At six weeks of pregnancy, the developing embryo measures roughly 2 to 5 millimeters long, about the size of a lentil or a small pea seed. It doesn’t look like a baby yet. It’s a curved, tadpole-like shape with a visible tail, a disproportionately large head, and tiny bumps where the arms and legs will eventually grow.
Size and Shape at Six Weeks
Crown-rump length, the standard measurement from head to bottom, ranges from about 1 millimeter at the start of week six to around 7.5 millimeters by the end of the week. Growth happens fast during this window: the embryo roughly doubles in size every two to three days. By comparison, a single grain of rice is about 7 millimeters long, so even at the end of week six, the embryo is barely that length.
The overall shape is a C-curve. The head end is noticeably bulged because the brain is developing rapidly and takes up a large proportion of the body. A small tail-like structure extends from the bottom, a leftover from the way the spinal column forms. This tail shrinks and disappears over the next few weeks.
Why It’s Called an Embryo, Not a Fetus
At six weeks, the correct medical term is “embryo.” It doesn’t become a fetus until the end of the 10th week of pregnancy (eight weeks after fertilization). The embryonic period is when all the major organ systems are being laid down for the first time. The fetal period, by contrast, is when those structures grow and mature. This distinction matters because the embryonic stage is when development is most sensitive to disruption.
The Heart and First Heartbeat
A primitive heart tube has already formed and begun to beat. At six weeks, the heart rate typically falls around 100 to 120 beats per minute. Rates below 90 bpm at this stage are associated with a higher risk of early miscarriage, though heart rate naturally increases as the weeks progress. The heart at this point is a simple tube that contracts rhythmically. It hasn’t yet divided into four chambers.
On a transvaginal ultrasound, this cardiac activity often shows up as a tiny flicker. It’s one of the key signs your provider looks for to confirm a viable pregnancy at this stage.
Facial Features Just Beginning
There’s no recognizable face yet, but the very first building blocks are forming. Small depressions mark where the eyes will develop (called optic vesicles), and shallow pits indicate the future nostrils. The two sides of the lower jaw fuse together during week six, creating the earliest outline of a mouth. Dark spots on either side of the head are the beginnings of the inner ears. These features are so rudimentary that, to the naked eye, they would look like faint bumps and indentations rather than anything resembling facial features.
Limb Buds and Body Structure
Arm buds appear first, looking like small paddle-shaped bumps on either side of the body. Leg buds follow shortly after and are slightly less developed. Neither has fingers or toes yet. Cartilage for the limbs, hands, and feet starts to form this week, though it won’t harden into bone for several more weeks.
The neural tube, the structure that becomes the brain and spinal cord, closed during weeks three and four. By week six, distinct regions of the brain are starting to differentiate, and the spinal cord runs the length of the body. The embryo may begin making very slight involuntary movements, though they’re far too small to feel.
Organ Systems Taking Shape
Week six sits in the middle of organogenesis, the critical period when all major organs begin forming. The liver is starting to develop and will soon take over early blood cell production. The kidneys are beginning to form. The lungs and heart are descending into the chest cavity. The pancreas is starting to produce the earliest traces of insulin. Even the tongue is forming, with the first taste-related structures appearing near the back of the mouth.
The digestive tract is a simple tube at this point, and the intestines are so large relative to the tiny body that they temporarily bulge into the base of the umbilical cord. Glands like the thymus and the adrenal glands are also starting to take shape from surrounding tissue. None of these organs are functional yet in any meaningful way. They’re being patterned and positioned so they can grow into working systems over the coming months.
How the Embryo Gets Nutrients
The placenta isn’t fully operational at six weeks. Instead, the yolk sac handles the critical job of delivering oxygen and nutrients from the mother to the embryo. It works through a network of tiny blood vessels: a primitive version of the embryo’s circulatory system sends blood out to the yolk sac, where it picks up nutrients and oxygen, then channels it back. This system, called vitelline circulation, is the embryo’s lifeline until the placenta matures enough to take over, which happens gradually over the next several weeks.
The yolk sac is visible on ultrasound as a small, round structure inside the gestational sac. Its presence and normal appearance are reassuring signs during early pregnancy scans.
What You’d See on an Ultrasound
At six weeks, a transvaginal ultrasound (an internal scan, which provides much clearer images this early) can typically show three key structures: the gestational sac, the yolk sac, and the fetal pole, which is the first visible evidence of the embryo itself. In studies of normal early pregnancies, transvaginal ultrasound detected the fetal pole in about 93% of cases, compared to roughly 72% with a standard abdominal ultrasound. That’s why most early scans are done transvaginally.
The embryo appears as a small, bright line or dot adjacent to the yolk sac. If cardiac activity is present and the fetal pole is visible, the flickering heartbeat can usually be detected. The entire gestational sac at this stage is only about 1 to 2 centimeters across, so the embryo itself is a tiny speck within it. Don’t expect to see anything that looks remotely like a baby on screen. At this stage, it takes a trained eye to identify the structures, and your provider will point out what you’re seeing.
If your scan at six weeks doesn’t show a heartbeat or a fetal pole, that doesn’t automatically mean something is wrong. Dating can be off by several days, and a follow-up scan a week later often reveals normal development that simply wasn’t visible yet.