At 6 weeks pregnant, your baby is about the size of a lentil, measuring roughly 2 to 5 millimeters from head to bottom (called crown-rump length). That’s approximately a quarter of an inch, and it weighs about 0.04 ounces. Despite being tiny enough to sit on the tip of a pencil eraser, an extraordinary amount of development is already underway.
What 5 Millimeters Actually Looks Like
The most common comparison you’ll see is a lentil, and it’s a good one. A single dried lentil is flat and round, roughly 5 mm across, which maps closely to the upper end of what a 6-week embryo measures. At the lower end (2 mm), think of a single sesame seed. This measurement is taken from the top of the head to the base of the spine, since the legs are too small and curled to measure.
Clinically, this crown-rump length is the most reliable way to date a pregnancy in the first trimester. At exactly 6 weeks and 0 days, the typical measurement falls right around 5 mm, growing to about 9 mm by the end of the sixth week. That’s nearly doubling in size over just a few days, which gives you a sense of how rapidly things are changing at this stage.
What’s Forming Inside That Tiny Body
Six weeks is one of the most active periods of embryonic development. The embryo doesn’t look like a baby yet. It’s a curved, tadpole-like shape with a large head, a tail-like structure, and the very beginnings of limbs. But the internal blueprint is being laid down for nearly every major organ system.
The heart is the standout development this week. It’s already beating, typically at 100 to 120 beats per minute when first detectable on ultrasound around this time. The single outflow tract from the heart is beginning to divide into what will become two separate pathways: one to send blood to the lungs and one to send it to the rest of the body.
The brain is developing rapidly, with five distinct regions already taking shape inside the neural tube. Early structures for vision and hearing (the optic and otic vesicles) are forming, and clusters of nerve cells are appearing along the length of the spinal cord. The upper limb buds have grown enough that the ends look like tiny paddles, with faint ridges called digital rays marking where the fingers will eventually separate. The lower limbs are slightly behind but following the same path.
Other organs getting started this week include the thyroid, pituitary gland, adrenal glands, and thymus. The tongue is beginning to form, with early taste structures appearing on its surface. Even the reproductive organs are in their earliest stages, as primitive germ cells migrate into the developing tissue that will eventually become ovaries or testes.
What You Can See on an Ultrasound
If you have a transvaginal ultrasound at 6 weeks, your provider is looking for a few specific things. The gestational sac, a dark fluid-filled circle, is the first thing visible. Inside it, a small round structure called the yolk sac provides nutrients to the embryo at this stage. Next to the yolk sac, the embryo itself appears as a tiny thickening called the fetal pole.
A heartbeat is often detectable once the fetal pole reaches 5 to 7 millimeters, which typically happens right around 6 weeks. If the embryo measures smaller than that, a heartbeat may not be visible yet, and your provider will likely schedule a follow-up scan a week or two later. This is common and doesn’t necessarily signal a problem.
How You Might Be Feeling
While your baby is lentil-sized, your body is producing hormones at a remarkable rate. The pregnancy hormone hCG can range anywhere from about 1,080 to 56,500 mIU/mL at 6 weeks. That enormous range is normal, and it’s a big reason why symptoms vary so widely from person to person.
Morning sickness is one of the hallmark symptoms at this stage, though calling it “morning” sickness is misleading. It can strike at any time of day and is often worst when you first wake up. Alongside nausea, the NHS lists a broad range of symptoms common at 6 weeks: sore breasts, fatigue, mood swings, headaches, a metallic taste in your mouth, a heightened sense of smell, bloating, and needing to urinate more frequently. Some people notice new food cravings or aversions that seem to appear out of nowhere.
Less expected symptoms include light spotting, mild cramping similar to period pains, thicker or shinier hair, and skin changes like darkened patches on the face (sometimes called the “mask of pregnancy”). A thin, white vaginal discharge is also normal. Not everyone experiences all of these, and some people at 6 weeks feel relatively fine. The intensity of your symptoms has no bearing on whether the pregnancy is progressing normally.
Week-by-Week Size in Context
It helps to see 6 weeks in relation to what comes before and after. At 5 weeks, the embryo is barely visible, roughly the size of a poppy seed. By 6 weeks, it’s a lentil. By 7 weeks, it’s about the size of a blueberry (around 10 mm). The growth rate during these early weeks is proportionally faster than at any other point in pregnancy. Your baby is roughly doubling in size every week or so during this stretch, a pace that slows considerably as the pregnancy progresses into the second trimester.