At six weeks of pregnancy, an embryo is tiny, roughly the size of a sweet pea, and looks nothing like a baby yet. It appears as a small curved structure called a fetal pole, measuring about 5 to 7 millimeters long, nestled inside a fluid-filled gestational sac. Despite its size, major developments are already underway beneath the surface.
Gestational Age vs. Actual Age
When doctors say “six weeks pregnant,” they’re counting from the first day of your last menstrual period, not from the day of conception. Because ovulation typically happens about two weeks into a cycle, the embryo’s actual age at the six-week mark is closer to four weeks. This two-week gap matters because descriptions of embryonic development sometimes reference fertilization age, which can create confusion about what’s happening and when.
What You See on Ultrasound
On a transvaginal ultrasound at six weeks, you’ll see a dark, round gestational sac. Inside it sits the fetal pole, a small bright spot, alongside the yolk sac, a tiny pouch that provides nutrients to the embryo at this stage. The embryo itself doesn’t have recognizable human features. It looks more like a small grain or tadpole-shaped structure with a slight curve.
A heartbeat is often detectable by the time the fetal pole reaches 5 to 7 millimeters in length, which typically happens around the six-week mark. The heart rate at this stage usually measures about 100 to 120 beats per minute. If a heartbeat isn’t visible yet, your provider may simply recommend a follow-up scan a week or so later, since timing can vary by a few days depending on when ovulation actually occurred.
The Heart and Circulatory System
The heart is the first functioning organ. By the end of the fifth week of pregnancy, it’s already pumping blood through a primitive circulatory system. At six weeks, the heart is still a simple tube-like structure that hasn’t yet divided into its four chambers, but it’s actively circulating blood throughout the embryo’s tiny body. The heartbeat is fast and rhythmic, visible as a flicker on ultrasound even though it’s far too faint to hear without specialized equipment.
Brain and Spinal Cord
The neural tube, a narrow channel running along the embryo’s back, is in the process of closing during this period. The upper portion of this tube will become the brain and skull, while the lower portion forms the spinal cord and the small bones of the back. This closure is one of the most critical milestones of early pregnancy. When the neural tube doesn’t close properly, it can lead to neural tube defects, which is why folate intake in the weeks before and after conception is so important.
At six weeks, the brain is just beginning to organize into distinct regions, though it’s still extremely primitive. The head end of the embryo is proportionally large compared to the rest of the body, a sign of how much neural development is happening.
Limbs, Eyes, and Ears
Tiny buds are forming where the arms and legs will eventually grow. At this stage, they look like small bumps on the sides of the embryo’s body, with no fingers or toes yet. Structures that will become the eyes, ears, and mouth are also beginning to take shape, though they’re not recognizable as facial features. The “eyes” at this point are small thickened patches of tissue on either side of the head, and the “ears” are shallow indentations. None of this is visible on a standard ultrasound, only under magnification.
Internal Organs Taking Shape
The embryo’s internal organs are developing from three distinct layers of cells that formed in the weeks prior. The middle layer is giving rise to the heart, bones, kidneys, and parts of the reproductive system. The inner layer is forming the lungs and intestines. None of these organs are functional yet (aside from the heart), but the blueprint is being laid down during this window. This is why the first trimester is considered the most sensitive period for organ development, and why exposure to certain substances or infections during these weeks carries more risk.
How Big Is It, Really?
At 5 to 7 millimeters, the embryo is smaller than a pencil eraser. The gestational sac surrounding it is larger, roughly 2 to 2.5 centimeters across, which is why it’s visible on ultrasound even though the embryo itself is so small. The entire structure could sit on the surface of a dime with room to spare. Despite this size, the pace of development is extraordinary. In just the next two weeks, the embryo will roughly double or triple in length, and features like fingers and a more defined face will start to emerge.