At 6 weeks of pregnancy, the embryo is about the size of a lentil, measuring roughly 5 to 9 millimeters long (about a quarter of an inch). It doesn’t look like a baby yet. It’s a curved, tadpole-like shape with a visible tail structure, tiny bumps where limbs will grow, and a cluster of cells that has already started to pulse as a primitive heart.
Size at 6 Weeks
Doctors measure early embryos from the top of the head to the bottom of the rump, called the crown-rump length. At exactly 6 weeks, the 50th percentile measurement is about 5 millimeters. By 6 weeks and 5 days, it’s closer to 8 millimeters. That’s roughly the width of a pencil eraser. The standard comparison is a lentil, and it’s a good one: a small, flat, roundish shape you could easily lose on a countertop.
Overall Shape and Appearance
The embryo at this stage is distinctly C-shaped, curled in on itself with a noticeable head end that’s disproportionately large compared to the rest of the body. A small tail-like extension protrudes from the bottom of the spine. This tail is completely normal and will gradually shrink over the next couple of weeks, eventually becoming the tailbone (coccyx) by around 8 weeks.
There’s no recognizable face yet. Small depressions are forming where the eyes will eventually sit, and tiny folds of tissue mark the future locations of the ears and jaw. The head is bent forward toward the chest, giving the embryo its characteristic curved posture. If you were to see a magnified image, it would look more like a tiny comma than anything resembling a newborn.
The Heart and Major Organs
One of the most dramatic developments at 6 weeks is the heart. The cells that will form the heart began clustering around week 5, and by week 6 they’re pulsing in a detectable rhythm. This isn’t a fully formed four-chambered heart yet. It’s a tube-like structure that contracts rhythmically, pushing blood through the embryo’s tiny circulatory system. By the end of week 5, it pulses about 110 times per minute, and a doctor can often detect this activity on a transvaginal ultrasound at the 6-week mark.
The neural tube, which becomes the brain and spinal cord, is in the process of closing along the embryo’s back. This closure actually begins during weeks 3 and 4 after conception (weeks 5 and 6 of pregnancy by standard dating). It’s one of the most critical stages of development, which is why folic acid intake matters so much in early pregnancy. The brain is starting to divide into its basic sections, and the spinal cord is taking shape, though both are extremely primitive at this point.
Other organs are beginning to form as well. Early versions of the lungs, liver, and kidneys are starting to develop, though none of them are functional yet. Most of the embryo’s major organ systems won’t take recognizable shape until around the end of week 8.
Limb Buds and Early Structures
At 6 weeks, the arms and legs exist only as small paddle-shaped buds protruding from the sides of the body. There are no fingers, toes, or joints yet. These buds will elongate and differentiate over the coming weeks, but right now they’re just rounded nubs of tissue, roughly the same size on top and bottom. It’s impossible to distinguish an arm bud from a leg bud by appearance alone at this stage.
What You’ll See on an Ultrasound
If you have an ultrasound at 6 weeks, what appears on the screen won’t look much like the detailed embryo images you might find in a textbook. A transvaginal ultrasound (the internal type, which provides more detail this early) will typically show three things: the gestational sac, which looks like a dark circular area within the uterus; the yolk sac, a smaller bright ring inside the gestational sac that nourishes the embryo before the placenta takes over; and the fetal pole, which is the first visible sign of the embryo itself. The fetal pole appears as a tiny bright spot adjacent to the yolk sac.
At 6 weeks, the embryo is so small that it may appear as little more than a thickening along the edge of the yolk sac. Your provider will check that the gestational sac is in the correct location inside the uterus and look for cardiac activity in the fetal pole. Detecting a heartbeat at 6 weeks is possible but not guaranteed. If one isn’t visible, a follow-up scan a week or two later often shows clear cardiac activity as the embryo grows.
An abdominal ultrasound (the external type) is less detailed at this stage. It can typically show the gestational sac by 6 or 7 weeks, but it may not reveal the fetal pole or heartbeat as reliably as the transvaginal approach.
Twins on a 6-Week Ultrasound
Six weeks is one of the earliest points at which a twin pregnancy can be identified. Sonographers look for the number of gestational sacs, yolk sacs, and heartbeats. Two separate gestational sacs indicate fraternal-type (dichorionic) twins, each with their own placenta. A single gestational sac with two heartbeats indicates identical-type (monochorionic) twins sharing a placenta. Determining the type of twin pregnancy before 10 weeks of gestation using this method is nearly perfectly accurate, which is why early ultrasounds are particularly valuable for multiple pregnancies.
How 6 Weeks Compares to Later Stages
The gap between what the embryo looks like at 6 weeks and what most people picture when they think of a baby is enormous. At 6 weeks, there are no recognizable facial features, no formed limbs, and the overall shape is more aquatic than human. By 8 weeks, the tail disappears, and the basic organ systems are in place. By 12 weeks, the fetus has a distinctly human profile with visible fingers and toes. The 6-week stage sits squarely in the period of most rapid transformation, when the embryo is changing dramatically from day to day but hasn’t yet taken on features you’d recognize as a baby.