At 31 weeks, a baby measures about 16 inches long and weighs roughly 3.3 pounds, comparable in size to a coconut. The thin, wrinkled appearance of earlier weeks is filling out as fat deposits accumulate under the skin, giving the baby a rounder, more newborn-like look. The skin is still pinkish-red and somewhat translucent but is becoming more opaque week by week as that fat layer thickens.
Size, Proportions, and Skin
A 31-week baby weighs approximately 1,500 grams (3.3 pounds) and stretches about 41 centimeters (16.2 inches) from crown to heel. The head is large relative to the body, accounting for roughly a third of overall length. Fingernails have grown to the tips of the fingers, and toenails are visible, though they haven’t fully reached the toe tips yet.
Fat accumulation accelerates early in the third trimester, with a noticeable growth spurt in soft tissue volume around 30 weeks. Between 70 and 90 percent of a baby’s total body fat is subcutaneous, meaning it sits just beneath the skin rather than around the organs. This fat is deposited in the upper arms, thighs, abdomen, cheeks, ribs, and buttocks. The result: limbs and the face look plumper than just a few weeks earlier, and the skin has fewer visible wrinkles. The cheeks in particular are filling out, giving the face a softer, rounder contour.
Lanugo, the fine downy hair that covered the body earlier in pregnancy, is starting to shed from some areas, though patches may remain on the shoulders and back. A waxy, white coating called vernix still covers much of the skin, protecting it from the amniotic fluid.
Brain Development and Sleep Cycles
The brain is growing rapidly at 31 weeks, adding billions of nerve connections. The surface of the brain is developing its characteristic folds and grooves, which dramatically increase surface area for processing information. Sleep cycles began forming between 26 and 28 weeks, and by 31 weeks the baby shifts between distinct states: quiet sleep (similar to deep, dreamless sleep), active sleep resembling REM, and quiet wakefulness. You may notice a pattern in your baby’s movements, with stretches of activity followed by periods of stillness as the baby cycles through these states.
Eyes, Hearing, and Reflexes
The eyes are open and can detect changes in light. If you shine a flashlight against your belly, the baby may turn toward or away from the light. A protein called melanopsin is active in the fetal eye at this stage, playing a direct role in retinal development as photons pass through the mother’s body. The irises can now constrict and dilate in response to light, though eye color isn’t set yet. Most babies are born with dark blue or gray-brown eyes that may change over the first year of life.
Hearing is well developed. The baby responds to voices, music, and sudden loud sounds. Taste buds are functional too, and the baby swallows amniotic fluid regularly, which is the primary way the body recycles that fluid. By the end of pregnancy, a baby’s kidneys produce enough urine (800 to 1,200 milliliters per day at term) to replace the entire volume of amniotic fluid every 12 to 24 hours. At 31 weeks, urine production is lower than that but steadily increasing as the kidneys mature.
Lungs and Breathing Practice
Lung development is one of the last major milestones before birth. At 31 weeks, the lungs are producing surfactant, a slippery substance that keeps the tiny air sacs from collapsing when a baby exhales. However, the amount is still limited. Clinically, lungs are generally considered mature enough to avoid significant breathing difficulty after about 36 weeks. A baby born at 31 weeks would likely need help breathing, but the lungs are far enough along that outcomes are very good with medical support.
The baby practices “breathing” by rhythmically moving the diaphragm and drawing amniotic fluid in and out of the lungs. These movements strengthen the muscles needed for that first real breath after birth. You might feel these practice breaths as small, rhythmic hiccups.
Position and Movement
At 31 weeks, many babies haven’t yet settled into their final birth position. Most babies move into a head-down (cephalic) position by 36 weeks, so there’s still plenty of time for turning. Some babies are already head-down at 31 weeks, while others are breech (feet or bottom first) or lying sideways. The baby is getting bigger relative to the available space, so movements may feel different than earlier in pregnancy. Instead of somersaults and flips, you’re more likely to feel stretches, rolls, and sharp jabs from elbows and knees pressing against your uterine wall.
If a Baby Is Born at 31 Weeks
Babies born between 31 and 34 weeks have a greater than 95 percent chance of survival. A 31-week baby would be considered moderately preterm and would need time in the NICU, primarily to support breathing, feeding, and temperature regulation. That thin but growing layer of subcutaneous fat isn’t yet thick enough to maintain body temperature independently, so an incubator or warming bed is standard.
Most preterm babies born at this stage stay in the hospital until around their original due date, roughly 38 weeks of gestational age, which means about 7 weeks in the NICU. The stay can be shorter or longer depending on how quickly the baby learns to feed by mouth and maintain stable breathing and body temperature. A 31-week baby looks like a smaller, thinner version of a full-term newborn. The features are fully formed, the eyes open and close, and the baby responds to touch and sound. The main visual differences are size, less body fat, and skin that appears redder and more delicate.
What You Might See on Ultrasound
On a 31-week ultrasound, you can see a baby that looks recognizably human and remarkably detailed. The face has filled out enough to see rounded cheeks, a defined nose, and lips that may be practicing sucking motions. You might catch the baby yawning, swallowing, or gripping the umbilical cord. 3D and 4D ultrasounds at this stage often produce striking images because there’s enough fat on the face to create soft, smooth contours rather than the skeletal look of earlier scans. Hair may be visible on the scalp, though the amount varies widely from baby to baby.