At 6 weeks of pregnancy, the embryo is tiny, measuring roughly 2 to 7 millimeters from top to bottom, about the size of a pomegranate seed. It doesn’t look like a baby yet. The body has a curved, C-shaped posture, an oversized head, and a small tail-like structure that gives it a shape often compared to a tadpole or seahorse.
Size and Shape
The standard measurement at this stage is called crown-rump length, which runs from the top of the head to the bottom of the torso. At the start of week 6, that length is around 1 to 2 millimeters. By the end of the week, it reaches about 7 to 7.5 millimeters. Growth happens remarkably fast during this window, with the embryo roughly quadrupling in size over just a few days.
The overall shape is distinctly curved. The head bends forward toward the chest, and the body curls into a C. A small tail-like extension protrudes from the lower end. This tail is a normal part of development and gradually recedes over the next couple of weeks as the lower spine takes shape.
Early Facial Features
There’s no recognizable face at 6 weeks, but the building blocks are forming. Dark spots mark where the eyes will eventually develop. These start as small outpouchings from the developing brain called optic vesicles, which began forming around week 3 and are now folding inward to create the early cup-shaped structures that will become the retinas. Small indentations on the sides of the head mark the beginnings of ear formation.
The head is disproportionately large compared to the rest of the body. That’s because the brain is one of the first organs to start developing. By week 6, the neural tube along the back is closing, and the front end is already dividing into the three regions that will become the major parts of the brain.
Limb Buds and Body
Tiny bumps called limb buds appear during week 6. These are the earliest precursors to arms. They look like small rounded paddles, with no fingers or distinct shape yet. Leg buds typically appear slightly later. The buds are so small they may not even be visible on an ultrasound at this point, but they’re actively forming beneath the surface.
The body itself is translucent. There’s no skin in the way we think of it. The outer layer is a thin sheet of cells, and internal structures are just beginning to organize beneath it.
The Heart at 6 Weeks
One of the most striking developments at this stage is the heart. It’s already beating, though it’s not the four-chambered organ it will eventually become. At this point it functions more like a tube that contracts rhythmically to push blood through the embryo’s tiny circulatory system.
A healthy heart rate at 6 weeks is at least 100 beats per minute. By the end of the sixth week and into the seventh, that lower limit rises to around 120 beats per minute. Heart rates below 90 bpm at this stage are associated with a higher risk of early pregnancy loss. If you have an early ultrasound, your provider may be able to detect this flicker of cardiac activity, though it’s not always visible this early depending on the equipment and the embryo’s exact age.
Organs Beginning to Form
While none of the internal organs are functional yet, many are taking their earliest shape during week 6. The digestive tract is dividing into three sections that will eventually become the esophagus and stomach, the small intestine and part of the colon, and the lower colon and rectum. The liver is beginning to form from a small bud of tissue at the junction of the upper digestive tract. It won’t start producing bile for several more weeks, but the cellular groundwork is being laid now.
The spinal cord is developing as the neural tube finishes closing. When this process goes smoothly, the tube seals completely, forming the protected channel that houses the spinal cord and, at the top, the brain. Problems with this closure are what lead to neural tube defects, which is one reason folic acid supplementation is recommended before and during early pregnancy.
What You’ll See on an Ultrasound
If you have a transvaginal ultrasound at 6 weeks, don’t expect to see anything that looks like a baby. The most prominent structure is the gestational sac, a dark, fluid-filled circle on the screen. Inside it, you’ll likely see the yolk sac, a small round structure with a bright rim that measures about 2 to 5 millimeters. The yolk sac is an important landmark because it confirms that the gestational sac is a true pregnancy rather than a fluid collection.
The embryo itself, sometimes called the fetal pole at this stage, may or may not be visible. At 6 weeks it can appear as a tiny thickening next to the yolk sac. If cardiac activity is detected, it shows up as a small flicker on the screen. Many providers prefer to wait until 7 or 8 weeks for a first ultrasound because everything is simply easier to see and measure by then. If an ultrasound at 6 weeks doesn’t show a clear embryo, it doesn’t necessarily mean something is wrong. It often means the pregnancy is a few days earlier than estimated.
A gestational sac with a mean diameter greater than 8 millimeters should contain a visible yolk sac. A sac larger than 16 millimeters without a visible embryo raises concern. These are the thresholds providers use when evaluating very early scans.
How 6 Weeks Is Counted
Pregnancy dating can be confusing because the 6-week mark is measured from the first day of your last menstrual period, not from conception. In reality, conception typically happened about 4 weeks ago. So when you read that an embryo is “6 weeks,” it has only been developing for roughly 4 weeks since the egg was fertilized. This distinction matters because some resources describe development by gestational age (from last period) and others by embryonic age (from conception), and the features described can seem mismatched if you’re comparing across sources.