At 5 weeks pregnant, your baby is about the size of a sesame seed. The embryo measures roughly 2 millimeters from end to end, so small it’s barely visible to the naked eye. But despite that tiny size, this is one of the most active weeks of early development, with the foundations of the brain, spinal cord, and heart already taking shape.
What the Embryo Looks Like at 5 Weeks
At this stage, the embryo doesn’t look like a baby yet. It’s a curved, tadpole-shaped cluster of rapidly dividing cells, and it has already organized itself into three distinct layers that will become every organ and tissue in your baby’s body. The outer layer develops into the nervous system, skin, and hair. The middle layer forms the skeleton, muscles, kidneys, and blood. The inner layer goes on to build the digestive system, respiratory system, and many hormone-producing glands.
This layered structure is the blueprint for everything that follows. By the end of week 5, the tube that will become your baby’s brain and spinal cord (called the neural tube) is forming along the length of the embryo. A tiny tube-shaped structure that will become the heart has also begun to pulse, beating around 110 times per minute by the end of this week. That’s nearly twice the resting heart rate of an adult, even though the “heart” is little more than a flickering cluster of cells at this point.
What Shows Up on an Ultrasound
Most providers won’t schedule an ultrasound at 5 weeks unless there’s a specific reason, like pain, bleeding, or a history of complications. If you do have one, a transvaginal ultrasound can typically show a gestational sac, which is the fluid-filled structure surrounding the embryo. Inside it, the yolk sac (a small round structure about 5 to 6 millimeters across) may also be visible. The yolk sac nourishes the embryo before the placenta takes over, and it’s sometimes spotted before the embryo itself can be seen.
The embryo at this size is often too small to pick up clearly on ultrasound, so don’t be alarmed if only the sac is visible. By weeks 6 to 7, the embryo and its heartbeat are much easier to detect.
What You Might Be Feeling
Week 5 is often when pregnancy symptoms first appear, because your hormone levels are climbing rapidly. The hormone hCG, which pregnancy tests detect, typically ranges from about 217 to 8,245 mIU/mL during week 5. That’s a wide range because hCG roughly doubles every 48 to 72 hours in early pregnancy, so levels vary dramatically depending on exactly when you test.
Those rising hormones are responsible for most of what you’re feeling. Common symptoms at this stage include:
- Extreme tiredness, often the first noticeable change
- Nausea, which can strike at any time of day despite being called “morning sickness”
- Sore or tender breasts
- Mood swings
- Bloating and cramping similar to period pains
- Needing to urinate more often
- A heightened sense of smell and sudden food aversions or cravings
- Light spotting, which can be implantation bleeding as the embryo attaches to the uterine lining
Some people experience most of these, and others feel almost nothing at 5 weeks. Both are normal. A metallic taste in the mouth, changes in hair thickness, and darkened patches of skin on the face are also reported in the first trimester, though these are less common this early.
Why Folic Acid Matters Right Now
Because the neural tube is actively forming during week 5, this is a critical window for folic acid. The CDC recommends 400 micrograms daily for anyone who could become pregnant, ideally starting before conception. Folic acid helps the neural tube close properly, reducing the risk of birth defects in the brain and spine. If you haven’t been taking a prenatal vitamin, starting now still provides benefit since the neural tube continues developing through about week 6. People who have had a previous pregnancy affected by a neural tube defect are generally advised to take a much higher dose of 4,000 micrograms daily, under the guidance of their provider.
Putting the Size in Perspective
It can be hard to believe that something the size of a sesame seed already has a beating heart tube and the beginnings of a nervous system. Growth accelerates quickly from here. By week 6, the embryo roughly doubles in size. By week 8, it reaches about the size of a raspberry and has the early structures of every major organ. The sesame seed stage is fleeting, but it’s laying the groundwork for everything that comes next.