At 5 weeks of pregnancy, the embryo is tiny, roughly the size of a sesame seed, measuring about 2 millimeters long. It doesn’t look like a baby yet. What you’d see, if you could see it at all, is a small curved shape resembling a tadpole, surrounded by a fluid-filled sac inside the uterus. Technically, it’s called an embryo at this stage, not a fetus. The American College of Obstetricians and Gynecologists defines a fetus as the stage beginning after 8 completed weeks of fertilization, so the term “fetus” won’t apply for several more weeks.
What’s Happening Inside at 5 Weeks
Week 5 is when the embryo transitions from a flat disc of cells into something with distinct layers and the earliest hints of a body plan. Three cell layers have formed, and each one is destined to build different parts of the body. The outer layer will eventually become the skin, nervous system, eyes, and inner ears. The middle layer gives rise to the heart, blood vessels, bones, muscles, and kidneys. The inner layer will form the lungs, liver, and digestive tract.
None of these organs exist yet in any recognizable form. What’s happening is more like a blueprint being laid down. Cells are migrating, folding, and signaling to each other to begin organizing into the structures that will take shape over the coming weeks.
The First Sign of a Heart
One of the most notable developments at 5 weeks is the very beginning of cardiac activity. The developing heart at this point is not a four-chambered organ. It’s essentially two tubes that have fused together in the middle, with four smaller tubes branching off. Cardiac tissue begins to contract, creating a flickering pulse that can sometimes be detected on a transvaginal ultrasound. This is often described as a “heartbeat,” though the heart itself hasn’t fully formed yet. It’s more accurate to think of it as cardiac tissue pulsing rhythmically.
The Neural Tube Is Forming
Around this time, the neural tube is beginning to take shape along what will become the baby’s back. By week 6, this tube closes, and it eventually develops into the brain and spinal cord. This is one of the most critical periods of early pregnancy, which is why folic acid intake before and during early pregnancy is so strongly emphasized. Disruptions to neural tube closure can lead to conditions like spina bifida.
At 5 weeks, the head end and tail end of the embryo are becoming distinguishable, giving it that curved, C-shaped appearance. But there are no visible facial features, no limbs, and no fingers or toes. Limb buds won’t start appearing until around week 6 or 7.
What You’d See on an Ultrasound
If you have an ultrasound at 5 weeks, don’t expect to see much. A transvaginal ultrasound (the type used this early, with a probe inserted vaginally rather than placed on the abdomen) can typically show a gestational sac, which looks like a small dark circle on the screen. Inside it, a yolk sac may be visible. The yolk sac is a small round structure that provides nutrients to the embryo before the placenta takes over. It’s one of the first signs that a pregnancy is developing normally.
The embryo itself is often too small to see clearly at exactly 5 weeks. Many providers prefer to wait until 6 to 8 weeks for the first ultrasound because there’s simply more to see. If you do go in at 5 weeks and only a gestational sac is visible, that’s typical and not necessarily a cause for concern. A follow-up scan a week or two later usually shows considerably more development.
How Big Is It, Really?
The embryo at 5 weeks is about 1 to 2 millimeters from end to end. That’s smaller than a grain of rice. The gestational sac surrounding it is larger, roughly the size of a small pea, which is what makes it visible on ultrasound even when the embryo itself is hard to spot. To put it in perspective, the entire structure could sit on your fingertip.
What’s Happening in Your Body
At 5 weeks, the hormone hCG (the one pregnancy tests detect) is rising rapidly. Typical levels at this point range from about 200 to 7,000 units per liter, a wide range that reflects how quickly levels double every couple of days. This surge is what triggers many early pregnancy symptoms: nausea, breast tenderness, fatigue, and frequent urination. Some people have noticeable symptoms by now, while others feel completely normal. Both are common.
The placenta is also beginning to develop but won’t fully take over its role of delivering oxygen and nutrients for several more weeks. In the meantime, the yolk sac handles that job.
Why 5 Weeks Feels So Early
Pregnancy weeks are counted from the first day of your last menstrual period, not from conception. So at “5 weeks pregnant,” the embryo has only been developing for about 3 weeks since fertilization. This is why so little is visible. Many people don’t even realize they’re pregnant until around this time, when a missed period prompts a test. The dramatic transformation from a cluster of cells into something recognizably human happens mostly between weeks 6 and 12, when facial features form, limbs grow, and organs begin functioning.