At 6 weeks of pregnancy, your baby is about the size of a lentil, measuring roughly 1 to 7.5 millimeters (about a quarter of an inch) from top to bottom. At this stage, it’s technically called an embryo, a term that applies from fertilization through the eighth week of pregnancy. It doesn’t look like a baby yet, but a surprising amount of development is already underway.
How Big Your Baby Is at 6 Weeks
Crown-rump length, the measurement from the top of the head to the bottom of the torso, grows rapidly during week six. At the start of the week (around 6 weeks, 1 day), the embryo measures just 1 to 1.5 millimeters. By the end of the week (6 weeks, 6 days), it reaches about 7 to 7.5 millimeters. That’s a dramatic jump in just a few days, though still small enough to sit on the tip of a pencil eraser.
What the Face Looks Like
There’s no recognizable face at 6 weeks, but the earliest building blocks are taking shape. Small folds of tissue that will become the jaw have enlarged and merged at the midline, though at this point they’re still just masses of soft tissue with no distinct lips or gums. A thickening of tissue called the dental lamina, the very first step toward tooth development, begins appearing along the edges of both developing jaws near the end of the sixth week.
The structures that will form the nose are also coming together. Paired tissue folds merge in the beginning of the sixth week to create the foundation for the bridge and tip of the nose, the nostrils, and the small groove between the nose and upper lip. Eyelids start forming during week six as two tiny folds of skin begin growing over where the eyes will be. At this point, the eyes are still positioned on the sides of the head rather than the front. They won’t shift to a more forward-facing position until around weeks seven and eight.
The Heart Is Already Beating
One of the most significant milestones at 6 weeks is a detectable heartbeat. The heart began as a simple tube that started contracting and folding on itself in earlier weeks, and by now it’s pumping with a measurable rhythm. A healthy heart rate at this stage is at least 100 beats per minute. After about 6 weeks and 3 days, that lower threshold rises to 120 bpm. Rates below 90 bpm between 6 and 8 weeks are associated with a higher risk of early pregnancy loss.
For context, that heart rate is significantly faster than an adult resting rate, which typically falls between 60 and 100 bpm. Over the coming weeks, your baby’s heart rate will continue climbing before eventually slowing later in pregnancy.
Arms, Legs, and Body Shape
Your baby has limb buds by week six, small paddle-like projections where the arms and legs will grow. The upper limb buds are slightly more developed than the lower ones at this point. Nerves for the hand, including the ones that will eventually let your baby grip and feel, have already started growing into the arm bud tissue. Muscle cells within the limb buds are elongating and aligning along the length of the developing limb, laying the groundwork for future movement.
The body itself has a curved, C-shaped posture, with a prominent head that makes up a large proportion of the overall size. A small tail-like structure is still visible at the bottom of the spine. This is completely normal and will gradually recede over the next couple of weeks as the lower spine finishes developing.
Brain and Spinal Cord Development
The neural tube, the structure that becomes the brain and spinal cord, finished closing by the end of week four. By week six, the front end of this tube is already dividing into the distinct regions that will become different parts of the brain. The back portion of the tube is developing into the spinal cord. This is why folic acid intake is so critical in the earliest weeks of pregnancy: neural tube defects occur between days 21 and 28 after conception, often before many people even know they’re pregnant.
Bone formation is also beginning. Cartilage rods called Meckel cartilages are forming in the jaw area between weeks five and eight, creating a framework that the actual jawbone will develop around. Throughout the embryo, a gradual process of replacing cartilage with bone starts around this time and continues through week twelve.
Organ Development Happening Inside
Week six falls squarely in the embryonic period, which runs from about the third week through the eighth week of pregnancy. This is the most intensive phase of organ formation. The basic structures of major organs, including the liver, lungs, and kidneys, are being established during these weeks. The embryo won’t be called a fetus until after the eighth week, when the major organ systems have their foundational structures in place and the focus shifts to growth and maturation.
What You’d See on an Ultrasound
If you have an ultrasound at 6 weeks, it’s typically done transvaginally because the embryo is too small to see clearly with an abdominal scan. On the screen, you’ll see a dark circle, which is the gestational sac. Inside it, a small round structure called the yolk sac is visible, looking like a tiny balloon. The yolk sac provides nutrients to the embryo at this early stage, and your provider will check its size and shape as an indicator of pregnancy health.
You may also see the fetal pole, which is the earliest visible form of the developing embryo. It appears as a small thickening along one edge of the yolk sac. At 6 weeks, your baby is only about a quarter of an inch long, so don’t expect to see much detail. In some cases, a flickering motion may be visible where the heart is beating, though it’s not always detectable this early depending on the exact day of development and the ultrasound equipment being used.
If the fetal pole or heartbeat isn’t visible at 6 weeks, your provider will often schedule a follow-up scan a week or two later. Pregnancy dating can be off by several days, which at this tiny scale makes a significant difference in what’s visible.