At 31 weeks pregnant, your baby weighs just over 3 pounds and is roughly the size of a coconut. You’re deep into the third trimester now, and both your body and your baby are changing fast. Here’s what’s happening this week and what to keep on your radar.
Your Baby at 31 Weeks
Your baby is putting on weight quickly at this stage, building up fat layers that will help regulate body temperature after birth. The lungs are still maturing, producing a substance that coats the air sacs and makes breathing possible outside the womb. Brain development is also accelerating, with billions of nerve connections forming every day.
At this point, your baby can respond to light, track sounds, and has regular sleep-wake cycles you may already recognize. You’ll likely notice distinct patterns of activity and rest throughout the day. The movements may feel different than they did earlier, less like flutters and more like definite rolls, pushes, and kicks as space in the uterus gets tighter.
If your baby were born this week, the outlook would be very good. Preterm babies born between 31 and 34 weeks have a greater than 95% chance of survival. Most would need some help with breathing, possibly extra oxygen or a ventilator, and would spend time in the NICU learning to feed and maintain body temperature. But the vast majority do well.
How Your Body Feels Right Now
The third trimester brings a distinct set of physical changes, and at 31 weeks many of them are in full swing. Shortness of breath is common because your growing uterus is pushing up against your rib cage, reducing how much space your lungs have to expand. You may find yourself winded just climbing a flight of stairs.
Backaches are another hallmark of this stage. Pregnancy hormones loosen the connective tissue in your pelvic area, and the expanding uterus stretches your abdominal muscles. Together, those shifts put extra strain on your lower back. Heartburn and constipation often ramp up too, since those same hormones slow digestion while your uterus presses on your intestines.
Frequent urination is likely increasing as the baby settles deeper into your pelvis and puts more pressure on your bladder. Leaking a little urine when you laugh, sneeze, or cough is normal at this point. You may also notice spider veins, varicose veins on your legs, or hemorrhoids, all caused by the increased blood volume your body is managing. Heart palpitations, little bouts of fluttering or skipped beats, can happen because your growing uterus slows blood returning to the heart.
Braxton Hicks Contractions
By 31 weeks, many women are experiencing Braxton Hicks contractions: brief tightening sensations across the belly that come and go without a predictable pattern. They tend to show up more often in the afternoon or evening, after physical activity, or after sex. They can range from barely noticeable to genuinely painful, which understandably causes some worry about preterm labor.
The key difference is regularity. True labor contractions come at consistent intervals, get closer together over time, and each one lasts about 60 to 90 seconds. Braxton Hicks contractions are irregular and vary in strength. A reliable test: if resting and drinking water makes them stop, they’re practice contractions, not the real thing.
Signs of Preterm Labor
While Braxton Hicks are harmless, certain symptoms at 31 weeks need immediate attention. Contact your provider right away if you experience any of the following:
- Regular or frequent contractions that don’t stop with rest
- A constant dull low backache that feels different from your usual pregnancy backache
- Pressure in your pelvis or lower belly
- Vaginal spotting or light bleeding
- A gush or steady trickle of fluid from the vagina, which could mean your water has broken
- Discharge that is watery, bloody, or mucus-like
These can be signs of preterm labor, and getting checked quickly gives your medical team the best chance to intervene if needed.
Tracking Your Baby’s Movements
Kick counts become an important daily habit around this time. The standard recommendation from ACOG is to time how long it takes to feel 10 movements, including kicks, rolls, flutters, and swishes. Ideally, you want to hit 10 within two hours. Most women reach that number much faster, especially if they count during a time when the baby is typically active.
Pick a consistent time each day, sit or lie down in a comfortable position, and pay attention. If two hours pass without 10 movements, contact your provider. It doesn’t necessarily mean something is wrong, but it’s worth checking.
What Happens at Prenatal Visits
During the third trimester, you’ll typically see your provider every two to four weeks. At each visit, your team will check your blood pressure, weight, and the baby’s heartbeat. They’ll ask about any symptoms you’ve noticed, particularly contractions, fluid leaks, or bleeding.
Around this time, your provider may discuss vaccinations. The Tdap vaccine is recommended between 27 and 36 weeks of pregnancy, which helps protect your newborn from whooping cough in the first few months of life. Flu, COVID-19, and RSV shots may also be suggested depending on the season and your situation. Expect to be tested for group B strep (GBS) at some point during the third trimester as well. GBS is a common bacterium that’s harmless to you but can cause problems for the baby during delivery if not treated.
Sleep and Comfort
Getting comfortable at night becomes increasingly difficult at 31 weeks. An NIH-funded study found that sleeping on your back or side through 30 weeks does not appear to increase the risk of stillbirth or other complications. However, the study did not evaluate sleep positions beyond 30 weeks, which is right where you are now. Most providers recommend side sleeping from this point forward, with the left side often preferred because it maximizes blood flow to the uterus.
A pillow between your knees, under your belly, or behind your back can help you stay on your side and reduce pressure on your hips. If you wake up on your back, don’t panic. Just roll to your side and settle back in.
Nutrition in the Third Trimester
Your baby is building bone, brain tissue, and blood supply at a rapid pace, so your nutritional needs are high right now. Iron is especially important because your blood volume has increased significantly and your baby is building iron stores for the first few months after birth. The World Health Organization recommends 30 to 60 mg of supplemental iron daily throughout pregnancy, with the higher end for women in areas where anemia is more common. If your prenatal vitamin contains iron, you’re likely covered, but it’s worth confirming the amount with your provider.
Calcium matters too, since your baby’s skeleton is hardening and will draw from your reserves if dietary intake falls short. Foods rich in calcium, like dairy, fortified plant milks, leafy greens, and canned fish with bones, help ensure both you and your baby get what you need. Staying well-hydrated also helps with many of the symptoms you’re managing now, from constipation to Braxton Hicks to swelling.