At 5 weeks pregnant, the embryo is roughly the size of a sesame seed, measuring about 2 millimeters (0.08 inches) from end to end. That’s tiny enough to sit on the tip of a pen, yet an extraordinary amount of development is already underway inside this speck-sized cluster of cells.
What the Embryo Looks Like at 5 Weeks
At this stage, the embryo doesn’t look anything like a baby yet. It’s a curved, tadpole-shaped structure with a distinct top and bottom, front and back. The body has already organized itself into three foundational cell layers that will become every organ and tissue in the body. The outer layer is destined to form the brain, spinal cord, nerves, and skin. The middle layer will develop into the heart, bones, muscles, kidneys, and blood. The inner layer becomes the digestive system, respiratory system, and major glands.
Even though the embryo is only about 2 mm long, this layered blueprint means that organ formation is already beginning in earnest.
The Heart Is Starting to Take Shape
One of the most remarkable things happening at 5 weeks is heart development. The heart begins as a simple, straight tube running along the front of the embryo. Over the course of this week, that tube lengthens and bends into a C-shaped loop, which is the very first step toward becoming a four-chambered heart.
Weak contractions can begin around this time. The earliest heartbeat activity has been detected as early as 34 gestational days (near the start of week 5), though for many embryos it doesn’t become detectable until closer to 49 gestational days (week 7). If you have an early ultrasound at 5 weeks, a heartbeat may or may not be visible, and that’s completely normal. At this point, the heart is just starting to push plasma through the embryo’s developing blood vessels; actual circulation of red blood cells comes slightly later.
The Brain and Spinal Cord Are Forming
The neural tube, the structure that becomes the brain and spinal cord, finishes closing by the end of week 4 and into week 5. This is one of the most critical windows in early pregnancy. Neural tube defects, which include conditions like spina bifida, occur when this tube fails to fully close, typically between days 21 and 28 after conception. This is why folic acid intake before and during early pregnancy is so important: it supports proper closure of this structure.
Once sealed, the top end of the neural tube begins expanding into what will eventually become distinct regions of the brain, while the lower portion develops into the spinal cord. At 5 weeks, these are still primitive structures, but the architectural plan is in place.
What You’d See on an Ultrasound
If you have a transvaginal ultrasound at 5 weeks, don’t expect to see much. The gestational sac, a small fluid-filled pocket in the uterus, is usually visible. Inside it, you may see the yolk sac, a round structure about 5 to 6 mm across that provides nutrients to the embryo before the placenta takes over. The yolk sac is often the first thing visible on ultrasound, sometimes appearing before the embryo itself can be spotted.
The embryo at this size is at the very edge of what ultrasound can detect. Many practitioners prefer to wait until 6 to 8 weeks for a first scan, when the embryo is larger and a heartbeat is more reliably visible. An ultrasound at 5 weeks that shows only a gestational sac and yolk sac is a normal finding, not a cause for concern.
What You Might Be Feeling
Five weeks is right around the time many people first realize they’re pregnant, often because of a missed period and a positive home test. The pregnancy hormone hCG, which is what home tests detect, typically ranges from 200 to 7,000 ยต/L at 5 weeks. That wide range is normal; levels vary significantly from person to person and rise rapidly, often doubling every two to three days.
Rising hormones trigger some of the earliest physical symptoms. Extreme fatigue is one of the most common complaints in the first trimester, and it can hit hard this week. You might also notice sore or swollen breasts, nausea (which can strike at any time of day, not just mornings), bloating, and cramping that feels similar to period pain. Some people experience a metallic taste in their mouth, stronger reactions to certain smells, or sudden new food preferences and aversions. Light spotting can also occur and is often harmless, though heavier bleeding warrants a call to your provider.
Mood swings, more frequent urination, and a heightened sense of smell round out the list of common early signs. Not everyone gets all of these, and some people feel almost nothing at 5 weeks. The intensity of symptoms doesn’t indicate how healthy the pregnancy is.
Putting the Size in Perspective
It can be hard to grasp how much is happening in something so small. At 2 mm, the embryo is roughly 1/12th of an inch. By comparison, a grain of rice is about 7 mm. Yet in this tiny structure, the groundwork for the nervous system, cardiovascular system, and digestive tract is already being laid. Over the next few weeks, the embryo will grow rapidly, roughly doubling in size each week through the first trimester, and distinct features like limb buds and facial structures will start to emerge by weeks 6 and 7.