At 5 weeks pregnant, your baby is about the size of a sesame seed, measuring roughly 2 millimeters (less than a tenth of an inch). That’s tiny, but a surprising amount of development is already underway inside that speck.
How Big Is the Embryo at 5 Weeks?
Week 5 marks the official start of the embryonic stage of development. Before this point, the growing cluster of cells was called a blastocyst. Now it’s an embryo, and while it’s barely visible to the naked eye, it already has a defined top and bottom, front and back. The sesame seed comparison is helpful because it captures both the size and the oval shape of the embryo at this stage.
To put that in perspective, the embryo is smaller than a grain of rice and would easily sit on the tip of a pen. It won’t be called a fetus until around week 10, when the major organ systems have taken their initial shape.
What’s Forming Inside That Tiny Embryo
Despite its size, the embryo at 5 weeks is in a period of rapid construction. The neural tube, the structure that becomes the brain and spinal cord, is folding and closing. The upper portion will eventually form the brain and skull, while the lower portion becomes the spinal cord and the bones of the back. This process actually began during weeks 3 and 4, but it’s completing now, which is one reason folic acid intake is so important in early pregnancy.
A primitive heart tube is also forming. By the end of week 5, this tiny tube will pulse about 110 times per minute. It doesn’t look like a heart yet (it’s literally a tube), but it’s already circulating blood. Cardiac tissue is among the first to become functional in the embryo. At the same time, the earliest parts of the face and inner ear are beginning to take shape, and small buds that will eventually grow into arms and legs are just starting to appear. The long tube that becomes the digestive tract is also forming.
What You’d See on an Ultrasound
If you have an early ultrasound at 5 weeks, it will almost certainly be transvaginal rather than abdominal, because the structures are too small to pick up through the abdomen. At this stage, a provider is looking for specific landmarks rather than a recognizable baby shape.
The most prominent structure is the gestational sac, a fluid-filled cavity inside the uterus that surrounds the embryo. Inside that sac, the yolk sac becomes visible around week 5. The yolk sac provides nutrients to the embryo before the placenta takes over. The embryo itself may appear as a tiny thickening called the fetal pole, positioned next to the yolk sac. In many cases, though, the fetal pole isn’t clearly visible until closer to week 6, so don’t be alarmed if your provider can only identify the gestational sac and yolk sac at this point.
What You Might Be Feeling
At 5 weeks, many people are just discovering they’re pregnant. Hormonal shifts are accelerating, and the pregnancy hormone hCG (the one detected by home tests) typically ranges between 200 and 7,000 units per liter during this week. That wide range is normal. Levels vary significantly from person to person and even between pregnancies in the same person.
Those rising hormones drive most of the symptoms you might notice:
- Fatigue: Many women describe extreme tiredness in the first trimester, sometimes hitting as early as week 5.
- Nausea: Morning sickness can start now, though it peaks later, around weeks 8 to 10.
- Breast soreness: Tenderness or a feeling of fullness is common.
- Heightened sense of smell: Hormonal changes affect taste and smell, which can trigger food cravings or aversions seemingly overnight.
- Frequent urination: Your body is already increasing blood volume, which means your kidneys are filtering more fluid.
- Light spotting or cramping: Mild cramps similar to period pains and light spotting can occur and are often normal.
Bloating, mood swings, a metallic taste in your mouth, and changes in your skin (including darker patches on the face) are also reported at this stage. Not everyone experiences all of these, and some people feel almost nothing at 5 weeks.
Why Size Varies at This Stage
A 2-millimeter measurement is an average. The actual size of the embryo can vary slightly depending on exactly when conception occurred, which doesn’t always line up neatly with the dating based on your last menstrual period. Embryos that measure a day or two ahead or behind are perfectly normal. Providers use gestational sac size and the appearance of the yolk sac as reassuring markers at this point, rather than relying heavily on embryo length, which is difficult to measure precisely when the structures are this small. Crown-to-rump length becomes a more reliable measurement starting around weeks 6 to 7, when the embryo is large enough to measure consistently on ultrasound.