At five weeks pregnant, the embryo is roughly the size of a sesame seed, measuring just 1.5 to 2 millimeters long. There’s no visible baby bump yet, and most changes are happening deep inside the uterus, invisible from the outside. But despite how tiny everything is, this is one of the most active weeks of early development.
What the Embryo Looks Like at Five Weeks
The embryo doesn’t look like a baby yet. It’s a tiny, curved structure, sometimes described as tadpole-shaped, with a distinct head end and tail end but no recognizable face or limbs. At just under 2 millimeters, you could place it on the tip of a pen and barely notice it. The outer layer of cells is burrowing into the uterine lining to establish a blood supply, while inside, three distinct cell layers are beginning to specialize into different body systems: one will become skin and the nervous system, another will form the digestive tract and lungs, and the third will give rise to muscles, bones, and the circulatory system.
What’s remarkable about this week is the speed of development. The neural tube, which eventually becomes the brain and spinal cord, is actively forming. A primitive heart tube is taking shape, and by the end of the fifth week it will pulse around 110 times per minute. The heart actually starts beating as early as three weeks and one day after fertilization, which falls right in this window. The embryo is also developing its own blood vessels, and a cluster of them is forming what will become the umbilical cord.
Gestational Age vs. Actual Development
One detail that confuses a lot of people: “five weeks pregnant” doesn’t mean the embryo is five weeks old. Pregnancy is dated from the first day of your last menstrual period, which is typically about two weeks before conception actually happened. So at five weeks pregnant, the embryo has only been developing for roughly three weeks. This dating system adds two weeks at the beginning when you weren’t actually pregnant, but it’s the standard used by doctors and pregnancy apps alike. A normal pregnancy lasts about 40 weeks under this system.
At this stage, the developing baby is technically called an embryo, a term that applies from fertilization through eight weeks of development. After that, it’s called a fetus.
What You’d See on an Ultrasound
If you had a transvaginal ultrasound at five weeks, you wouldn’t see anything that looks like a baby. What shows up is a gestational sac, a small, dark, fluid-filled circle within the uterus. Inside it, your provider may be able to spot the yolk sac, a smaller round structure that provides nutrients to the embryo before the placenta takes over. The embryo itself may or may not be visible yet, depending on the exact day and the quality of the equipment. Many providers prefer to wait until six or seven weeks for the first scan, when a heartbeat is more reliably detectable.
It’s common to feel anxious if an early ultrasound shows “only” a gestational sac. At five weeks, that’s completely normal. The embryo is so small that even a few days of growth can make the difference between seeing it and not.
What Your Body Looks Like
From the outside, nothing has changed. Your uterus is still roughly the size of a small pear, tucked behind the pubic bone, and won’t start pushing your abdomen outward for several more weeks. Any bloating you notice is from hormonal shifts, not uterine growth.
Inside, though, your body is working hard. Levels of hCG, the hormone that makes a pregnancy test turn positive, are climbing rapidly. At five weeks, hCG typically ranges from 200 to 7,000 µ/L, a wide range because levels can double every two to three days and vary significantly from person to person. This hormonal surge is behind most of the symptoms you may start to feel.
Symptoms You Might Notice
Five weeks is often when pregnancy starts to make itself known. Some people feel nothing yet, while others are already dealing with nausea, breast tenderness, fatigue, or heightened sensitivity to smells. Morning sickness, which can strike at any time of day, commonly begins between weeks four and six. You may also notice more frequent urination, mild cramping similar to period pain, or mood swings driven by rising hormone levels.
Not everyone experiences the same symptoms at the same intensity, and having fewer symptoms doesn’t indicate a problem. Some people sail through the first trimester with little more than fatigue and sore breasts, while others feel nauseated around the clock. Both are normal variations. The one near-universal sign at this point is a missed period, which is often what prompts people to test in the first place.
Why This Week Matters for Development
Weeks four through eight are a critical window. This is when the embryo’s major organ systems are laid down, making it the period most sensitive to disruption. The neural tube, which is actively closing during week five, is why folic acid supplementation is recommended before and during early pregnancy. If you’re five weeks along, the neural tube is forming right now, and adequate folate helps it close properly.
The circulatory system is also being built from scratch this week. The embryo’s blood vessels are connecting, the heart tube is beginning its rhythmic contractions, and the basic architecture of the cardiovascular system is being established. All of this is happening in a structure smaller than a grain of rice.