At 23 weeks, a fetus is about 11.4 inches long from head to heel and weighs roughly 1.1 pounds, making it comparable in size to a large mango. Every major body part is formed at this point, and the fetus looks recognizably human, though the skin is thin, wrinkled, and somewhat translucent. This is a stage of rapid refinement: organs are maturing, senses are switching on, and fat stores are just beginning to fill out the frame.
Size and Proportions
At just over 500 grams, a 23-week fetus is lean. The head is still large relative to the body, but the limbs have lengthened considerably and the proportions are shifting closer to what they’ll be at birth. The fingers and toes are fully separated, with tiny fingernails growing in. Arms and legs are thin because the layer of fat beneath the skin is still minimal. Over the coming weeks, the fetus will gain weight rapidly as subcutaneous fat develops, but at 23 weeks, the body looks bony and angular.
Skin, Hair, and Surface Features
The skin at 23 weeks is wrinkled and reddish, partly because blood vessels are visible through it. A fine, soft layer of hair called lanugo covers much of the body. This hair ranges from very light to dark and serves an important purpose: it helps a waxy coating called vernix stick to the skin. Vernix acts as a barrier, protecting the fetus’s delicate skin from constant exposure to amniotic fluid, which would otherwise cause damage over months of immersion.
Because fat deposits are still sparse, the skin hangs loosely. Over the next several weeks, the fetus will “fill out” as fat accumulates, smoothing those wrinkles and making the skin less see-through.
Facial Features
The face is well-defined at 23 weeks. The lips, nose, and eyelids are all formed, and the facial profile is clearly visible on ultrasound. The eyelids, which fused shut early in development, are in the process of reopening. This typically completes around 24 weeks, meaning at 23 weeks the eyes may still be sealed or just beginning to part. Behind those lids, the structures of the eye are developing but not yet ready to process light in any meaningful way.
Eyebrows are starting to appear, and the ears are taking on their final shape and position on the sides of the head.
What You’d See on an Ultrasound
A 23-week ultrasound can show remarkable detail. The anatomy scan performed between 18 and 23 weeks evaluates the skull shape, brain structures (including the cerebral ventricles and cerebellum), the four chambers of the heart and its outflow tracts, the spine in both lengthwise and cross-section views, and the profile of the face including the upper lip and eye sockets. The stomach, liver, kidneys, and bladder are all visible. Individual arm and leg bones are measured, and the hands and feet, including joint movement, can be observed.
You may also catch the fetus sucking a thumb, stretching, or changing position. At this stage, there’s still enough room in the uterus for full-body flips, so the fetus might be head-up one scan and head-down the next.
Movement and What It Feels Like
Most pregnant people feel their first fetal movements between 16 and 24 weeks. By 23 weeks, those early flutters have often given way to more distinct sensations: kicks, rolls, and jerky movements. Each fetus has its own activity pattern, and there’s no set number of movements to expect per day. What matters is getting familiar with the rhythm over time.
Inside the uterus, the fetus is quite active at this stage. It stretches, grasps, hiccups, and practices swallowing amniotic fluid. Imaging studies have documented swallowing movements between 22 and 25 weeks, though they’re often uncoordinated at this point, with fluid sometimes flowing in both directions due to the immature swallowing reflex.
Hearing and Brain Development
Around 23 weeks, the fetus begins responding to sounds from outside the womb. At first, it can only detect low-frequency noises, like a dog barking or a lawn mower, but speech and music are also reaching the inner ear at this stage. These sounds aren’t just background noise. Specific parts of the auditory system depend on them to develop properly, which is why the sound environment during pregnancy plays a role in future hearing.
The brain is growing rapidly, adding neurons and forming connections. Electrical brain activity is present at 22 to 23 weeks in the form of short bursts of oscillation separated by quiet periods. This is the earliest detectable brain wave pattern in a viable fetus. However, organized sleep states, like the cycles of active and quiet sleep that newborns display, haven’t emerged yet. Clear differentiation between those sleep phases doesn’t appear on brain recordings until around 26 to 27 weeks.
Lung Development and Viability
The lungs are in a critical phase of development at 23 weeks. The airways are branching into smaller passages, and the cells lining them are beginning to differentiate into two types: one that will form the thin walls of the air sacs, and another that will produce surfactant, a substance that keeps the air sacs from collapsing after birth. Surfactant production starts around 24 weeks, meaning at 23 weeks the lungs are on the cusp of this milestone but not quite there yet.
This is why 23 weeks sits right at the edge of viability. Infants born before 24 weeks generally have less than a 50 percent chance of survival, and outcomes depend heavily on the medical resources available. Those who do survive at 23 weeks typically require intensive respiratory support and face a higher risk of long-term complications. Each additional day in the womb at this stage meaningfully improves the odds.
Organ Development at a Glance
Beyond the lungs, the other organs are reaching varying levels of maturity. The heart has been beating since early in the first trimester and now pumps blood through a fully formed circulatory system. The kidneys are producing urine, which contributes to the amniotic fluid. The digestive tract is practicing its function through swallowing, though it won’t need to process food until after birth. The liver and pancreas are developing but won’t take on their full roles for weeks.
The bones are hardening, transitioning from soft cartilage to calcified bone, though they’ll remain somewhat flexible even at birth. The skeletal structure is complete in form, with all the bones of the arms, legs, hands, feet, ribs, and spine in place and continuing to strengthen.