At 5 weeks pregnant, your baby is tiny, roughly the size of a sesame seed or the tip of a pen. It doesn’t look like a baby yet. The embryo is a small cluster of rapidly dividing cells that has just started organizing itself into distinct layers, each destined to become different body systems. Despite being almost too small to see with the naked eye, some remarkable things are already underway.
How Big Is the Embryo at 5 Weeks?
At this stage, the embryo measures roughly 1 to 2 millimeters long. To put that in perspective, it’s about the width of a single grain of rice. By the end of week 7 (which is 5 weeks after conception, since pregnancy is dated from your last menstrual period), the embryo will have grown to about half an inch, or 11 to 14 millimeters. But right now at week 5, you’re at the very beginning of that growth spurt.
What It Actually Looks Like
The embryo at 5 weeks doesn’t resemble a person at all. It’s taking on a C-shaped curve, almost like a tiny comma. There’s a visible bulge where the head will form, and the opposite end tapers into a small tail-like structure that will eventually disappear as development continues. The entire body is translucent and flat, with no recognizable facial features, limbs, or fingers.
Along the back, a critical structure called the neural tube is in the process of closing. This tube will become your baby’s brain and spinal cord. It’s one of the first major structures to take shape, which is why folic acid intake is so important in these early weeks.
Three Layers Building a Body
Even though the embryo looks like a simple speck, it’s already organized into three distinct cell layers that will give rise to every organ and tissue in the body. The outer layer will eventually form the brain, nervous system, and skin. The middle layer is responsible for building the heart, blood vessels, muscles, and bones. The inner layer will become the lungs, liver, and digestive tract. All of this differentiation is happening simultaneously in a space smaller than a peppercorn.
The Heart Is Already Starting
One of the most striking developments at 5 weeks is the heart. It’s not a fully formed organ yet. Instead, it’s a tiny tube-shaped structure that has just begun to pulse. By the end of week 5, this primitive heart tube beats approximately 110 times per minute. You won’t be able to hear it on a standard ultrasound yet, but the rhythmic contractions are already circulating the embryo’s first blood cells through a simple network of vessels.
What You’d See on an Ultrasound
If you have an early ultrasound at 5 weeks (usually transvaginal, since the embryo is too small for an abdominal scan to detect), you won’t see anything that looks like a baby. What you will see is a gestational sac, a small dark circle of fluid inside the uterus. Inside that sac, a yolk sac may be visible as a tiny white ring. The yolk sac provides nutrients to the embryo before the placenta takes over, and it’s one of the first reassuring signs that a pregnancy is developing normally.
The embryo itself is often too small to distinguish from the yolk sac at this point. Many providers prefer to wait until 6 to 8 weeks for the first ultrasound, when a heartbeat flicker and a more defined embryo (called the fetal pole) become visible.
What Twins Look Like at 5 Weeks
If you’re carrying fraternal twins, an ultrasound at 5 weeks would show two completely separate gestational sacs, each containing its own yolk sac. Identical twins look different: there’s typically one gestational sac with two yolk sacs visible inside it. In either case, the embryos themselves are still too small to see clearly at this stage, so the number and arrangement of sacs and yolk sacs are what your provider uses to identify a multiple pregnancy this early.
What’s Happening With Your Body
Your body is producing a hormone called hCG, which is what home pregnancy tests detect. At 5 weeks, hCG levels typically range from 200 to 7,000 units per liter. That’s a wide range because levels vary enormously between individuals and can double every 48 to 72 hours in a healthy pregnancy. A single hCG number doesn’t tell you much on its own; what matters is whether it rises appropriately over time.
You may already be experiencing early pregnancy symptoms like breast tenderness, fatigue, nausea, or frequent urination. These are all driven by the sharp rise in hCG and progesterone. Some people feel nothing at all at 5 weeks, which is also completely normal.
The Placenta and Umbilical Cord
The placenta is still in its earliest stages at week 5. Right now, it exists as a network of cells burrowing into the uterine lining, building the blood vessel connections that will eventually supply the baby with oxygen and nutrients for the rest of pregnancy. The umbilical cord is forming as a simple stalk connecting the embryo to this developing placenta. Neither structure is fully functional yet, which is why the yolk sac is still doing the heavy lifting for nutrition at this point.