At 31 weeks, your baby is about the size of a coconut, measuring over 10.5 inches from crown to rump and weighing close to 3 pounds (1,300 grams). Head to toe, that’s roughly 16 inches. The baby looks remarkably like a newborn at this point, with defined facial features, plumping skin, and increasingly coordinated movements you can feel from the outside.
Size and Proportions
The 3-pound mark is a meaningful milestone. Your baby has been gaining weight rapidly over the past several weeks, and that pace is about to accelerate even more. Much of the weight gain from here until delivery comes from fat deposits under the skin, which are filling out the previously wrinkled, translucent appearance. The limbs look rounder, the cheeks are fuller, and the body proportions are shifting closer to what you’d expect in a full-term baby, though the head is still relatively large compared to the body.
Skin, Hair, and Outer Appearance
Earlier in pregnancy, your baby’s skin was see-through enough to show blood vessels underneath. By 31 weeks, layers of fat have changed that. The skin is becoming more opaque and taking on a pinkish tone regardless of the baby’s eventual skin color (pigmentation deepens after birth).
A fine, downy hair called lanugo has covered your baby’s body for months. It begins shedding around this stage, typically falling off between 33 and 36 weeks. As it sheds, the hair mixes into the amniotic fluid and eventually becomes part of meconium, the baby’s first bowel movement. Meanwhile, a waxy white coating called vernix still covers the skin, protecting it from the constant exposure to amniotic fluid. Some babies are born with patches of vernix still visible, especially in skin folds.
Fingernails and toenails have grown to the tips of the fingers and toes. The hair on the baby’s head, if there is any, is getting thicker.
Facial Features and Senses
Your baby’s face at 31 weeks is fully formed and surprisingly expressive. On a 3D ultrasound, you could see distinct features: a nose shape, lip contours, and eyelids that open and close. The irises can now react to light. If a bright light is shone on your belly, the baby may turn toward or away from it.
All five senses are actively developing. Your baby can hear your voice, your heartbeat, and muffled sounds from outside the womb. Taste buds are functional, and the baby swallows amniotic fluid regularly, which means flavors from your diet subtly reach them. Touch is well developed too. Babies at this stage will grasp the umbilical cord, suck their thumbs, and react to pressure on the uterine wall.
Brain Development
The brain is in one of its most intense growth phases during the early 30s of pregnancy. Billions of nerve connections are forming rapidly, and the brain’s surface is developing its characteristic wrinkles and folds. These folds dramatically increase the brain’s surface area, allowing for more complex processing. This is why the third trimester is so critical for neurological development. The brain at 31 weeks is processing information from all the senses, coordinating increasingly complex movements, and beginning to regulate body temperature (though not well enough yet to manage without help outside the womb).
Lungs and Internal Organs
The lungs are one of the last organs to fully mature, and at 31 weeks they’re still working on it. Specialized cells in the lungs are actively producing surfactant, a slippery substance that keeps the tiny air sacs from collapsing when a baby breathes. These cells develop between 24 and 34 weeks, so your baby is right in the middle of that window. The lungs are not considered clinically mature until around 35 weeks.
Your baby is already practicing breathing movements, rhythmically moving the chest and diaphragm. These practice breaths don’t bring in air; instead, they move small amounts of amniotic fluid in and out of the airways. This rehearsal strengthens the muscles needed for breathing after birth. Other organs, including the kidneys, liver, and digestive system, are functioning and continuing to refine their performance.
Bones and Skeleton
The skeleton has been hardening throughout pregnancy as cartilage gradually converts to bone. By 31 weeks, the bones in the arms, legs, and spine are becoming increasingly solid. The skull, however, remains intentionally flexible. The skull bones haven’t fused together yet, and won’t for some time after birth. This flexibility allows the head to compress slightly during delivery and accommodates the rapid brain growth that continues after the baby is born. You’ll notice this as the soft spots (fontanelles) on a newborn’s head.
Movement and Sleep Patterns
At 31 weeks, your baby cycles through distinct behavioral states similar to a newborn’s. Research tracking fetal activity has identified at least three main states: quiet sleep, active sleep, and active wakefulness. Babies at this stage spend most of their time, roughly 83%, in active sleep. During active sleep, you’ll feel gentle movements, twitches, and rolling. About 13% of the time is spent in quiet sleep, when movement stops almost entirely and you might worry something is wrong (it’s not). Only about 4% of the time is spent in a fully active, awake state with vigorous kicking and stretching.
That most active state tends to peak in the evening and early nighttime hours, between about 9 p.m. and 1 a.m., which is why many pregnant people report the strongest kicks right when they’re trying to fall asleep. Maternal position also matters. Babies tend to be more active when the mother is lying on her side and quieter when she’s on her back.
Space inside the uterus is getting tighter. You’ll notice the nature of movements changing from the somersaults of earlier weeks to more defined pushes, kicks, and stretches. You might see a foot or elbow press visibly against your belly.
If a Baby Is Born at 31 Weeks
A baby born at 31 weeks is premature and would need time in a neonatal intensive care unit, but outcomes at this gestational age are very good. Data from a large study tracking preterm births found that among babies born at 31 weeks who survived, 93.5% had no severe complications. The small percentage who did experience complications most commonly had issues related to lung immaturity, intestinal inflammation, or bleeding in the brain, but even these occurred at rates of just 1 to 2% each.
A 31-week baby would likely need help with breathing, temperature regulation, and feeding, since the sucking and swallowing reflexes aren’t fully coordinated yet. Hospital stays typically last several weeks, often until close to the original due date. But the overall picture at 31 weeks is reassuring: the major organs are formed and functioning, and the remaining weeks of pregnancy are primarily about growth, fat accumulation, and final maturation of the lungs and brain.