At 13 weeks of pregnancy, your baby is about 7.4 centimeters long, roughly the size of a kiwi fruit. This is the very start of the second trimester, and the fetus has shifted from a curled, tadpole-like shape into something that looks unmistakably human. The head is still large relative to the body, but the rest is catching up fast.
Size and Proportions
That 7.4-centimeter measurement is taken from the crown of the head to the rump, since the legs are still tucked up and difficult to measure consistently. To picture it, think of your baby fitting comfortably inside a closed fist. The head makes up about a third of the total body length at this point, which gives the fetus a top-heavy appearance on ultrasound. Over the coming weeks, the body will grow rapidly to balance things out.
Facial Features
By week 13, the eyes have moved from the sides of the head toward the front of the face, though the eyelids are fused shut and will stay that way for several more months. The ears are close to their final position on the sides of the head. A small nose bridge and chin are visible in profile, and the mouth can open and close. If you were to see a 3D ultrasound image, you’d recognize a tiny face with features that are distinctly human but still very delicate.
Skin, Bones, and Body
The skin at 13 weeks is thin and almost completely transparent. Blood vessels are visible through it, giving the fetus a reddish appearance. There’s virtually no fat beneath the skin yet, so the body looks lean and bony.
This is also the week when the skeleton begins to harden. Bones in the skull and the long bones of the arms and legs start the process of turning from soft cartilage into actual bone tissue. This process, called ossification, will continue throughout pregnancy and even after birth, but at 13 weeks it’s just getting underway. On an ultrasound, the limbs are clearly visible: arms, legs, fingers, and toes are all formed and can be seen moving.
Organs and Internal Development
Most of the major organs are in place by 13 weeks, though they still have a lot of maturing to do. The intestines, which earlier in development were partly outside the body in the umbilical cord, have now moved fully into the abdomen. The kidneys are beginning to produce small amounts of urine, which gets released into the amniotic fluid. The liver is starting to produce bile.
A significant shift is happening behind the scenes at this stage. During the first trimester, your body relied on a structure in the ovary called the corpus luteum to produce the progesterone needed to sustain the pregnancy. Around 13 weeks, the placenta takes over that job. This hormonal handoff is one reason many women start to feel better as the second trimester begins, with less nausea and fatigue.
Movement
Your baby is moving constantly at 13 weeks, stretching, flexing, and even making sucking motions. The muscles and nervous system are developed enough to allow coordinated movement, though the fetus is still far too small for you to feel any of it. Most women don’t feel movement until somewhere between 16 and 22 weeks. On ultrasound, though, you might catch your baby kicking, turning, or bringing a hand up toward the face.
Can You Tell the Sex?
External genitalia are developing at 13 weeks, but identifying the sex on ultrasound at this stage is unreliable. In male fetuses, the phallus can sometimes be visualized as early as 10 to 11 weeks, but it’s easy to mistake at this size. In female fetuses, the labia aren’t clearly visible until around 15 weeks. Most ultrasound technicians won’t attempt a definitive sex determination until the anatomy scan, typically done between 18 and 22 weeks. If you want an earlier answer, a blood test that screens for fetal DNA can identify chromosomal sex as early as 10 weeks with high accuracy.
What You’ll See on an Ultrasound
If you have an ultrasound around 13 weeks, you’ll likely see a recognizable baby shape: a round head, a visible spine, and limbs that may be moving during the scan. In many countries, the first routine ultrasound happens between 11 and 14 weeks as part of screening for chromosomal conditions. During this scan, the technician measures the fluid at the back of the baby’s neck and confirms the due date based on the baby’s size.
Standard 2D ultrasound images at this stage show a clear silhouette in profile, and you can usually make out the forehead, nose, and chin. 3D images, if available, reveal more surface detail, though the baby’s features are still quite small and may appear somewhat abstract. Many parents find 13-week images surprisingly baby-like compared to the earlier scans, where the embryo looked more like a blob than a person.