At 5 weeks of pregnancy, the embryo is roughly the size of a sesame seed, measuring about 2 millimeters (0.08 inches) from end to end. That’s tiny enough to sit on the tip of a pen. Despite its small size, this is a period of rapid and critical development.
What 2 Millimeters Actually Looks Like
Two millimeters is hard to visualize, so comparisons help. The embryo at this stage is about the width of the lead in a mechanical pencil, or roughly the size of a single grain of coarse sand. It wouldn’t be visible to the naked eye in any meaningful detail. On an early ultrasound, the embryo itself is often too small to see clearly at 5 weeks, though the gestational sac (the fluid-filled space surrounding it) may be visible as a small dark circle measuring around 5 to 6 millimeters.
A Note on How Weeks Are Counted
Pregnancy is dated from the first day of your last menstrual period, not from conception. This means that at “5 weeks pregnant,” the embryo has only been developing for about 3 weeks since fertilization. Ovulation and conception typically happen around day 14 of a 28-day cycle, so gestational age always runs about two weeks ahead of the embryo’s actual age. This dating system assumes a regular cycle, and the American College of Obstetricians and Gynecologists notes it doesn’t account for irregular cycles or variations in ovulation timing. If you conceived through IVF, your provider will calculate the due date based on the embryo transfer date instead.
What’s Developing at 5 Weeks
The embryo at this stage doesn’t look like a baby. It resembles a tiny curved tube or tadpole shape, organized into three distinct layers of cells that will become every organ and tissue in the body. The middle layer is already forming a primitive heart and circulatory system. The outer layer will eventually become the skin, the central and peripheral nervous systems, the eyes, and the inner ears. The inner layer is destined to become the lungs, intestines, and other internal organs.
The heart is one of the first organs to begin functioning. By the end of week 5 or early in week 6, a rudimentary heart tube starts to beat, sometimes detectable on a transvaginal ultrasound. The neural tube, which will become the brain and spinal cord, is in the process of forming and typically closes by around week 6. This is one reason folic acid intake is so important in early pregnancy: it supports proper neural tube closure.
How Size Changes in the Coming Weeks
Growth during the first trimester is exponential. To put the 5-week size in context, here’s how quickly things change:
- Week 5: about 2 mm (sesame seed)
- Week 6: about 4 to 5 mm (lentil)
- Week 7: about 10 mm (blueberry)
- Week 8: about 16 mm (kidney bean)
- Week 12: about 5 to 6 cm (lime)
The embryo roughly doubles in size each week during this early stretch. By week 8, it transitions from being called an embryo to a fetus, once the basic structures of all major organs are in place.
What You Might Experience at 5 Weeks
At this point, many people are just discovering they’re pregnant. A home pregnancy test picks up the hormone hCG in urine, and at 5 weeks, blood levels of hCG typically range from 200 to 7,000 units per liter. That wide range is normal because hCG levels double roughly every 48 to 72 hours in early pregnancy, so the exact number depends on when implantation occurred.
Symptoms at 5 weeks vary widely. Some people feel nothing at all. Others notice breast tenderness, fatigue, nausea, or frequent urination. These are driven by rising hormone levels, not by the physical size of the embryo, which is far too small to cause any pressure or visible changes to your body. Your uterus at this stage is still about its normal size, roughly the size of a small pear.
What an Ultrasound Shows at 5 Weeks
If you have an early ultrasound at 5 weeks, it will almost always be transvaginal rather than abdominal, because the embryo is so small that an external scan can’t pick it up. What your provider looks for is the gestational sac and possibly a yolk sac, a small circular structure that nourishes the embryo before the placenta takes over. The embryo itself may or may not be visible yet, and a heartbeat is not reliably detectable this early. If nothing is seen at 5 weeks, your provider will typically schedule a follow-up scan a week or two later rather than drawing any conclusions, since even a few days of growth makes a significant difference at this scale.