At 31 weeks pregnant, your baby is about 16 inches long from head to heel and weighs roughly 3.3 pounds (1,502 grams). That’s about the size of a coconut. Over the next several weeks, your baby will gain weight rapidly, but most of the major structural development is already well underway.
Weight, Length, and Growth Rate
The 3.3-pound mark at week 31 represents a baby that has done most of its lengthening but still has significant filling out to do. From here until birth, weight gain accelerates considerably. Starting around week 35, babies typically put on 8 to 12 ounces per week, which is the fastest period of weight gain during the entire pregnancy. By full term, the average newborn weighs between 6 and 9 pounds, meaning your baby will roughly double or triple its current weight before delivery.
The 16-inch crown-to-heel length is already close to final birth length, which averages around 19 to 21 inches. Most of the remaining growth is about adding fat and muscle rather than getting longer. That fat layer is important: it helps regulate body temperature and provides energy reserves for the first days after birth.
What Your Baby Can Do Right Now
At 31 weeks, your baby’s brain is maturing rapidly and can process more information and stimuli than just a few weeks ago. You may notice more distinct sleep and wake cycles, with predictable windows of activity and quiet. The brain’s ability to regulate body temperature is also coming online, though it won’t be fully reliable for several more weeks.
The lungs have been producing surfactant, a slippery substance that keeps the air sacs from collapsing, since around week 26. At 31 weeks, surfactant production is still ramping up. The lungs are functional enough that survival outside the womb is very likely, but most babies born at this stage still need some breathing support.
How Movement Changes at This Stage
Your baby is getting big enough that the uterus is starting to feel crowded. You’ll likely still feel strong movements, but the character of those movements shifts during the third trimester. Expect more rolls, stretches, and pushes rather than the sharp kicks you felt earlier. Some parents mistake this change for reduced movement, but it’s simply a different type of motion as the baby runs out of room to wind up for a full kick.
Movements typically get stronger as the third trimester progresses. A common guideline is to feel at least 10 movements (kicks, flutters, swishes, or rolls) within two hours, though many babies hit that number in under an hour. Tracking movement becomes especially useful from week 28 onward, since a noticeable drop in activity can signal that the baby is under stress.
If Your Baby Were Born at 31 Weeks
Babies born between 31 and 34 weeks have a greater than 95% chance of survival. That’s reassuring, but a 31-week baby is still premature and typically needs time in the NICU to finish developing. The most common challenges are breathing, temperature regulation, feeding, and infection risk.
Breathing is usually the biggest concern. Because surfactant production isn’t fully mature, many babies born at this stage develop respiratory distress syndrome and need supplemental oxygen or a ventilator. The brain’s breathing center also isn’t fully developed until around 36 weeks, which can cause brief pauses in breathing called apnea.
Temperature control is another hurdle. Premature babies have very little body fat and an immature internal thermostat, so they lose heat quickly and typically need an incubator to stay warm. Feeding can also be difficult because the coordination required to suck, swallow, and breathe simultaneously doesn’t mature until closer to 34 weeks. Many 31-week babies receive breast milk or formula through a tube until they’re ready to feed on their own. Their immune systems are also underdeveloped, making them more vulnerable to infections while in the hospital.
What You Might Notice in Your Own Body
Your uterus has grown to match your baby’s size. At 31 weeks, your fundal height (the distance from your pubic bone to the top of your uterus) should measure close to 31 centimeters, give or take 2 centimeters. Your provider checks this at prenatal visits as a quick gauge of whether your baby’s growth is on track. A measurement significantly above or below that range sometimes prompts an ultrasound for a closer look, though minor variations are normal and can reflect your build, the baby’s position, or fluid levels.
With over 3 pounds of baby plus amniotic fluid and placenta pressing on your organs, you’re likely feeling the physical weight of the third trimester. Shortness of breath, lower back pressure, and frequent bathroom trips are all common as your baby takes up more space.