At 6 weeks pregnant, your baby is about 5 millimeters long, roughly the size of a lentil or a small pea. That’s measured from the top of the head to the bottom of the torso (called crown-rump length), since legs are far too tiny to factor in at this stage. Despite being smaller than a pencil eraser, your baby is in the middle of one of the most rapid periods of growth in the entire pregnancy.
What 5 Millimeters Actually Looks Like
Five millimeters is about one-fifth of an inch. If you placed the embryo next to a grain of rice, it would be roughly the same length. At 6 weeks, the body has a distinct C-shaped curve, almost like a tiny comma. There are no recognizable arms or legs yet, just small buds on either side of the body that will eventually become limbs. The whole structure is called an embryo at this point, not yet a fetus, and it won’t earn that label until around week 10.
What’s Developing Inside
Size alone doesn’t capture what’s happening at 6 weeks. This is when the body’s most critical structures begin forming. The neural tube along the back is closing, and the brain and spinal cord develop from it. The heart and other major organs are starting to take shape. In fact, a flickering heartbeat often becomes detectable right around this time, though it can take another week or so to show up clearly on ultrasound.
Tiny structures that will eventually form the eyes and ears are also developing. The small arm buds that appear this week will grow rapidly over the coming weeks into fully formed limbs with fingers. It’s a staggering amount of construction happening inside something the size of a lentil.
What You’d See on an Ultrasound
If you have an early ultrasound around 6 weeks, it will likely be done vaginally rather than on your belly. A transvaginal approach produces much clearer images at this stage because the embryo is so small. What the technician looks for is a fetal pole, which is the first visible sign of the developing embryo, sitting alongside a yolk sac inside the gestational sac.
Don’t be alarmed if the image looks like a small dot or blob. At 5 millimeters, there isn’t much detail to see. Some people go in expecting to see a baby-shaped outline and are surprised by how abstract the image looks. A heartbeat may or may not be visible yet. If it isn’t detected at exactly 6 weeks, your provider will typically schedule a follow-up scan a week or two later, since timing can vary by a few days depending on when you actually ovulated.
How Weeks Are Counted
One thing that confuses many people is that “6 weeks pregnant” doesn’t mean the embryo has been developing for six full weeks. Pregnancy is dated from the first day of your last menstrual period, which is roughly two weeks before conception actually happened. So at 6 weeks pregnant, the embryo has really only been growing for about four weeks. This is why the embryo is so much smaller than you might expect for something described as six weeks old.
What You Might Be Feeling
While your baby is lentil-sized, your body is already responding to the pregnancy in ways that feel anything but small. Rising hormone levels drive most of the symptoms that peak during these early weeks. Nausea, commonly called morning sickness, often kicks in between weeks 4 and 9 and can hit at any time of day, not just mornings. Breast tenderness and swelling are common as hormonal shifts increase blood flow and sensitivity. Fatigue can feel overwhelming because rising progesterone levels have a strong sedating effect.
You might also notice changes in your appetite or sense of smell. Foods you normally enjoy might suddenly seem repulsive, while unexpected cravings pop up. Progesterone slows digestion, which can lead to constipation and heartburn. The valve between your stomach and esophagus relaxes under the influence of pregnancy hormones, letting acid creep upward. All of this is happening because your body is redirecting enormous resources toward building a placenta and supporting the embryo’s rapid development.
How Quickly Size Changes From Here
Growth accelerates fast after week 6. By week 7, the embryo roughly doubles in length. By week 8, it reaches about 15 to 20 millimeters, around the size of a kidney bean. By the end of the first trimester at 12 weeks, your baby will be about 6 centimeters long (over 2 inches) and will have shifted from the embryonic stage to the fetal stage, with recognizable facial features, fingers, and toes. The jump from 5 millimeters to 6 centimeters in just six weeks gives you a sense of how extraordinary the pace of growth is during this period.