At 6 weeks of pregnancy, the embryo measures roughly 4 to 6 millimeters long, or about 1/6 to 1/4 of an inch. That’s approximately the size of a pea. It weighs less than 0.04 ounces, making it far too small to feel or notice physically, but big enough to detect on a transvaginal ultrasound.
How Size Is Measured at 6 Weeks
At this stage, the embryo is measured from the top of the head to the bottom of the torso, a measurement called crown-rump length. There are no measurable legs yet, so this head-to-rump number is the standard way to estimate size and confirm how far along a pregnancy is. According to clinical dating charts from the British Medical Ultrasound Society, an embryo at exactly 6 weeks and 0 days typically has a crown-rump length of about 5 mm. By 6 weeks and 6 days, it reaches roughly 9 mm.
That rapid growth within a single week matters. An embryo can nearly double in length over just a few days during this period, which is why ultrasound measurements taken even a day or two apart can look noticeably different.
Gestational Age vs. Actual Age
When you hear “6 weeks pregnant,” that number counts from the first day of your last menstrual period, not from conception. Since ovulation and fertilization typically happen around two weeks into a cycle, the embryo itself is closer to 4 weeks old in actual developmental terms. This is why some sources describe slightly different sizes for “6 weeks.” They may be using gestational age (from last period) or embryonic age (from fertilization). Most pregnancy apps and doctor visits use gestational age, so the measurements here follow that convention.
What’s Developing at 6 Weeks
Despite being the size of a pea, the embryo at 6 weeks is in the middle of an intense building phase. The neural tube, which will become the brain and spinal cord, has already formed during week 5. By week 6, structures that will eventually become the eyes, ears, and mouth are starting to take shape. Tiny buds appear where arms and legs will grow.
Blood cells are forming and basic circulation begins. A cluster of cells that will become the heart is already pulsing. By the end of week 5, this early heart tube beats around 110 times per minute, and a healthcare provider can often detect those pulses on a transvaginal ultrasound during week 6. It’s worth noting that this isn’t a fully formed heart with four chambers yet. It’s a tube-like structure that will continue developing over the coming weeks.
What You Might See on an Ultrasound
If you have an early ultrasound around 6 weeks, the embryo appears as a tiny bright spot inside the gestational sac. At 5 mm or so, it’s small enough that it can be difficult to spot on an abdominal ultrasound, which is why a transvaginal ultrasound is more common at this stage. The probe gets closer to the uterus, making it easier to see such a small structure.
You may or may not see a flickering motion where the heart is developing. Whether that’s visible depends on the exact day of development, the equipment, and the embryo’s position. Not seeing it at 6 weeks and 0 days doesn’t necessarily indicate a problem, since even a day or two of additional growth can make the difference. Many providers will schedule a follow-up scan a week later if the heartbeat isn’t visible yet.
How Quickly Size Changes From Here
Growth accelerates rapidly after week 6. By week 7, the embryo roughly doubles in size to about 10 mm. By week 8, it reaches around 16 mm, comparable to a kidney bean. By the end of the first trimester at 12 weeks, the crown-rump length is typically 5 to 6 centimeters, about the length of a lime. So while 4 to 6 mm sounds tiny, the embryo is in one of the fastest growth phases of development, with significant changes happening every few days.