At 25 weeks of pregnancy, your fetus is roughly 13¼ inches long from head to heel and weighs about 1¾ pounds, comparable in size to an acorn squash. The body is fully formed in miniature but still visibly immature: the skin is thin and wrinkled, the limbs are lean, and a fine layer of hair covers nearly every surface. Over the next few weeks, fat deposits will begin filling out the frame and smoothing the skin.
Skin, Hair, and Body Proportions
The most striking feature of a 25-week fetus is its skin. It is wrinkled, somewhat see-through, and tends to look pink or reddish because blood vessels are visible beneath the surface. There is very little subcutaneous fat at this stage, which is why the skin appears loose and saggy, almost like clothing a few sizes too big. Fat accumulation is just getting underway and will accelerate through the third trimester, gradually giving the skin a smoother, more opaque appearance.
Covering the skin is a greasy, white coating called vernix caseosa. It acts as a barrier against the amniotic fluid, protecting the delicate skin from chapping and hardening during the remaining weeks of pregnancy. Holding that coating in place is a layer of fine, downy hair called lanugo, which covers most of the body by this point. The lanugo typically sheds before birth or shortly after.
The head is still large relative to the body, and the facial features are well defined. Eyebrows and eyelashes are present. The eyelids, which were fused shut earlier in development, are beginning to separate. By about 26 weeks the eyelids are partially open, and by 28 weeks the eyes are fully open.
What’s Happening Inside the Lungs
The lungs are in a critical phase of development at 25 weeks. This period, known as the canalicular stage, runs from roughly 16 to 26 weeks. During it, the primitive gas-exchange regions of the lungs are forming and the lining of the airways is starting to differentiate into specialized cell types. Among those are the cells responsible for producing surfactant, a substance that keeps the tiny air sacs from collapsing. Surfactant-producing cells differentiate between 24 and 34 weeks, so at 25 weeks the process is in its early stages.
Your baby is also practicing breathing movements. These aren’t real breaths since the lungs are filled with fluid, not air. Instead, the fetus rhythmically moves fluid in and out of the airways and into the amniotic fluid. These practice movements help the lungs develop the muscle coordination they’ll need after birth.
Brain and Nervous System
The brain is growing rapidly at 25 weeks, and the nervous system is actively maturing. Cortical neurons, the cells that will eventually handle thought, sensation, and movement, are migrating outward from deeper brain structures toward the surface layer called the cortical plate. Once they arrive, they begin forming connections with neighboring neurons through branching networks and early synapses. At this gestational age the brain surface is still relatively smooth. The characteristic folds and grooves that increase the brain’s surface area develop over the coming weeks, with significant structural changes occurring between 20 and 35 weeks.
This rapid neural development is laying the groundwork for sensory processing. The fetus can already detect sound, and studies show that the visual system is also starting to come online. Pupil constriction and light detection develop around 31 weeks, but the structural wiring is being assembled right now.
Movement and Sensory Responses
If you’re 25 weeks pregnant, you’ve likely been feeling movement for several weeks. By now those flutters have become more definite kicks, rolls, and stretches. Your baby is developing recognizable sleep-wake patterns, which means there will be periods of activity followed by stretches of quiet. Many people notice their baby is most active when they themselves are resting, partly because movement during the day can rock the fetus to sleep.
The fetus can hear at this stage. Loud sounds from outside the womb may trigger a startle response. The earliest eye movements have been detectable on ultrasound since around 14 weeks, though coordinated eye-opening and responses to light come later, closer to 28 to 33 weeks.
What Happens If a Baby Is Born at 25 Weeks
Twenty-five weeks is considered the edge of viability. Survival rates for babies born at this gestational age range from 59% to 86%, depending on the hospital, available neonatal care, and individual factors like birth weight. That’s a meaningful jump from just one week earlier, when survival rates at 24 weeks range from 31% to 78%.
Babies born this early face significant medical challenges. The lungs are not yet producing enough surfactant to function independently, so most 25-week preemies need breathing support. The brain is still in a vulnerable stage of development, and the risk of long-term neurodevelopmental difficulties is real, though outcomes vary widely. A stay in the neonatal intensive care unit typically lasts several months, often until close to the original due date. The wide range of survival and outcome statistics reflects the enormous difference that access to specialized neonatal care can make.
How 25 Weeks Compares to Neighboring Weeks
Development at this stage moves fast. At 24 weeks, the skin is at its most translucent and wrinkled. By 27 weeks, the nervous system has matured further and fat deposits are noticeably smoothing the skin. The jump in survival rates from week to week during this period is among the steepest in all of pregnancy, which is why each additional week in the womb carries outsized importance for babies at risk of preterm birth.
At 25 weeks, your baby is roughly two-thirds of the way through pregnancy by time but only about a quarter of the way to full birth weight. The bulk of weight gain happens in the third trimester, when fat accumulation accelerates and the organs finish maturing. What you’d see on an ultrasound right now is a baby that looks unmistakably human, with defined fingers and toes, visible facial expressions, and active limb movements, but one that is still visibly thin and fragile compared to a full-term newborn.