At 6 weeks pregnant, the embryo measures about 5 to 9 millimeters long, roughly the size of a pomegranate seed. This is a week of rapid, foundational development: the neural tube has closed, the heart is beginning to beat, and many women are just starting to notice their first pregnancy symptoms. It’s also the earliest point when a transvaginal ultrasound can detect meaningful signs of a viable pregnancy.
What the Embryo Looks Like at 6 Weeks
The embryo is measured from crown to rump (head to bottom) since the legs are too small and curled to factor in. At exactly 6 weeks, the average crown-rump length is about 5 mm, growing to around 9 mm by the end of the week. That’s a nearly doubled size in just days, which gives you a sense of how fast cell division is happening.
The shape is still curved, often described as a C or comma. A distinct head end and tail end are visible, but the embryo doesn’t look recognizably human yet. Small paddle-shaped upper and lower limb buds are forming, which will eventually become arms and legs. The optic vesicles, the earliest structures of the eyes, have developed and sit close to the surface of the embryo. They won’t become functional eyes for months, but the blueprint is already in place.
Major Organ Development Underway
Week 6 falls in the middle of organogenesis, the critical window when all major organs begin to form. The neural tube, which becomes the brain and spinal cord, normally finishes closing by the end of week 4. By week 6, the brain has already divided into three primary vesicles, the early compartments that will become the forebrain, midbrain, and hindbrain. This is why folic acid intake matters so much in early pregnancy. The CDC recommends 400 mcg daily for all women who could become pregnant, ideally starting at least a month before conception, because neural tube defects happen in these first few weeks, often before you even know you’re pregnant.
The heart is one of the first organs to function. By 6 weeks, cardiac activity is often detectable on ultrasound. At this stage, the heart is a simple tube that pulses rather than the four-chambered organ it will become, but it’s already circulating blood through the embryo’s tiny body. The digestive tract, lungs, and kidneys are also beginning to take shape, though none of them are functional yet.
What You Might Be Feeling
Six weeks is when early pregnancy symptoms tend to arrive in force. Nausea, commonly called morning sickness, typically starts between weeks 4 and 6. Despite the name, it can hit at any time of day. Breast tenderness and swelling often begin in the same window, sometimes as early as two weeks into pregnancy. You may also notice fatigue that feels disproportionate to your activity level, frequent urination, and mood swings.
These symptoms are driven largely by a hormone called hCG (human chorionic gonadotropin), which your body produces after implantation to support the pregnancy. At 6 weeks, hCG levels typically range from about 152 to 32,177 mIU/mL. That’s an enormous range, and it’s normal. HCG roughly doubles every 48 to 72 hours in early pregnancy, so levels vary widely depending on the exact day and the individual. Higher hCG levels are associated with more intense nausea, which is why symptoms can feel like they escalate quickly this week.
Some women feel almost nothing at 6 weeks. A lack of symptoms doesn’t mean something is wrong. Symptom intensity varies dramatically from person to person and even between pregnancies.
What Shows Up on a 6-Week Ultrasound
If you have an early ultrasound at 6 weeks, it will almost always be transvaginal rather than abdominal, since the embryo is too small to see clearly through the abdomen. Three key structures should be visible: the gestational sac (a fluid-filled space in the uterus), the yolk sac (a small pouch that provides nutrients to the embryo before the placenta takes over), and the fetal pole, which is the earliest visible form of the embryo itself.
A fetal heartbeat is often detectable once the fetal pole reaches about 5 to 7 mm, which typically happens around 6 weeks. Seeing a heartbeat at this stage is a reassuring sign. One Australian study found that the risk of miscarriage at 6 weeks is about 9.4%, dropping to 4.2% by week 7. Once a heartbeat is confirmed on ultrasound, the odds of the pregnancy continuing improve significantly.
That said, it’s also common for a 6-week ultrasound to show a gestational sac and yolk sac but no detectable heartbeat yet, especially if your dating is off by even a few days. Providers will often schedule a follow-up scan a week or two later before drawing any conclusions.
How Twins Are Detected This Early
Six weeks is one of the earliest points when a multiple pregnancy can be identified. Sonographers look at the number of gestational sacs, yolk sacs, and amniotic sacs to determine not just whether you’re carrying twins, but what type of twin pregnancy it is.
Two separate gestational sacs indicate a dichorionic pregnancy, meaning each twin has its own outer membrane and, most often, its own placenta. These twins can be identical or fraternal. A single gestational sac with two heartbeats indicates a monochorionic pregnancy, where the twins share a placenta and are always identical. Determining this distinction before 10 weeks has nearly perfect accuracy, and it matters because monochorionic twins require closer monitoring throughout pregnancy due to shared blood supply.
What to Focus on Right Now
If you haven’t already started a prenatal vitamin with at least 400 mcg of folic acid, now is the time. The neural tube has likely already closed by week 6, but folic acid continues to support healthy development throughout the first trimester. If you’ve had a previous pregnancy affected by a neural tube defect, the recommended dose is much higher at 4,000 mcg daily, something to discuss with your provider.
Nausea can make eating feel impossible some days. Small, frequent meals tend to be easier to tolerate than three large ones. Staying hydrated matters more than eating perfectly balanced meals right now. Many women find that bland, carb-heavy foods sit better during the worst of the nausea.
Most providers schedule the first prenatal appointment between weeks 8 and 12. If you haven’t called to book that appointment yet, 6 weeks is a good time to get on the schedule, since popular practices can fill up quickly.