At six weeks of pregnancy, the embryo is roughly the size of a lentil or a small pea, measuring about 2 to 5 millimeters long. It doesn’t yet look like a baby. It’s a curved, tadpole-shaped structure with a bulging head end, a tail-like extension, and tiny bumps where limbs will eventually grow. Most people first encounter what a six-week embryo looks like on an early ultrasound, where it appears as a small bright spot next to a round pouch inside a dark fluid-filled sac.
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 when conception actually happened. Because ovulation and fertilization typically occur about two weeks into the cycle, a six-week embryo has really only been developing for about four weeks. This distinction matters because some pregnancy resources describe features by gestational age and others by embryonic age, which can create confusion about what’s happening when.
What the Embryo Looks Like
At this stage, the embryo is C-shaped, curling in on itself with a prominent head region that makes up a large portion of its total length. The body has a soft, translucent quality since skin and bone haven’t formed yet. There are no recognizable facial features, but small depressions are beginning to form where the eyes and ears will eventually develop. The neural tube, the structure that becomes the brain and spinal cord, is in the process of closing along the back.
Tiny limb buds are just starting to appear, looking more like small paddles or bumps on the sides of the body than anything resembling arms or legs. There are no fingers or toes. A thin tail-like structure extends from the lower end of the body, which will gradually recede over the coming weeks. Overall, the embryo at six weeks is not easily distinguishable from the early embryos of many other species.
What’s Developing Inside
Despite its small size, significant internal development is underway. The most notable event at six weeks is the heart. A tiny tube-shaped cluster of cells has begun rhythmically pulsing, beating around 110 times per minute by the end of week five into week six. This isn’t yet a four-chambered heart like the one you have. It’s a simple tube that will fold and divide into chambers over the next several weeks.
The brain is forming from three primary bulges at the head end of the neural tube, which will eventually become the forebrain, midbrain, and hindbrain. These are rudimentary structures at this point, not yet capable of processing sensory information. The early digestive and respiratory systems are also taking shape. A primitive gut tube runs through the body, and tiny lung buds have branched off from the tube that will become the windpipe. The liver is beginning to form as well, though none of these organs are functional yet.
What You See on Ultrasound
If you have a transvaginal ultrasound at six weeks, you’ll see a dark circular area on the screen: the gestational sac, which is the fluid-filled space surrounding the embryo. Inside the sac, there are typically two visible structures. One is the yolk sac, a small round pouch that provides nutrients to the embryo before the placenta takes over. The other is the fetal pole, which is the earliest visible form of the embryo itself. It appears as a small thickened area next to the yolk sac.
At six weeks, the fetal pole is often just 2 to 5 millimeters long. Your provider may be able to detect a flickering on the ultrasound, which represents the early cardiac activity. A heartbeat is usually visible once the fetal pole reaches about 5 millimeters, though in 5 to 10 percent of viable pregnancies, cardiac activity isn’t detectable when the embryo measures between 2 and 4 millimeters. That doesn’t necessarily mean something is wrong. It can simply mean the embryo is a few days earlier in development than expected.
At this early stage, it’s often not possible to tell the head end from the tail end on ultrasound. Providers take a “greatest length” measurement rather than a true crown-to-rump measurement, which becomes more precise in the weeks that follow. Because the embryo is typically curled tightly at this point, with its chin tucked to its chest, even the length measurement is an approximation.
How Size Is Measured This Early
The standard measurement used in early pregnancy is called the crown-rump length, or CRL. It’s taken from the top of the head to the bottom of the torso, excluding the legs (which barely exist at this point). At six weeks, the embryo is so small and so tightly flexed that the measurement is really more of a neck-to-rump length, though clinicians still call it a CRL. This measurement is used to estimate gestational age and confirm that the pregnancy is developing on schedule.
A transvaginal ultrasound provides a much clearer image than an abdominal one at this stage because the embryo is so tiny. Providers magnify the image until the embryo fills most of the screen and place electronic calipers at each end to record the length. They take care to avoid including the yolk sac in the measurement, which sits right next to the fetal pole and could inflate the number.
What Changes in the Coming Weeks
Growth between weeks six and eight is rapid. By week seven, the limb buds begin to lengthen and separate into arm and hand segments. By week eight, the tail recedes, the face starts to take a more recognizable shape with nostrils and upper lip visible, and the embryo roughly doubles in size. The transition from “embryo” to “fetus” officially happens at the end of week ten (gestational age), by which point all major organ systems have at least begun to form and the body plan looks distinctly human.
At six weeks, though, the embryo is still in its most foundational phase. It’s building the basic architecture that every later stage of development depends on, laying down the central nervous system, establishing a circulatory loop, and setting the pattern for limbs and organs that won’t be fully functional for months.