At 7 weeks pregnant, your baby is roughly 1 centimeter long, or about the size of a blueberry. That’s measured from the top of the head to the bottom of the rump (called “crown-to-rump length”), since the legs are too small and curled to factor into the measurement. Despite being tiny, the embryo is in the middle of a rapid building phase, with major organs and structures taking shape every day.
What a Blueberry-Sized Embryo Looks Like
At this stage, the embryo doesn’t look much like a baby yet. The head is large compared to the rest of the body, making up nearly half the total length. A small tail is still visible, which is why many people say the embryo resembles a tadpole or seahorse. That tail will recede over the coming weeks as the lower spine finishes developing.
Bones are just beginning to replace the soft cartilage that formed the embryo’s early skeleton. Tiny arm and leg buds are present but haven’t separated into fingers and toes yet. Facial features are starting to organize: dark spots mark where the eyes will be, and small indentations show the future nostrils and ears. Everything is still very rudimentary, but the basic blueprint of a human body is being laid down.
What’s Happening Inside
The heart is the standout performer at 7 weeks. It’s already beating at 120 to 154 beats per minute, roughly twice the rate of an adult heart. A rate below 120 bpm at this point can signal a problem, while rates above 155 bpm are considered unusually fast. Most prenatal providers look for a heartbeat within that 120 to 154 range as a reassuring sign.
The brain is growing quickly too, though it’s still a simple structure. On ultrasound, a small fluid-filled area is sometimes visible at the top of the embryo. This is one of the earliest brain chambers, which will eventually develop into the structures that control balance and coordination. The rest of the brain’s architecture is forming but won’t be distinguishable for several more weeks.
Genitals are also beginning to form internally at this stage, though it’s far too early to determine sex on an ultrasound. That typically becomes possible around 18 to 20 weeks.
What You’d See on an Ultrasound
If you have a 7-week ultrasound, it will almost certainly be transvaginal rather than abdominal, since the embryo is so small that an external probe can’t pick it up reliably. On the screen, you’ll see a dark, round gestational sac. Inside it, the embryo appears as a small bright shape, usually with a flickering dot that represents the heartbeat.
You’ll also see the yolk sac, a small circular structure that sits near the embryo but outside the thin membrane (the amnion) that’s beginning to surround it. The yolk sac provides nutrients until the placenta is fully functional. It’s a normal and expected finding at this stage. The embryo itself will look fairly featureless on the screen. Don’t expect to make out arms, legs, or a face. At 1 centimeter, the detail just isn’t there yet.
Your provider will measure the crown-to-rump length to confirm or adjust your due date. A measurement of about 12 millimeters, for example, corresponds to roughly 7 weeks and 3 days. These early measurements are the most accurate way to date a pregnancy, often more reliable than counting from your last period.
How Quickly Things Change
Growth at this stage is exponential. Just one week earlier, at 6 weeks, the embryo was roughly half its current size. By 8 weeks, it will nearly double again, reaching about 1.5 to 2 centimeters. To put that in perspective, the embryo is growing approximately 1 millimeter per day during this stretch, which is an enormous rate relative to its total size.
This speed is why the period between weeks 5 and 10 is considered the most critical window for organ development. The heart, brain, spinal cord, and limbs are all forming simultaneously. By the end of week 8, the embryo will officially be reclassified as a fetus, marking the transition from building new organ systems to refining and growing the ones already in place.
Size in Context
It can be hard to wrap your head around how small 1 centimeter really is. A blueberry is the most common comparison, but you could also think of it as the width of your pinky fingernail or a single pea. The entire gestational sac, the fluid-filled space the embryo sits in, is only about 2 to 3 centimeters across at this point, roughly the size of a cherry.
If you’re tracking your pregnancy week by week, keep in mind that size estimates are averages. A measurement a few millimeters above or below 1 centimeter at 7 weeks is perfectly normal. What matters more than a single number is whether the embryo is growing consistently over time, which is why follow-up ultrasounds are sometimes scheduled a week or two apart in early pregnancy.