At 16 weeks of pregnancy, a fetus is about 4¾ inches (12 centimeters) long from crown to rump and weighs roughly 4 ounces (110 grams), making it about the size of an avocado. The body is becoming more proportional, the face is taking on recognizably human features, and the limbs are long enough to move freely. Here’s a closer look at what’s developing.
Overall Size and Proportions
At the start of pregnancy, a fetus’s head accounts for nearly half its total length. By week 16, the body and limbs have caught up significantly. The legs are now longer than the arms, and the torso has lengthened so the head, while still large relative to an adult’s proportions, no longer dominates the silhouette. The neck is more defined, allowing the head to lift away from the chest. If you held a 16-week fetus in your hand, it would fit comfortably in your palm.
Facial Features
The face looks distinctly human at this point. The eyes have moved from the sides of the head toward the front, though the eyelids remain fused shut and won’t open for several more weeks. Eyebrows and eyelashes are beginning to form as fine hairs. The ears have shifted from the neck area to nearly their final position on the sides of the head, though they aren’t functional yet. Actual sound perception doesn’t begin until around 23 weeks of pregnancy.
The nose, lips, and chin are well defined on ultrasound. The mouth can open and close, and the muscles of the face are developed enough for small expressions, though these are reflexive rather than intentional.
Skin, Hair, and Protective Layers
The skin at 16 weeks is still extremely thin and nearly translucent. Blood vessels are clearly visible beneath the surface, giving the skin a reddish appearance. There’s virtually no fat underneath yet, so the fetus looks lean and angular compared to a newborn.
This is also when lanugo, a layer of soft, fine, feathery hair, begins developing across the body. Lanugo appears between 16 and 20 weeks and serves an important purpose: it helps a waxy coating called vernix stick to the skin. Vernix acts as a waterproof barrier that protects the fetus’s delicate skin from the amniotic fluid it’s floating in. Without this system, the fluid could irritate or damage the skin over the remaining months of pregnancy.
Bones and Skeleton
The skeleton is transitioning from soft cartilage to harder bone. This hardening process, called ossification, follows a predictable pattern. By 16 weeks, most of the long bones in the arms and legs are beginning to calcify, and the spine is progressively hardening in a downward direction. The lowest lumbar vertebrae are ossifying around this time, with each additional vertebral level solidifying roughly every two to three weeks after week 16. On an ultrasound, these hardening bones show up as bright white lines and shapes against the darker soft tissue.
Movement at 16 Weeks
A 16-week fetus is surprisingly active. The arms and legs are long enough, and the joints flexible enough, for kicking, stretching, and even somersaulting in the amniotic fluid. The fetus can make fists, flex its fingers, and curl its toes. Sucking and swallowing reflexes are developing, and the fetus regularly swallows amniotic fluid as a form of practice for feeding after birth.
Most people who are pregnant can’t feel these movements yet at 16 weeks, especially in a first pregnancy. The fetus is still small enough that its kicks and rolls don’t reach the uterine wall with enough force to notice. First-time mothers typically feel movement between 18 and 22 weeks, while those who’ve been pregnant before sometimes notice it a few weeks earlier.
Organ Development
Several organ systems hit functional milestones around week 16. The kidneys are now producing urine, and from this point forward, fetal urination is responsible for more than half of the amniotic fluid volume. The fetus swallows this fluid, processes it through the digestive system, and urinates it back out in a continuous cycle that helps maintain the right amount of fluid in the uterus.
The heart is fully formed and pumping about 25 quarts of blood per day. The circulatory system is complete enough that the heartbeat is easily detected on a standard Doppler device at a prenatal visit. The liver and spleen are beginning to produce blood cells, and the intestines are starting to accumulate meconium, the dark substance that will become the baby’s first bowel movement after birth.
What You Can See on Ultrasound
If you have an ultrasound around 16 weeks, you’ll see a fetus that looks recognizably like a baby. The profile view typically shows a clear forehead, nose, and chin. You can often see the hands and feet, count fingers and toes, and watch the fetus move in real time. The four chambers of the heart are visible, and the spine appears as a distinct row of bright dots.
Many parents want to know the sex at this stage. While the external genitalia are formed by 16 weeks, the accuracy of sex determination depends on the fetus’s position during the scan. Studies of ultrasound predictions between 14 and 16 weeks show high accuracy when the anatomy is clearly visible, though most providers prefer to confirm at the standard anatomy scan around 18 to 20 weeks, when the structures are larger and easier to evaluate.
Sensory Development So Far
At 16 weeks, the sensory systems are still in early construction. The inner ear structures are forming but not yet connected to the brain in a way that allows hearing. True sound responsiveness won’t begin until around 23 weeks, and consistent reactions to sound across all fetuses don’t appear until 28 to 30 weeks. The eyes can sense light and dark to a limited degree through the closed eyelids, but vision remains the least developed sense at this stage. Touch receptors, however, are fairly well established. The fetus responds to pressure on the uterine wall and will often move away from the probe during an ultrasound.