At six weeks of pregnancy, the embryo is roughly the size of a lentil or a small pea, measuring about 5 to 7 millimeters long. It has a distinctly curved, C-shaped body with a visible head end and a narrow tail-like structure at the bottom. It looks nothing like a baby yet. To the naked eye, it would appear as a tiny, translucent grain nestled inside a fluid-filled sac.
Size and Overall Shape
The embryo’s most striking feature at this stage is its pronounced C-shaped curl. The head is disproportionately large compared to the rest of the body, bending forward toward a bulging midsection. A small tail-like projection extends from the lower end, which will gradually shrink and disappear over the coming weeks as the spine develops. The entire structure is translucent and pale, with visible ridges along the back where tissues are folding into place.
At roughly a quarter of an inch, it’s easy to underestimate how much is happening inside that tiny form. The body is segmented into repeating bulges called somites, which will eventually become the vertebrae, ribs, and muscles. Small paddle-shaped limb buds have started to emerge where the arms and legs will grow.
The Heart and Circulatory System
One of the most remarkable developments at six weeks is the heart. It isn’t a four-chambered organ yet. Instead, it’s a simple tube that has begun to loop and fold on itself, creating the earliest version of separate chambers. This tiny tube pulses around 110 times per minute by the end of week five, and the rate continues to climb through week six. A heartbeat can often be detected on ultrasound once the embryo reaches 5 to 7 millimeters, which typically happens right around this time.
Blood vessels are also forming rapidly, creating a basic circulatory network that connects the embryo to the yolk sac, its primary source of nutrients at this stage.
Brain and Nervous System
The neural tube, which will become the brain and spinal cord, has closed by six weeks. This is a critical milestone. The front end of the tube is already swelling into three distinct bulges that will develop into the major regions of the brain: the forebrain, midbrain, and hindbrain. These look like a series of bumps at the head end of the embryo, giving the head its oversized appearance relative to the rest of the body.
Nerve cells are multiplying at an extraordinary rate and beginning to form early connections. The spinal cord runs the length of the embryo’s back, and the tail-like structure at the bottom is actually an extension of this developing spine that hasn’t yet been absorbed.
Early Facial Features
A face is just beginning to take shape, though it wouldn’t be recognizable as one. Small depressions called optic pits have formed on either side of the head, marking where the eyes will eventually develop. Similar shallow pits indicate the future location of the nostrils. Tiny thickenings of tissue on the sides of the head are the earliest precursors of the ears. The structures needed to form the eyes and ears are actively developing, but they won’t look like distinct features for several more weeks.
The mouth area is forming from several tissue folds that will eventually fuse together. At this point, the jaw, cheeks, and chin don’t exist as separate structures yet.
What You See on an Ultrasound
If you have an ultrasound at six weeks, you won’t see most of these details. What’s visible on a transvaginal ultrasound is a dark, round gestational sac inside the uterus. Within that sac, you can typically see two structures: the yolk sac (a small, round pouch that provides nutrients to the embryo) and the fetal pole, which is the earliest visible form of the embryo itself. The fetal pole appears as a tiny, bright spot or thickening next to the yolk sac.
At this stage, the fetal pole is often just barely large enough to measure. Your provider may or may not detect a heartbeat, depending on whether the embryo has reached the 5-to-7-millimeter threshold. If no heartbeat is visible at exactly six weeks, it doesn’t necessarily signal a problem. A follow-up scan a week or two later often shows a strong heartbeat once the embryo has grown slightly larger. The timing can also vary because gestational age is estimated from the last menstrual period, which doesn’t always match the actual date of conception.
How Six Weeks Compares to Later Stages
The difference between a six-week embryo and even an eight-week embryo is dramatic. At six weeks, the limb buds are featureless paddles with no fingers or toes. The face is a collection of pits and folds. The tail is still present. By eight weeks, the tail has receded, fingers and toes are starting to form (though still webbed), and the face has recognizable features including eyelids and the tip of a nose. The embryo will have grown to roughly 16 millimeters, more than double its six-week size.
At six weeks, the embryo is in one of its most active periods of development. Nearly every major organ system is being established, even though very little is visible from the outside. The kidneys, liver, and lungs are all in their earliest stages of formation. The digestive tract is a simple tube running through the center of the body. Over the next two to four weeks, all of these systems will become more complex and defined, and the embryo will start to look increasingly human.