At six weeks pregnant, the embryo measures about 5 to 9 millimeters, roughly the size of a pomegranate seed or a small lentil. It’s far too small to change the way your belly looks, and most people at this stage show no visible signs of pregnancy on the outside. What’s happening inside, though, is remarkably busy: major organs are starting to form, and the embryo is just becoming visible on ultrasound for the first time.
How Big the Embryo Actually Is
Crown-rump length, the standard measurement from top to bottom, ranges from 5 mm at exactly 6 weeks to about 9 mm by the end of the week. That’s smaller than a standard pencil eraser. Growth happens fast at this stage: the embryo can nearly double in size over just a few days, which is why ultrasound dating during this window can pinpoint gestational age to within two or three days.
What’s Forming Inside the Embryo
Week six falls in the middle of organogenesis, the period when the body’s major systems are being built from scratch. The neural tube along the embryo’s back is closing, and the brain and spinal cord are developing from it. This is one reason folic acid matters so much in early pregnancy: it supports the closure of that tube.
Small buds appear on the sides of the body that will eventually become arms. Leg buds typically follow a few days later. The embryo at this point has a curved, C-shaped posture with a visible tail-like structure that will recede as development continues. Dark spots mark where the eyes will form, and shallow pits on either side of the head are the earliest beginnings of the ears. A rudimentary heart tube is already beating, though it looks nothing like a four-chambered heart yet.
The digestive system is also taking its earliest shape. A simple tube runs through the center of the embryo that will eventually differentiate into the stomach, intestines, and other organs. Blood vessels are forming and connecting to what will become the placenta.
What You See on an Ultrasound
A six-week ultrasound is almost always done transvaginally rather than on the abdomen, because the embryo is so small that an abdominal probe can’t pick it up reliably. On screen, three structures tell the story.
The gestational sac appears as a dark, round or oval pocket within the uterine lining. It first becomes visible around weeks four to five, so by six weeks it’s well established and easier to measure. Inside it, the yolk sac shows up as a small, bright ring. The yolk sac provides nutrients to the embryo before the placenta takes over, and its presence inside the gestational sac is the definitive confirmation that the pregnancy is located in the uterus.
The fetal pole, a thickening along the edge of the yolk sac that represents the embryo itself, typically becomes visible during weeks six to seven. At exactly six weeks, some scans pick it up clearly while others don’t, depending on whether the embryo has grown enough to distinguish from the yolk sac. A flickering heartbeat can sometimes be detected in this same window, often appearing as a rapid fluttering on the screen. If the fetal pole or heartbeat isn’t visible at six weeks, your provider will often schedule a follow-up scan a week later, since a few days of growth can make a significant difference.
If you’re carrying twins, two separate gestational sacs (or one sac with two yolk sacs, in the case of identical twins) can sometimes be identified at six weeks on a transvaginal ultrasound. It’s one of the earliest points at which a multiple pregnancy can be detected, though confirmation usually comes at a later scan.
What Your Body Looks Like From the Outside
Your uterus at six weeks is still about the size of a plum, tucked behind your pelvic bone. There’s no visible bump yet. Some people notice mild bloating from rising progesterone levels, which can make pants feel tighter, but this is gas and fluid retention rather than uterine growth.
Breast changes are often the most noticeable physical sign. Your breasts may feel fuller, heavier, or tender, and the areolas can start darkening slightly. Some people notice more prominent veins on their chest as blood volume increases.
What Six Weeks Feels Like
The pregnancy hormone hCG is climbing rapidly at this point, with levels typically ranging from about 1,080 to 56,500 mIU/mL. That enormous range is normal because hCG roughly doubles every 48 to 72 hours, so levels vary widely depending on the exact day and the individual pregnancy. This hormonal surge is what drives most early pregnancy symptoms.
Nausea often kicks in right around week six, sometimes with vomiting, sometimes just as a persistent queasy feeling that worsens on an empty stomach. Fatigue can feel overwhelming, far beyond ordinary tiredness, because progesterone has a strong sedating effect. You may also notice food aversions, heightened sense of smell, frequent urination, or mood swings. Some people at six weeks have very few symptoms at all, which is also normal and not a sign that anything is wrong.
Light spotting can occur around this time as the embryo implants more deeply into the uterine lining or as the cervix becomes more sensitive from increased blood flow. It’s common, but heavy bleeding or cramping that worsens over time is worth reporting to your provider.