At 6 weeks of pregnancy, an 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 adding approximately 1 millimeter per day in length.
How the Embryo Is Measured
Embryo size in the first trimester is tracked using something called crown-rump length, which is the distance from the top of the head to the bottom of the torso. At 6 weeks and 1 day, that measurement starts around 1 millimeter. By 6 weeks and 6 days, it reaches about 7.5 millimeters. That range reflects how rapidly things change from one day to the next during this window of development.
It’s also worth knowing that “6 weeks pregnant” is counted from the first day of your last period, not from conception. The embryo itself is actually only about 4 weeks old at this point. This is why medical charts and pregnancy apps sometimes seem to describe different stages for the same week. Every ultrasound measurement and growth milestone tied to “6 weeks” uses this period-based dating system.
What Develops During Week 6
Despite being smaller than a pencil eraser, the embryo at 6 weeks already has a surprising amount going on internally. The neural tube along the back is closing, which is the earliest foundation of the brain and spinal cord. The heart, which established its four chambers around week 4, is now undergoing further development as its outflow tracts separate and the heart begins descending into the chest cavity alongside the developing lungs.
Limb buds for the arms and legs are present and continuing to grow. Early precursors to the eyes (called optic cups) are forming as part of brain development. The digestive system is also taking shape: the midgut begins to push outward through the umbilical opening around this time, a normal part of development. Even the pancreas is undergoing a precise rotation around the developing intestine. All of this is happening in a structure you could rest on the tip of your finger.
What a 6-Week Ultrasound Shows
If you have a transvaginal ultrasound at 6 weeks, the first thing your provider looks for is a gestational sac containing a yolk sac inside the uterus. The yolk sac is a small circular structure that nourishes the embryo before the placenta takes over, and it’s typically visible by 5 to 6 weeks.
Around 6 to 7 weeks, the fetal pole becomes visible. This is the first recognizable sign of the developing embryo on the screen, appearing as a small thickening next to the yolk sac. A heartbeat may also be detectable in this same timeframe, though seeing it depends on the exact day, the quality of the equipment, and the embryo’s position. At the very start of week 6, the embryo can be too small to pick up on ultrasound at all, which is why many providers prefer to schedule the first scan closer to 7 or 8 weeks.
Heart Rate at 6 Weeks
When a heartbeat is detected at 6 weeks, it’s typically beating between 100 and 120 times per minute. That’s slower than it will be in just a week or two, because the heart is still maturing. Research from early pregnancy studies found that the lower limit of normal is about 100 beats per minute up to 6 weeks and 2 days, rising to 120 beats per minute from 6 weeks and 3 days onward. Heart rates below 90 beats per minute at this stage have been associated with a higher risk of miscarriage in the first trimester.
These numbers change quickly. By 8 to 9 weeks, the embryonic heart rate typically peaks around 170 to 180 beats per minute before gradually settling into a steadier range for the rest of pregnancy.
How Size Varies at This Stage
A difference of even one or two days can mean a noticeable change in crown-rump length at 6 weeks, because the embryo is growing at about 1 millimeter per day. That means an embryo measured on Monday could look meaningfully different from one measured on Thursday of the same week. This rapid daily growth is why a measurement that seems “small” on an early ultrasound isn’t necessarily a concern. Your provider will look at whether the size matches the estimated gestational age within a reasonable margin, and a follow-up scan a week or two later can confirm that growth is on track.
Ovulation timing also matters. If you ovulated a few days later than the standard assumption (day 14 of your cycle), the embryo may measure smaller than expected for your period-based due date. In those cases, the due date is often adjusted based on the ultrasound measurement rather than your last period.