At six weeks of pregnancy, the embryo measures roughly 1 to 7.5 millimeters from top to bottom, depending on exactly which day of that week you’re in. That’s smaller than a pea. Medically, this stage is still called an embryo, not a fetus. The transition to “fetus” doesn’t happen until about 8 weeks after fertilization, according to the American College of Obstetricians and Gynecologists. Despite its tiny size, a surprising amount of development is already underway.
How Big Is It, Exactly?
The embryo’s length is measured from the top of the head to the bottom of the torso, a measurement called crown-rump length. At 6 weeks and 1 day, that length is about 1 to 1.5 mm. By 6 weeks and 6 days, it reaches around 7 to 7.5 mm. Growth is rapid and measurable day by day. For perspective, the embryo at the start of week six is roughly the size of a single lentil, and by the end of the week it’s closer to a small blueberry.
What It Looks Like to the Naked Eye
At six weeks, the embryo doesn’t look like a baby yet. It has a curved, C-shaped body with a prominent head region that makes up a large proportion of its total size. A tail-like structure is still present at the bottom, which will gradually disappear over the coming weeks. Small bumps called limb buds are forming where the arms and legs will eventually grow, but there are no fingers or toes yet.
Early facial structures are just beginning to take shape. Dark spots mark where the eyes will develop (these are the optic cups forming beneath the surface). Tiny pits where the nostrils will be are starting to appear. The outer ears, eyelids, and recognizable face contours won’t emerge until around week 7 or later. At this stage, the embryo looks more like a small curved tadpole than anything recognizably human.
What’s Happening Inside
The most dramatic development at six weeks is happening internally. The brain is dividing into distinct regions that will eventually become the structures responsible for hormone regulation, vision, and basic body functions. The neural tube, which forms the foundation of the brain and spinal cord, has already closed by this point.
The heart is one of the first organs to function. At six weeks, it has begun beating with a regular rhythm. The normal heart rate at this stage is at least 100 beats per minute. By 6 weeks and 3 days, a healthy rate rises to 120 beats per minute or above. Rates below 90 bpm at this stage are associated with a higher risk of miscarriage. The heart at this point is still a simple tube-like structure, not yet divided into four chambers, but it is actively pumping blood.
Other organ systems are in their earliest stages of formation. The liver, lungs, and kidneys are beginning to develop from clusters of specialized cells, though none of them are functional yet. The digestive tract is forming as a simple tube running through the body.
What You See on an Ultrasound
If you have a transvaginal ultrasound at six weeks, the embryo itself appears as a tiny bright line or dot inside the gestational sac, sometimes called the fetal pole. The gestational sac looks like a small dark circle on the screen. Inside the sac, the yolk sac (a small round structure that provides early nourishment) is usually visible as well.
In most normal pregnancies, the fetal pole is detectable by transvaginal ultrasound around this time, and a flickering heartbeat can often be seen. In one study, fetal heart motion was observed in over 91% of cases where the fetal pole was visible. The heartbeat appears as a rapid flicker on the screen, and your provider may or may not be able to let you hear it depending on the equipment.
Don’t be alarmed if the embryo is hard to see or smaller than expected. The crown-rump length changes by a full millimeter or more every day during this week, so being off by even two or three days in your dating can make a noticeable difference in what shows up on screen.
Why the Embryo Might Not Show Up
Sometimes a six-week ultrasound doesn’t show much, and that isn’t always a cause for concern. The most common reasons include being earlier in the pregnancy than calculated (if you ovulated later than expected, the embryo may only be at five weeks of development), having a high BMI that makes imaging more difficult, or having uterine fibroids that interfere with the ultrasound picture. In these cases, a follow-up scan one to two weeks later typically provides a clearer image.
More serious reasons for not seeing an embryo include miscarriage or an ectopic pregnancy, where the embryo implants outside the uterus. Your provider will use a combination of ultrasound findings and blood hormone levels to distinguish between a pregnancy that’s simply too early to see and one that isn’t progressing normally. A single inconclusive scan at six weeks is very common and usually leads to a repeat scan rather than an immediate diagnosis.
Embryo vs. Fetus at This Stage
People often use the word “fetus” from the moment of a positive pregnancy test, but medically the first eight weeks after fertilization are the embryonic period. This distinction matters because the embryonic period is when all major organs and body structures are first laid down, making it the most sensitive window for developmental disruption. After eight weeks post-fertilization (about 10 weeks of gestational age, counted from the last menstrual period), the developing baby is reclassified as a fetus, and the focus shifts from forming new structures to growing and maturing the ones already in place.