At 5 weeks of pregnancy, the embryo is about 2.5 millimeters long, roughly the size of a sesame seed. On an ultrasound, you won’t see anything that looks like a baby yet. What you will see, if anything shows up at all, is a small dark circle called a gestational sac, possibly with a tiny round structure inside it. Here’s what’s actually happening at this stage, both inside the uterus and in your own body.
What a 5-Week Ultrasound Shows
If you have a transvaginal ultrasound at 5 weeks, the most prominent feature is the gestational sac: a small, fluid-filled dark circle within the uterine lining. Inside that sac, a round structure called the yolk sac may be visible. The yolk sac is typically 5 to 6 millimeters across and sometimes appears before the embryo itself can be detected. It looks like a small bright ring inside the darker gestational sac, and it serves as the embryo’s earliest source of nutrition before the placenta takes over.
A fetal pole, the first visible form of the embryo on ultrasound, is generally not detectable until around six weeks. In some cases, it doesn’t appear on imaging until as late as nine weeks. At 5 weeks, the embryo is simply too small to show up clearly. A heartbeat is similarly not expected yet; it typically becomes detectable once the fetal pole reaches 5 to 7 millimeters, which happens closer to six weeks. If you get an early scan and nothing beyond a gestational sac is visible, that’s normal for this stage and not automatically a cause for concern.
How Big the Embryo Actually Is
The crown-rump length of the embryo at 5 weeks measures a median of 2.5 millimeters, with a typical range between 1.8 and 4.3 millimeters. That’s smaller than a grain of rice. At this point, the embryo doesn’t have recognizable features like limbs or a face. It looks more like a tiny curved shape, sometimes compared to a tadpole or a comma, nestled within the gestational sac.
What’s Forming Inside the Embryo
Despite its minuscule size, the embryo at 5 weeks is in one of the most active phases of early development. Three foundational cell layers established during week 3 are now differentiating into the building blocks of every organ system. The outer layer gives rise to the brain, spinal cord, and skin. The middle layer forms the heart, blood vessels, muscles, bones, and the lining of the lungs. The inner layer becomes the digestive organs, including the liver, which begins a period of rapid growth between weeks 5 and 10.
The most dramatic event at 5 weeks is the formation of the heart tube. It starts as a short, straight tube running along the front of the developing gut. Over the course of this stage, the tube lengthens and curves into a C-shape, a process called looping. This primitive heart begins generating traveling mechanical waves that push plasma through the embryo’s tiny blood vessels. True circulation of red blood cells starts around the transition from this stage to the next, once the C-shaped loop connects to the earliest blood vessels. This isn’t a heartbeat you could hear or see on ultrasound yet, but the basic pumping mechanism is already at work.
The neural tube, which will become the brain and spinal cord, is also closing during this period. By the end of this stage, the brain is beginning to divide into its earliest compartments, the precursors to distinct brain structures that will continue developing through week 8.
What You Might Be Feeling
Five weeks is right around the time many people first realize they’re pregnant, often because a period is late. Some notice light spotting, which can be implantation bleeding as the embryo settles into the uterine lining. This is easy to mistake for a light period.
Hormonal shifts are already producing noticeable symptoms for many people at this stage. The pregnancy hormone hCG, which home tests detect, typically ranges from 200 to 7,000 units per liter at 5 weeks. That wide range is normal; levels vary significantly between individuals and even between pregnancies in the same person. Rising hCG is what drives many of the symptoms you may already feel:
- Fatigue: Extreme tiredness is one of the earliest and most common first-trimester symptoms.
- Nausea: Often called morning sickness, but it can strike at any time of day.
- Breast tenderness: Soreness or swelling from increased blood flow and hormonal changes.
- Frequent urination: Your body is already increasing blood volume and fluid processing.
- Food aversions or cravings: Hormonal changes alter your senses of taste and smell, sometimes dramatically.
- Mood swings: Rapid hormonal fluctuations affect neurotransmitter activity.
- Bloating and mild cramping: The uterus is beginning to change, and cramping similar to period pain is common.
Some people also notice a metallic taste in their mouth, a heightened sense of smell, or a thin milky white vaginal discharge. Skin changes like darkened patches on the face can appear this early, though they’re more common later in pregnancy. Not everyone experiences all of these symptoms, and some people feel almost nothing at 5 weeks. Both experiences fall within the normal range.
If Your Scan Doesn’t Show Much
Many people who get an ultrasound at 5 weeks feel anxious when they don’t see a clear embryo or heartbeat. At this stage, the most you can typically expect to see is the gestational sac and possibly the yolk sac. The embryo is so small that even high-resolution transvaginal ultrasound may not pick it up yet. If the fetal pole isn’t visible or appears smaller than expected, it often means the pregnancy is a few days earlier than estimated. Your provider will typically schedule a follow-up scan a week or two later to reassess, and in many cases, the embryo and heartbeat become clearly visible by then.