At 5 weeks of pregnancy, the embryo is roughly the size of a sesame seed, measuring about 2 millimeters (less than one-tenth of an inch) from end to end. That’s tiny enough to sit on the tip of a pen, but an enormous amount of development is already underway inside it.
One important note on terminology: at 5 weeks, this is technically an embryo, not a fetus. The embryonic stage lasts from fertilization through about 8 weeks. After that point, the developing baby is called a fetus. But regardless of the label, what’s happening at week 5 is remarkable for something so small.
What 5 Weeks Actually Looks Like
The embryo itself isn’t visible to the naked eye in any meaningful detail. If you were to have a transvaginal ultrasound at this stage, you wouldn’t see anything resembling a baby. What shows up on the screen is the gestational sac, a fluid-filled cavity inside the uterus that surrounds the embryo. Inside that sac, a provider may be able to spot the yolk sac, a small circular structure that nourishes the embryo before the placenta takes over. The yolk sac typically measures 3 to 5 millimeters across.
Many women who get an early ultrasound at 5 weeks are surprised by how little there is to see. It’s common to spot only the gestational sac at this point, with no visible heartbeat or embryo yet. That doesn’t necessarily signal a problem. The timing can vary by a few days depending on when ovulation and implantation actually occurred, so a follow-up scan a week or two later often reveals much more.
What’s Developing at This Stage
Despite being barely visible, the embryo at 5 weeks is in the middle of some of the most critical development of the entire pregnancy. The nervous system is actively forming. The brain and spinal cord are taking shape from a structure called the neural tube, which folds and closes during weeks 3 and 4. By week 5, this process is wrapping up, which is why adequate folate intake before and during early pregnancy is so important for preventing neural tube defects.
The tiny heart is also starting to form and will beat for the first time around this week. It doesn’t look like a four-chambered heart yet. At this point it’s more of a tube-shaped structure that begins rhythmic contractions to push blood through the embryo’s developing circulatory system. The embryo already has some of its own blood vessels, and a set of them is forming what will become the umbilical cord.
Three layers of cells are differentiating into the foundations of every organ system: one layer will become the skin and nervous system, another will form the digestive and respiratory organs, and the third gives rise to the heart, muscles, and bones. All of this is happening in a structure smaller than a grain of rice.
How Size Changes Week by Week
Growth in early pregnancy is exponential. At 4 weeks, the embryo is barely a millimeter. At 5 weeks, it roughly doubles to about 2 millimeters. By week 6, it reaches around 5 to 6 millimeters, and by week 8, it can be 14 to 20 millimeters long (about the size of a kidney bean). This rapid scaling is why early ultrasound measurements can estimate gestational age so precisely. Even a difference of a few millimeters tells a provider a lot about how far along the pregnancy is.
The measurement used in early pregnancy is called the crown-rump length, which is exactly what it sounds like: the distance from the top of the head to the bottom of the torso. At 5 weeks, though, there’s no distinguishable “crown” or “rump” yet, so the measurement is more of a general length of the embryonic disc.
What You Might Be Feeling
At 5 weeks, the hormone that pregnancy tests detect (hCG) is rising quickly. Typical levels at this stage range from about 200 to 7,000 units per liter, a wide range that reflects how rapidly things change day by day. This hormonal surge is responsible for many of the earliest pregnancy symptoms: nausea, breast tenderness, fatigue, and frequent urination. Some women feel all of these at 5 weeks, while others notice almost nothing yet. Both are normal.
If you’ve just gotten a positive pregnancy test, you’re likely right around this stage. The embryo is microscopic, but the biological groundwork being laid this week, particularly in the brain, spinal cord, and heart, makes it one of the most developmentally significant points in the entire pregnancy.