At 13 weeks, a fetus measures about 7 to 8 centimeters from head to rump, roughly the size of a lemon. It has a recognizably human shape, with a head that’s becoming more proportional to the body, visible arms and legs, and thin, transparent skin that reveals developing blood vessels underneath.
Size and Proportions
The average crown-to-rump length at 13 weeks is about 6.8 centimeters at the start of the week, growing to nearly 7.9 centimeters by the end. That measurement covers only the head to the bottom, not the legs. The head, which dominated the body in earlier weeks, is starting to look more balanced relative to the torso. Arms, legs, fingers, and toes are all formed and increasingly well-defined, though the limbs are still short compared to what they’ll become later.
Skin, Bones, and Features
The skin at this stage is thin and transparent. You could see blood vessels and developing organs through it if you were looking directly. There’s no fat layer yet, so the fetus appears lean and somewhat fragile.
Bones are beginning to harden, especially in the skull and the long bones of the arms and legs. The spine is also ossifying; vertebral column hardening starts around weeks 10 to 11 and is well underway by week 13. This is the shift from soft cartilage to actual bone tissue, a process that will continue for years after birth but is visibly progressing now on imaging.
Facial features are becoming more distinct. The eyes, which started on the sides of the head, have moved closer together toward the front of the face. Ears are close to their final position. On ultrasound, you can often see a clear profile: forehead, nose, lips, and chin.
Organ Development
Internal organs are not just present but starting to function. The liver and pancreas are beginning to work, and vocal cords are forming in the throat, though they won’t be used until birth. The kidneys are producing small amounts of urine, which gets released into the amniotic fluid.
The placenta is now well established. It has been developing since the earliest days of pregnancy, building an increasingly dense network of tiny branches (villi) that deliver oxygen and nutrients from the mother’s blood supply while carrying waste away. By 13 weeks, the placenta is transitioning from its early low-oxygen environment to a more active role supporting rapid fetal growth.
Movement
A 13-week fetus is surprisingly active. It can stretch, yawn, suck its thumb, and make facial expressions. These movements are reflexive rather than intentional, driven by the developing nervous system testing out new connections. Most mothers can’t feel any of this yet. The fetus is still small enough that its movements don’t register against the uterine wall. That sensation typically comes several weeks later, usually between weeks 16 and 25.
Genitalia and Sex
The internal sex organs are fully developed by week 13, but external genitalia are just beginning to form. A small structure called the genital tubercle is present, and it will eventually become either a penis or a clitoris. At this stage, it’s generally too early for a standard ultrasound to reliably identify the sex. Some practitioners use what’s called the “nub theory,” examining the angle of that tubercle relative to the spine. An angle above 30 degrees suggests male, while below 30 degrees suggests female. But most providers wait until 18 to 20 weeks for a confident determination.
What You’ll See on an Ultrasound
If you have an ultrasound at 13 weeks, the technician will measure the crown-to-rump length to confirm or adjust your due date. You’ll likely see clearly defined arms and legs, a proportionate head, and possibly the fetus moving around. The scan also evaluates bone development, organ formation, heart rate, blood flow through developing vessels, amniotic fluid levels, and placental health.
This is also the window for nuchal translucency screening, where the technician measures a small fluid-filled space at the back of the fetus’s neck. That measurement, combined with blood work, helps assess the risk of certain chromosomal conditions. It’s a routine part of first-trimester screening and needs to be done before 14 weeks.
How You Might Be Feeling
Week 13 marks the beginning of the second trimester, and many women notice a shift. The intense nausea and fatigue of the first trimester often start to ease. Your belly may be visibly growing as the uterus expands, and your breasts will likely continue getting larger.
New symptoms can replace the old ones. Lower back pain is common as hormonal changes loosen your joints and your center of gravity shifts. You might notice skin changes: darker patches on your face, a dark line running down your belly, or the beginnings of stretch marks. Nasal congestion and nosebleeds can show up as increased blood volume causes swelling in your nasal passages. Leg cramps, dizziness from blood pressure changes, and increased vaginal discharge are all typical for this stage.
The second trimester is often described as the most comfortable stretch of pregnancy. The fetus isn’t yet large enough to press on your organs or make movement difficult, and the hormonal turbulence of the first trimester is settling down.