At 5 weeks pregnant, the embryo is roughly the size of a sesame seed and too small to see on most ultrasounds. What is visible, if you do get an early scan, is a tiny fluid-filled gestational sac measuring about 6 millimeters across. Inside the sac, a small white circle called the yolk sac may also appear. The embryo itself, though, is just beginning to take shape.
What an Ultrasound Shows at 5 Weeks
On a transvaginal ultrasound at 5 weeks, the most prominent feature is the gestational sac, which appears as a small dark (black) circle within the uterine lining. Inside that sac, the yolk sac often shows up as a tiny white ring. At this point, the embryo is too small to be detected visually on the screen.
This can feel anticlimactic if you’re expecting to see a recognizable baby shape. Many providers won’t schedule an ultrasound this early for that reason. If you do have one, seeing a gestational sac in the correct location inside the uterus is considered a reassuring sign, even without a visible embryo. By 6 to 7 weeks, the embryo and its heartbeat typically become detectable.
What’s Happening Inside the Embryo
Even though the embryo is barely visible, development is moving fast. The neural tube, which will become the brain and spinal cord, is forming during week 5. This is one of the most critical stages of early development, and it’s the reason folic acid matters so much. Neural tube defects happen in these first few weeks, often before you even know you’re pregnant. The CDC recommends 400 micrograms of folic acid daily before and during early pregnancy to reduce that risk.
The heart is also taking shape. A primitive heart tube begins to pulse around 22 to 23 days after conception. By the end of the fifth week, that tiny tube beats roughly 110 times per minute. It’s not yet a four-chambered heart, just a tube that contracts rhythmically, but it’s already circulating the earliest blood cells.
The Yolk Sac’s Role
Before the placenta is fully established, the yolk sac does the heavy lifting. It serves as the primary exchange point between you and the embryo, handling nutrient transfer, early blood cell production, and waste removal. Think of it as a temporary life-support system. As the placenta develops over the coming weeks, it gradually takes over these functions, and the yolk sac shrinks away.
What You Might Be Feeling
Week 5 is often when pregnancy symptoms first show up, since this is around the time of a missed period. The NHS lists a wide range of possible symptoms at this stage: extreme tiredness, sore breasts, nausea (which can strike at any time of day, not just mornings), mood swings, bloating, and needing to urinate more often. Some women notice a metallic taste in their mouth, a heightened sense of smell, or sudden shifts in food preferences. Light spotting and mild cramping similar to period pain can also occur and are common in early pregnancy.
Not everyone experiences all of these, and some women feel nothing unusual at 5 weeks. Hormonal changes are driving these symptoms, particularly the rapid rise in hCG (the hormone pregnancy tests detect). At 5 weeks, hCG levels in the blood typically range from 200 to 7,000 units per liter. That’s a wide range because levels vary significantly from one pregnancy to another and can double every 48 to 72 hours during this period. The same hormonal surge that causes a positive test result is what triggers nausea, fatigue, and breast tenderness.
How Big Is the Embryo
The embryo at 5 weeks measures about 1 to 2 millimeters from end to end. It doesn’t look like a baby yet. At this stage it resembles a curved, layered disc more than anything recognizable. Three distinct cell layers are forming that will eventually become different body systems: the outer layer becomes the skin and nervous system, the middle layer develops into the heart, bones, and muscles, and the inner layer forms the lungs and digestive tract. The overall shape is sometimes compared to a tiny tadpole, with a rounded head end and a tapered tail.
By the end of this week, the embryo is still enclosed within that small gestational sac, floating in fluid, connected to the yolk sac, and just beginning the explosive growth that will make it visible on ultrasound within the next week or two.