At 13 weeks, a fetus measures roughly 68 to 79 millimeters (about 2.7 to 3.1 inches) from crown to rump, depending on the exact day of the week. 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.
Crown-to-Rump Length Day by Day
Fetal size at 13 weeks changes noticeably even from one day to the next. International growth standards place the median crown-to-rump length at 67.8 mm at the start of week 13, climbing steadily to 78.8 mm by the end of the week. That’s more than a centimeter of growth in just seven days.
Crown-to-rump length is the standard measurement used during first-trimester ultrasounds because the legs are curled up against the body, making a head-to-toe measurement unreliable. If your ultrasound report lists a CRL, this is the number it refers to. By later in the second trimester, providers switch to measuring individual body parts like the head circumference and thigh bone length instead.
Keep in mind that these numbers represent the 50th percentile. A fetus measuring somewhat smaller or larger is completely normal. Growth restriction is only a clinical concern when estimated size falls below the 10th percentile for gestational age.
What’s Happening Inside the Body
Week 13 marks the very beginning of the second trimester, and the fetus is hitting several developmental milestones at once. Bones are starting to harden, particularly in the skull and the long bones of the arms and legs. Up to this point, the skeleton has been mostly soft cartilage. This hardening process, called ossification, will continue throughout pregnancy, but it’s a noticeable shift at 13 weeks because it makes the skeleton visible on ultrasound in a new way.
The skin is still thin and nearly transparent. Underlying blood vessels are clearly visible through it, though the skin will begin thickening soon. Facial features are becoming more defined. The eyes, which started on the sides of the head, have moved closer together toward the front of the face, and the ears are settling into their final position.
The kidneys reach a major milestone around this time: they begin producing urine. This is significant because fetal urine becomes the primary contributor to amniotic fluid, the protective liquid surrounding the baby in the womb. Before this point, amniotic fluid comes mainly from the mother’s body. From 13 weeks onward, the fetus is actively swallowing amniotic fluid and filtering it through the kidneys in a continuous cycle that helps both kidney development and fluid balance.
The Placenta Takes Over
Something important is happening behind the scenes around this time. Earlier in pregnancy, a small structure called the corpus luteum (the remnant of the follicle that released the egg) was responsible for producing the hormones needed to sustain the pregnancy. By around week 10, the placenta gradually takes over that job. By week 13, the placenta is firmly in control of hormone production, and this transition is one reason many people start to feel noticeably better as the first trimester ends. Nausea and fatigue often ease as hormone levels stabilize under the placenta’s more consistent management.
Changes You Might Notice
At 13 weeks, your uterus has grown enough to rise just above the pubic bone. For many people, this is the point where a small bump first becomes visible, though it varies widely depending on body type, muscle tone, and whether this is a first pregnancy. The uterus was previously tucked entirely within the pelvis, so this upward shift is what creates the visible change.
Because the uterus is now pressing less on the bladder (it’s growing upward rather than pressing downward), many people find that the constant need to urinate from the first trimester eases a bit. At the same time, you may notice your waistband getting tighter even if the bump isn’t dramatic yet. Round ligament discomfort, a quick sharp pain on one or both sides of the lower abdomen, can start around this time as the ligaments supporting the uterus stretch to accommodate its growth.
What Ultrasound Shows at This Stage
If you have an ultrasound at 13 weeks, you’ll likely see a fetus that looks recognizably human. The head is still disproportionately large compared to the body, making up about a third of the total length, but the body is catching up. Arms and legs are clearly visible and moving, though most people can’t feel those movements yet since the fetus is still so small. You may see the fetus stretching, flexing, or bringing hands toward the face.
Many providers perform a nuchal translucency scan between 11 and 14 weeks, so 13 weeks falls right in the window for this screening. This ultrasound measures a small pocket of fluid at the back of the fetal neck and, combined with blood work, helps assess the likelihood of certain chromosomal conditions. The scan also gives a detailed look at the fetus’s overall anatomy and confirms that growth is on track.