At 13 weeks, a baby measures roughly 2.6 to 3 inches from crown to rump, about the size of a lemon, and weighs around 3 ounces. This is the very beginning of the second trimester, and the baby has shifted from a tiny, top-heavy figure 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
Crown-to-rump length at 13 weeks ranges from about 67 to 80 millimeters, depending on the exact day of the week. That translates to roughly 2.6 to 3.1 inches, not counting the legs. The body is starting to lengthen, and the limbs are more proportionate than they were just a few weeks ago. The head, which dominated the profile during the first trimester, is beginning to grow in better proportion to the torso.
Facial Features
By 13 weeks, the eyes have moved from the sides of the head toward the front of the face, giving the baby a more recognizably human appearance. The eyelids are fused shut and will stay that way for several more weeks. The ears are close to their final position on the sides of the head, though they’ll continue shifting slightly over the next couple of weeks. Vocal cords are forming inside the tiny throat this week. A distinct nose, lips, and chin are all visible on ultrasound, and facial expressions may already faintly resemble the parents’.
Skin and Bones
The skin at 13 weeks is paper-thin and nearly translucent. Blood vessels are clearly visible beneath the surface, which gives the baby a reddish appearance. There’s almost no fat underneath the skin yet, so the body looks lean and angular.
Inside, the skeleton is hitting a major milestone. Bones are beginning to harden, particularly in the skull and the long bones of the arms and legs. Before this point, the skeleton was made almost entirely of soft cartilage. This hardening process, called ossification, is why the baby’s skeleton starts showing up more clearly on ultrasound around this time. Tiny ribs are also becoming visible.
Organ Development
One of the most important changes at 13 weeks is happening in the abdomen. Earlier in development, some organs, including the intestines, actually grew outside the baby’s abdominal cavity. By around 11 weeks, those organs migrate back inside, and by 13 weeks the abdominal wall has typically closed around them. The intestines are now settling into place and beginning to function.
The placenta has fully taken over hormone production by this stage. During the first several weeks of pregnancy, a structure in the ovary called the corpus luteum handled that job. Starting around weeks 6 to 8, the placenta gradually assumed responsibility for producing progesterone and estrogen, the hormones that sustain the pregnancy. By week 13, the placenta is firmly in control, which is one reason many people notice their nausea and fatigue starting to improve around this time.
Movement and Reflexes
The baby is actively moving at 13 weeks, stretching, turning, and flexing newly formed muscles. These movements are still far too small for a pregnant person to feel, but they’re clearly visible on ultrasound. The baby can open and close its fists and may bring its hands toward its face. Swallowing reflexes are developing as the baby begins taking in small amounts of amniotic fluid, which helps the digestive system practice for life outside the womb.
What You See on Ultrasound
If you have an ultrasound at 13 weeks, you’ll see a recognizable baby shape: a rounded head, a defined profile with a nose and chin, arms and legs that move, and a visible spine. The baby may be curled up or stretched out, and you might catch it waving its hands or kicking.
This is also the earliest point where sex can be reliably identified on ultrasound. All babies develop a small structure between the legs called a genital tubercle, sometimes informally called “the nub,” between weeks 11 and 13. The angle of this nub relative to the spine gives a clue: angled more than 30 degrees typically indicates a boy, while parallel to the spine suggests a girl. Accuracy at 13 weeks reaches close to 100%, though most providers prefer to confirm at the anatomy scan around 18 to 20 weeks. Some parents also learn the sex earlier through noninvasive prenatal blood testing, which can be done as early as 10 weeks.
How 13 Weeks Compares to Earlier Stages
The difference between 8 weeks and 13 weeks is dramatic. At 8 weeks, the embryo was about the size of a raspberry, with paddle-shaped limb buds and a massive head making up nearly half its total length. By 13 weeks, fingers and toes are fully separated, joints bend, and the face has distinct features. The tail-like structure present in early embryonic development is long gone. What you’re looking at now is a small but complete human form, with all major organs in place and a skeleton that’s just beginning to solidify.