By the end of the fourth week of gestation (which overlaps with the start of week 5 in pregnancy dating), the embryo’s primitive heart tube begins to beat. This isn’t a fully formed heart with four chambers. It’s a simple tube that contracts rhythmically, pushing blood through a developing circulatory system that connects the embryo to the chorionic villi, the structures that interface with your uterine lining to exchange oxygen and nutrients.
You’re unlikely to hear or see this heartbeat on ultrasound at exactly 5 weeks. Most providers can detect cardiac activity between 6 and 7 weeks, when the heart is beating more strongly and the embryo is large enough to visualize clearly.
Brain and Spinal Cord Formation
The neural tube, a narrow channel that runs along the length of the embryo, folds and closes during weeks 3 and 4 of gestation. By week 5, this process is completing. The upper portion of the neural tube is forming the earliest structures of the brain and skull. The lower portion is becoming the spinal cord and the bones that will protect it.
This is one of the most critical windows in embryonic development. If the neural tube doesn’t close properly, it can lead to neural tube defects affecting the brain or spine. Folic acid plays a protective role during this exact period, which is why prenatal vitamins containing folic acid are recommended starting before conception. Researchers still don’t fully understand the mechanism behind this protection, but the evidence that it reduces risk is strong enough to be one of the most important findings in birth defect prevention.
What’s Happening in Your Body
Your body is producing rapidly increasing levels of hCG, the hormone that triggered your positive pregnancy test. At 5 weeks, typical hCG levels range from about 200 to 7,000 µ/L, a wide range because levels vary significantly from one pregnancy to another and even from one day to the next. This hormone is partly responsible for early pregnancy symptoms like nausea, breast tenderness, and fatigue, though many women at 5 weeks don’t feel much of anything yet.
There’s no visible bump. Your uterus is still about the same size it was before pregnancy, and the embryo is so small it occupies only a fraction of the gestational sac. Externally, nothing has changed. Internally, the pace of development is extraordinary: cells are dividing, migrating, and specializing faster during these early weeks than at any other point in pregnancy.
Week 5 Compared to Later Weeks
It helps to have a sense of scale. At 5 weeks, the embryo is a 2-millimeter curved structure with no limbs and no distinguishable face. By week 8, it will be about the size of a raspberry, with tiny arm and leg buds, the beginnings of ears, and a more recognizable head shape. By week 12, all major organs will have formed in basic structure, and the embryo will officially be called a fetus.
Week 5 sits right at the start of this cascade. What you’re carrying doesn’t look like a baby in any visual sense, but the groundwork being laid right now, the neural tube closing, the heart tube pulsing, the three cell layers organizing into future organs, is setting the trajectory for everything that follows.