At 6 weeks of pregnancy, your baby measures about 5 millimeters long, roughly the size of a lentil. That’s measured from the top of the head to the bottom of the torso (there are no measurable legs yet), a measurement called crown-rump length. Despite being tiny, a surprising amount of development is already underway.
What 5 Millimeters Looks Like
Five millimeters is just under a quarter of an inch. A single lentil is the most common comparison, and it’s a good one: flat, rounded, and easy to lose between your fingers. At this stage, the embryo doesn’t look like a miniature baby. It’s a curved, tadpole-like shape with a visible tail structure that will eventually be absorbed as the spine develops.
If you have a transvaginal ultrasound at 6 weeks, what you’ll see on the screen is a small dark circle (the gestational sac) containing a tiny bright spot. That bright spot is the fetal pole, which is your developing embryo, sitting next to a small pouch called the yolk sac that supplies its nutrients. At this size, a heartbeat can often be detected. Providers typically look for cardiac activity once the fetal pole reaches 5 to 7 millimeters.
What’s Forming at 6 Weeks
The neural tube, which becomes the brain and spinal cord, is closing along the embryo’s back during this week. This is one of the most critical milestones in early pregnancy. The tube actually begins folding shut during weeks 3 and 4 after conception, and by week 6 the process is wrapping up. This is why folic acid intake matters most in the weeks before and just after conception, often before many people even know they’re pregnant.
The heart and other organs are also starting to form. The heart at this stage is a simple tube that has begun to beat and pump blood, though it won’t develop its four chambers for a few more weeks. Small buds have appeared on either side of the body that will become arms. Leg buds follow shortly after. Tiny pits mark where the eyes and ears will eventually develop, and the beginnings of the digestive and respiratory systems are taking shape.
What You Might Be Feeling
While the embryo is barely visible to the naked eye, the hormonal shifts driving its growth are already producing noticeable symptoms. The hormone progesterone rises sharply in early pregnancy, which commonly causes deep fatigue, constipation, and slowed digestion. Many people describe the tiredness at 6 weeks as unlike anything they’ve felt before, more like an overwhelming need to sleep than ordinary end-of-day exhaustion.
Nausea typically kicks in between weeks 4 and 9, so at 6 weeks you may be right in the thick of it or just starting to notice queasiness. Despite being called “morning sickness,” it can hit at any hour. Your breasts may feel tender or swollen, your sense of smell may seem heightened, and foods you normally enjoy might suddenly seem unappealing. Heartburn can also show up this early because pregnancy hormones relax the valve between your stomach and esophagus, allowing acid to creep upward.
Pregnancy hormone levels at 6 weeks span a wide range, anywhere from 200 to 32,000 units per liter in the blood. That enormous spread is normal. Two people at exactly 6 weeks can have vastly different hormone levels and both have perfectly healthy pregnancies, which is why a single blood draw doesn’t tell you much without context.
How Growth Accelerates From Here
The jump from week 6 to week 8 is dramatic. The embryo roughly triples in size over those two weeks, going from lentil-sized to about the size of a kidney bean (around 15 to 16 millimeters). Facial features become more defined, fingers begin to form on the arm buds, and the tail structure disappears. By the end of week 8, the embryo officially transitions to being called a fetus, and all major organ systems have at least begun to develop. The rapid pace of growth at this stage is why the first trimester is considered the most sensitive period for development.