At 13 weeks, a fetus measures roughly 6.8 to 7.9 centimeters (about 2.7 to 3.1 inches) from the top of the head to the bottom of the torso. That’s close to the size of a kiwi fruit. Weight at this stage is approximately 23 to 25 grams, or just under an ounce.
How a 13-Week Fetus Is Measured
Fetal size at 13 weeks is measured using crown-rump length (CRL), which runs from the top of the skull to the base of the spine. Legs aren’t included because they’re tightly curled against the body at this stage, making a head-to-toe measurement unreliable. CRL is the standard sizing method from about 6 weeks through 14 weeks of pregnancy and is one of the most accurate ways to confirm gestational age during the first trimester.
After 14 weeks, ultrasound technicians typically switch to other measurements: head circumference, the width of the skull, thigh bone length, and abdominal circumference. These individual measurements give a more detailed picture of growth once the fetus is large enough that CRL becomes harder to capture in a single image.
Day-by-Day Size During Week 13
Growth at this stage is fast enough that the fetus measurably changes within a single week. Based on international fetal size standards from the University of Oxford, the median crown-rump length increases by about 1.5 to 2 millimeters per day during week 13:
- 13 weeks, 0 days: 67.8 mm (2.67 inches)
- 13 weeks, 1 day: 69.6 mm (2.74 inches)
- 13 weeks, 2 days: 71.4 mm (2.81 inches)
- 13 weeks, 3 days: 73.3 mm (2.89 inches)
- 13 weeks, 4 days: 75.1 mm (2.96 inches)
- 13 weeks, 5 days: 76.9 mm (3.03 inches)
- 13 weeks, 6 days: 78.8 mm (3.10 inches)
These are 50th percentile values, meaning half of all fetuses at that exact gestational age will measure larger and half smaller.
What’s Normal: The Range of Sizes
Not every 13-week fetus measures the same, and a measurement that falls above or below the median isn’t automatically a concern. Data from the National Institutes of Health shows that at 13 weeks, head circumference ranges from about 79 mm at the 10th percentile to roughly 92 mm at the 90th percentile. Thigh bone length ranges from about 7.5 mm to 11.2 mm across that same spread. These ranges hold remarkably consistent across different populations.
A shorter-than-expected crown-rump length in the first trimester has been linked to lower birth weight, so your provider may note it and schedule a follow-up scan. But a single measurement that falls slightly outside the middle range is common and usually just reflects natural variation in growth timing.
What’s Developing at This Size
Week 13 marks the very beginning of the second trimester, and the fetus is transitioning from basic structure-building to refinement. Bones in the skeleton are beginning to harden, particularly in the skull and the long bones of the arms and legs. Up to this point, much of the skeleton has been soft cartilage. The skin is still thin and transparent enough that developing blood vessels are visible beneath it.
The head, which dominated the body’s proportions during the first trimester, is starting to look more balanced relative to the rest of the body. The intestines, which temporarily developed inside the umbilical cord because the abdomen was too small to hold them, have now moved into the abdominal cavity. Fingers and toes are fully separated, and the fetus can make small movements, though most pregnant people won’t feel them for several more weeks.
What You Might Notice in Your Own Body
As the fetus reaches this size, the uterus grows upward and begins to rise out of the pelvic cavity. For many people, a small bump becomes visible around this time. One practical upside: because the uterus is shifting away from the bladder, the frequent urge to urinate that marks early pregnancy often eases during this stretch. It typically returns later as the uterus grows large enough to press on the bladder again from above.
How Size Relates to Gestational Dating
Crown-rump length is considered the most reliable way to date a pregnancy between 6 and 14 weeks. If your ultrasound measurement doesn’t match your expected due date based on your last menstrual period, your provider may adjust the due date to align with the CRL. This matters because accurate dating affects the timing of screenings, growth monitoring, and decisions about delivery later in pregnancy. After 14 weeks, dating based on size becomes less precise because individual growth rates start to diverge more significantly.