At six weeks of pregnancy, the embryo measures about 5 millimeters from top to bottom, roughly the size of a lentil. That’s less than a quarter of an inch. Despite being tiny enough to sit on the tip of a pencil, the embryo is already undergoing rapid and complex development.
How the Measurement Works
Embryo size at this stage is measured as “crown-rump length,” which is the distance from the top of the head to the bottom of the torso. There are no legs long enough to measure yet, so this head-to-rump number is the standard. At exactly six weeks and zero days, the 50th percentile (average) crown-rump length is 5 mm. Some embryos measure slightly smaller or larger, and that’s normal. The range spans from about 3 mm on the low end to around 7 mm on the high end.
It’s also worth knowing that “six weeks pregnant” refers to gestational age, which is counted from the first day of your last menstrual period. The embryo was actually conceived about four weeks earlier. So at six weeks of pregnancy, the embryo has really only been developing for roughly four weeks since fertilization.
What’s Happening Inside the Embryo
A 5 mm embryo doesn’t look like much on an ultrasound screen, but the amount of construction happening inside it is staggering. By this point, the neural tube, which becomes the brain and spinal cord, has already closed. The heart has descended into the chest cavity and is forming its internal structure, with valve leaflets beginning to take shape. The lungs are settling into the chest as well.
Limb buds are present and developing rapidly. Nerves are growing into what will become the hands, and early muscle cells are aligning along the length of each tiny limb bud. The components of the upper lip are forming, with the nasal and jaw structures moving into position. Small grooves above and below each eye mark the earliest beginnings of eyelids. Several hormone-producing glands, including the adrenal glands, thyroid-related structures, and the pituitary gland, are all in early stages of formation.
In short, the embryo at six weeks is laying the groundwork for nearly every major organ system, even though it’s smaller than a pea.
The Heartbeat at Six Weeks
One of the most notable milestones around six weeks is cardiac activity. A heartbeat can usually be detected once the embryo reaches 5 to 7 mm in length, which lines up with this stage. If detected, the heart rate at this point tends to be on the slower side of the normal fetal range (110 to 160 beats per minute), since it’s just getting started. It’s common for the heartbeat to be visible on ultrasound at six weeks, but if it isn’t picked up yet, your provider will typically schedule a follow-up scan a week or two later before drawing any conclusions.
What You’d See on an Ultrasound
If you have a transvaginal ultrasound at six weeks, you’ll see the gestational sac first. It’s a dark, fluid-filled circle on the screen. Inside that sac, two structures are usually visible: the yolk sac and the fetal pole. The yolk sac is a small pouch that provides nutrients to the embryo at this early stage. The fetal pole is the embryo itself, appearing as a tiny thickening next to the yolk sac.
At 5 mm, the fetal pole is barely distinguishable. Don’t expect to see anything that resembles a baby. It looks more like a small bright spot or a slight curve of tissue. On some scans, a flickering motion within the fetal pole indicates cardiac activity. That flicker is often the most recognizable thing on the screen at this point.
What’s Happening in Your Body
At six weeks, the hormone hCG (the one pregnancy tests detect) is rising sharply. Typical levels range from 200 to 32,000 ยต/L, which is an enormous spread. That wide range is normal because hCG levels vary significantly from person to person and can double every two to three days during this period. This rapid hormone surge is what drives many early pregnancy symptoms: nausea, breast tenderness, fatigue, and frequent urination often kick in around this time or intensify if they’ve already started.
The placenta is not yet fully functional, which is why the yolk sac is still doing the heavy lifting for embryo nutrition. Over the coming weeks, the placenta will gradually take over that role.
How Quickly Size Changes From Here
Growth at this stage is exponential. The embryo roughly doubles in size every week during the first trimester. By week seven, it’s typically around 10 mm. By week eight, it reaches about 16 mm, and by the end of the first trimester at 12 weeks, the crown-rump length is usually around 55 to 60 mm (just over two inches). So the lentil-sized embryo you have at six weeks will be roughly 12 times longer in just six more weeks.