At six weeks of pregnancy, the embryo measures roughly 3 to 5 millimeters from top to bottom, about the size of a lentil. That’s small enough to sit on the tip of a pencil eraser, yet a surprising amount of development is already underway.
Actual Measurements at Six Weeks
The standard measurement used this early in pregnancy is called crown-rump length, which is simply the distance from the top of the embryo to its bottom (there are no legs to measure yet). At exactly six weeks of gestational age, that length is typically around 3.2 millimeters. By six weeks and two days, it’s already closer to 5.3 millimeters. That rapid change reflects a growth rate of about 1 millimeter per day, a pace that continues through roughly the ninth week of pregnancy.
To put those numbers in perspective, 5 millimeters is about the width of a pencil. The entire embryo, nestled inside a fluid-filled gestational sac, is smaller than a pea.
Gestational Age vs. Fertilization Age
One detail worth understanding: “six weeks pregnant” is measured from the first day of your last menstrual period, not from the day of conception. Since ovulation typically happens about two weeks into a cycle, the embryo itself is closer to four weeks old in actual developmental time. Nearly all pregnancy timelines, apps, and ultrasound reports use gestational age, so when your provider says six weeks, this is what they mean.
What’s Forming Inside
Despite being barely visible to the naked eye, a six-week embryo is in the middle of some of its most critical development.
The heart is the headline. A primitive heart tube has begun beating, and at six weeks it typically pulses at about 100 to 120 beats per minute. This is often the earliest point at which a heartbeat can be detected on a transvaginal ultrasound, usually once the embryo reaches 5 to 7 millimeters in length. That heart rate will climb over the next few weeks before settling into a steadier rhythm later in pregnancy.
The neural tube, which will become the brain and spinal cord, has already closed by this point. It finishes folding and sealing during weeks three and four. The upper portion is now starting to divide into what will eventually become distinct parts of the brain, while the lower portion is forming the early spinal cord and the small bones that will surround it.
Tiny limb buds are present and growing. These small bumps on the sides of the embryo are the very beginnings of arms and legs. They don’t look like limbs yet, but over the coming weeks nerves will grow into them, muscles will start forming, and by around eight weeks they’ll begin to take recognizable shape.
Early facial structures are also taking form. The optic vesicles, which will eventually become the eyes, have moved close to the surface of the embryo. The pharyngeal arches, a series of small ridges, are developing into what will later become the jaw, ears, and parts of the neck. None of these look like a face yet. At this stage, the embryo’s head end is a cluster of folds and bulges that will gradually reorganize over the next several weeks.
What a Six-Week Ultrasound Shows
If you have an ultrasound at six weeks, it will be transvaginal rather than abdominal, because the embryo is too small to pick up through the abdomen at this stage. On the screen, the most obvious structure is the gestational sac, a dark, round pocket of fluid. Inside it, you’ll typically see a yolk sac, a small ring-shaped structure that provides early nourishment before the placenta takes over.
The fetal pole, which is the first visible sign of the embryo itself, often appears at about six weeks. It looks like a tiny bright line or thickening next to the yolk sac. If the embryo has reached 5 millimeters or so, your provider may also be able to detect a flickering heartbeat on the ultrasound. If no heartbeat is visible yet, that’s not necessarily a concern. Growth at this stage happens so quickly that a follow-up scan a few days later often shows clear cardiac activity.
How Fast Growth Happens From Here
The 1-millimeter-per-day growth rate that begins around week six continues through about week nine and a half. That means in just one week, the embryo will roughly double in length. By week eight, it will measure around 14 to 20 millimeters, roughly the size of a kidney bean, and will officially transition from “embryo” to “fetus.” By the end of the first trimester at 12 weeks, it will be about 6 centimeters long, nearly 20 times its size at six weeks.
This early stretch is when organ systems are being laid down, which is why the first trimester is the most sensitive window for developmental disruption. The structures forming now, the heart, brain, spinal cord, and limb foundations, are doing the bulk of their initial construction in these weeks, even though the embryo itself is still barely the size of a small seed.