At five weeks pregnant, the embryo is roughly the size of a sesame seed, about 2 millimeters long. From the outside, your body shows no visible changes. But inside, development is moving fast: the foundations of the brain, heart, and spinal cord are already forming. If you’ve just gotten a positive pregnancy test, here’s what’s actually happening at this stage.
What You’d See on an Ultrasound
If you had an ultrasound at five weeks, you wouldn’t see anything that looks like a baby. What shows up is a small, dark, fluid-filled circle called a gestational sac, nestled in the lining of the uterus. It measures only a few millimeters across.
Around the end of week five or the beginning of week six, a tiny bright spot called a yolk sac may become visible inside the gestational sac. The yolk sac provides nutrients to the embryo before the placenta takes over. A heartbeat is not yet detectable at this point. That typically becomes visible around week six or seven. Many providers won’t schedule a first ultrasound until weeks seven through nine for this reason. An early scan that shows only a gestational sac without a yolk sac isn’t necessarily a problem; it may simply mean you’re a few days earlier than estimated.
What’s Developing Inside the Embryo
Week five is when the embryo transitions from a cluster of cells into something with distinct layers that will become organs. Three cell layers are now specializing: one will form the skin and nervous system, one the muscles and bones, and one the digestive and respiratory organs.
The neural tube, which becomes the brain and spinal cord, is closing during this week. The brain end of the tube is already folding into sections that will eventually become different brain regions. Three distinct bends have formed in the neural tube, shaping the earliest architecture of the brain stem and spinal cord. This is why folic acid intake matters so much right now. The CDC recommends 400 micrograms of folic acid daily for anyone who could become pregnant, because it’s the only form of folate proven to help prevent neural tube defects. Most prenatal vitamins contain 400 to 800 micrograms.
The heart is also taking shape. It starts as a simple tube and is beginning to divide into chambers. By the end of week five, the heart tube may start producing faint electrical activity, though it’s too early to pick up on ultrasound. The circulatory system is one of the first organ systems to function in an embryo.
What Your Body Feels Like
Five weeks is when many people first notice pregnancy symptoms, though some feel nothing yet. The hormone hCG, which triggered your positive test, is rising rapidly. At five weeks, hCG levels range widely from 18 to 7,340 mIU/mL. That enormous range is normal. What matters more than a single number is that levels roughly double every two to three days in a healthy early pregnancy.
Common symptoms at this stage include nausea (which can hit at any time of day, not just mornings), breast tenderness, fatigue that feels disproportionate to your activity level, and frequent urination. Some people also notice bloating, mild cramping, or food aversions. Others feel completely normal. The absence of symptoms at five weeks doesn’t indicate a problem.
Spotting and Warning Signs
Light spotting in early pregnancy is common and often harmless. It can result from the embryo implanting into the uterine lining or from increased blood flow to the cervix. However, certain combinations of symptoms warrant immediate medical attention.
Ectopic pregnancy, where the embryo implants outside the uterus (usually in a fallopian tube), can produce symptoms that overlap with normal early pregnancy: a positive test, nausea, breast tenderness. The first distinguishing signs are often light vaginal bleeding paired with pelvic pain on one side. Shoulder pain or a sudden urge to have a bowel movement can also signal that blood is leaking internally. If a fallopian tube ruptures, symptoms escalate to severe abdominal pain, heavy bleeding, extreme lightheadedness, or fainting. These are medical emergencies.
About 10 to 20 percent of ectopic pregnancies can produce what looks like a normal gestational sac on ultrasound, which is one reason providers are cautious about confirming a healthy pregnancy from a very early scan alone.
How Big Your Belly Looks
Your belly looks the same as it did before pregnancy. The uterus at five weeks is still entirely within the pelvis, roughly the size of a small pear. Visible belly changes from uterine growth don’t typically begin until 12 to 16 weeks. Any bloating you notice now is from hormonal shifts slowing digestion, not from the size of the embryo or uterus.
What Happens Next
Week five sits at the very beginning of a critical developmental window. Over the next three weeks, the embryo will grow from 2 millimeters to about 16 millimeters. Limb buds will appear, facial features will start to form, and the heart will develop a regular rhythm visible on ultrasound. By week eight, every major organ system will have begun forming, and the embryo transitions to being called a fetus. The rapid pace of development during weeks five through eight is why this period is considered the most sensitive to disruptions from alcohol, certain medications, and nutritional deficiencies.