At 5 weeks pregnant, your baby is roughly the size of a sesame seed, measuring about 2 millimeters (0.08 inches) from end to end. That’s tiny, but an extraordinary amount of development is already underway inside that speck-sized embryo.
What Your Baby Looks Like at 5 Weeks
At this stage, your baby doesn’t look like a baby yet. The embryo resembles a small curved tadpole, with a head end and a tail end but no recognizable limbs or features. It’s made up of three distinct layers of cells that will eventually become every organ and tissue in your baby’s body. The outer layer becomes the nervous system and skin. The middle layer forms the heart, bones, muscles, and circulatory system. The inner layer develops into the lungs, liver, and digestive tract.
Despite being barely visible to the naked eye, the embryo is already organized with a clear top-to-bottom axis. A groove running along the back marks where the spinal cord is forming.
The Heart Is Already Starting to Work
One of the most remarkable things happening at 5 weeks is early heart development. The cells that form the heart begin to cluster together and can already pulse. By the end of the fifth week, this tiny heart tube beats around 110 times per minute. It’s not a fully formed heart yet, just a simple tube pushing fluid through the embryo, but it’s the first functioning organ your baby develops.
Neural Tube Closure
The neural tube, a narrow channel that will become your baby’s brain and spinal cord, folds and closes during weeks three and four. By week five, this process is typically finishing up. The upper portion of the tube shapes the brain and skull, while the lower portion forms the spinal cord and the bones that protect it.
This is why folic acid matters so much in early pregnancy. Folic acid is the only form of folate proven to help prevent neural tube defects, which are serious birth defects of the brain and spine. These defects happen in the first few weeks of pregnancy, often before you even know you’re pregnant. The CDC recommends 400 micrograms of folic acid daily for all women who could become pregnant. If you’ve had a previous pregnancy affected by a neural tube defect, the recommendation jumps to 4,000 micrograms daily, starting a month before conception and continuing through the first trimester.
What You’d See on an Ultrasound
If you have an early ultrasound at 5 weeks, don’t expect to see much. A transvaginal ultrasound can usually detect the gestational sac, a fluid-filled cavity inside the uterus that surrounds the embryo. Inside that sac, the yolk sac may also be visible. The yolk sac is a small circular structure that nourishes the embryo before the placenta takes over.
The embryo itself is often too small to see clearly at this point. It’s common for providers to schedule a follow-up ultrasound at 6 to 8 weeks, when a heartbeat can be detected and the embryo is large enough to measure. Seeing only a gestational sac at 5 weeks is completely normal and not a reason to worry.
Pregnancy Hormone Levels at 5 Weeks
At 5 weeks, levels of hCG (the hormone detected by pregnancy tests) typically range from 200 to 7,000 µ/L. That’s a wide range because hCG levels vary enormously between pregnancies and double roughly every 48 to 72 hours in early weeks. A single measurement matters less than the trend over time. If your provider checks your hCG, they’ll likely draw blood twice, a couple of days apart, to confirm the level is rising appropriately.
These rising hormone levels are also why many women start feeling pregnancy symptoms around week 5: nausea, breast tenderness, fatigue, and frequent urination. Some women feel nothing yet, which is also normal.
How Size Changes Over the Next Few Weeks
Growth accelerates quickly from here. At 5 weeks your baby is about 2 millimeters. By week 6, tiny limb buds appear, blood cells take shape, and circulation begins. By week 8, the embryo reaches roughly 16 millimeters (about the size of a kidney bean) and has early versions of all major organs. By the end of the first trimester at 12 weeks, your baby will be about 6 centimeters long, nearly 30 times bigger than it is right now.
Week 5 is one of the most active periods of development in the entire pregnancy. Almost every major organ system is being established, even though the embryo is smaller than a grain of rice.