At 5 weeks pregnant, your baby is about the size of an apple seed, measuring less than a quarter of an inch long. That’s roughly 2 millimeters from end to end. Despite being tiny, this is a week of rapid and critical development, with the foundations of the brain, spinal cord, and heart all taking shape.
What “5 Weeks” Actually Means
Pregnancy is dated from the first day of your last menstrual period, not from the day you conceived. Since conception generally happens about two weeks after the start of that cycle, your baby at 5 weeks gestational age has really only been developing for about three weeks. This distinction matters because you’ll see some sources reference “fetal age” or “embryonic age,” which would put development at roughly 3 weeks. When your doctor, an app, or a pregnancy website says “5 weeks,” they almost always mean gestational age.
How Big the Baby Is
At this stage, the embryo is a tiny oval shape, not yet recognizable as anything resembling a baby. It’s smaller than a grain of rice. The gestational sac surrounding it, which is the fluid-filled structure that protects the embryo, is the main thing visible on an ultrasound right now. Most of the “size” you’d see on a scan is this sac rather than the embryo itself.
If you have an early ultrasound at 5 weeks, it would typically be a transvaginal scan rather than an abdominal one. At this point, a gestational sac may be visible, and a yolk sac (the structure that nourishes the embryo before the placenta takes over) sometimes appears around 5 to 6 weeks. The embryo itself is often too small to measure clearly this early.
What’s Developing This Week
Size doesn’t tell the whole story at 5 weeks. This is one of the most active periods of early development, even though the embryo is barely visible to the naked eye.
The neural tube is forming. This structure eventually becomes the brain, spinal cord, and the rest of the central nervous system. It’s one of the earliest and most important systems to develop, which is why folic acid intake before and during early pregnancy is so strongly emphasized. Defects in neural tube closure happen in these first few weeks, often before many people even know they’re pregnant.
The heart is also beginning to take shape. At 5 weeks, cardiac tissue starts to pulse, and by the end of the week, that tiny tube of heart tissue beats around 110 times per minute. It’s not a fully formed heart yet. There are no chambers or valves. But the rhythmic contracting of cardiac cells marks the very beginning of circulatory function. On a transvaginal ultrasound, this flickering may sometimes be detectable at 5 to 6 weeks, though it’s common for it not to show up until a bit later.
What You Might Feel at 5 Weeks
Five weeks is right around the time many people first suspect they’re pregnant, often because of a missed period. A home pregnancy test picks up the hormone hCG in your urine, and at 5 weeks, hCG levels typically range from about 200 to 7,000 units per liter. That wide range is normal. Levels vary significantly from person to person and even between pregnancies in the same person.
Rising hCG is what drives most early pregnancy symptoms. At 5 weeks, you might notice breast tenderness, fatigue, frequent urination, or the very first hints of nausea. Some people feel almost nothing at this point, and that’s also completely normal. Symptoms tend to intensify over the next few weeks as hormone levels climb.
Why Size Varies in Early Pregnancy
If you’ve looked at multiple sources, you may have noticed slightly different size estimates for 5 weeks. That’s partly because ovulation and conception timing vary. Two people who are both “5 weeks pregnant” by last-period dating may have conceived a few days apart, which at this scale makes a real difference. A couple of days of growth can nearly double the size of an embryo when it’s only 2 millimeters long.
This is also why doctors don’t rely heavily on size measurements at 5 weeks to date a pregnancy. The standard dating scan, where your baby’s length is measured to estimate a due date, typically happens between 8 and 12 weeks, when the embryo is large enough to measure accurately and growth follows a more predictable pattern. If you have an ultrasound at 5 weeks and the measurements seem smaller or larger than expected, it often just means your ovulation timing was slightly different than assumed.