At 6 weeks pregnant, the embryo measures roughly 1 to 7.5 millimeters from top to bottom, depending on the exact day of the week. That’s about the size of a lentil. Growth happens fast at this stage, with the embryo nearly doubling in length every few days.
How Big the Embryo Is Day by Day
Because the embryo grows so rapidly during week 6, even a day or two makes a measurable difference. At the start of the week (6 weeks, 1 day), the crown-rump length is around 1 to 1.5 mm. By mid-week, it reaches about 4 to 5 mm. And by 6 weeks, 6 days, it can measure 7 to 7.5 mm. Crown-rump length is the standard measurement at this stage, taken from the top of the head to the base of the spine, since the legs are too small and curled to include.
In terms of weight, the embryo weighs less than 0.04 ounces, which is roughly 1 gram. To put that in perspective, a single paperclip weighs about a gram. Most of what you’d see on an ultrasound at this point isn’t the embryo itself but the fluid-filled gestational sac surrounding it.
What Develops at This Size
Despite being barely visible to the naked eye, the embryo at 6 weeks is already undergoing major changes. The most notable is the start of cardiac activity. The heart begins beating during this week, typically detectable on a transvaginal ultrasound once the embryo reaches about 5 to 7 mm in length. Early heart rates vary quite a bit, ranging from around 80 to 130 beats per minute depending on the exact day. These rates climb quickly over the following weeks.
The embryo at this point has a distinct head end and tail end but doesn’t look recognizably human yet. Tiny buds that will eventually become arms and legs are just starting to form. The neural tube, which develops into the brain and spinal cord, is closing. Small pits on the sides of the head mark where the eyes will eventually develop, and similar structures are forming where the ears will be.
What You’d See on an Ultrasound
If you have an ultrasound at 6 weeks, it’s almost always done transvaginally rather than on the abdomen, because the embryo is still too small to pick up through layers of tissue. Three structures are typically visible. The gestational sac appears first, a dark circular area filled with fluid. Inside it sits the yolk sac, a smaller round structure that provides the embryo with nutrients before the placenta takes over. The fetal pole, which is the embryo itself, appears as a tiny thickened area next to the yolk sac.
Not everyone will see all three structures at exactly 6 weeks. If your dates are off by even a few days, the fetal pole may not be visible yet, and that’s normal. Providers typically wait until the gestational sac reaches a certain size before drawing conclusions about whether the pregnancy is developing as expected. A follow-up scan a week or two later often resolves any uncertainty, since the embryo grows dramatically between weeks 6 and 8.
Why Size Varies at 6 Weeks
Gestational age is counted from the first day of your last menstrual period, not from conception. That means “6 weeks pregnant” actually corresponds to about 4 weeks of embryonic development. If your cycle is longer or shorter than 28 days, or if you ovulated later than expected, your embryo may measure smaller or larger than the averages listed above. This is one reason early ultrasound measurements can shift your estimated due date by several days.
At this stage, most embryos of the same gestational age are remarkably similar in size. Individual variation in growth becomes more pronounced later in pregnancy. So if your 6-week measurement falls within the expected range of 1 to 8 mm, it’s a reliable indicator that development is on track. The measurement becomes less useful for dating after the first trimester, when babies start growing at more individual rates.