At 31 weeks pregnant, you’re deep into the third trimester with roughly nine weeks to go. Your baby is about 16 inches long and weighs around 3.3 pounds, roughly the size of a coconut. This is a phase of rapid weight gain for the baby and increasing physical demands on your body, so here’s what’s actually happening and what you’ll likely feel.
How Your Baby Is Developing
Your baby is putting on fat quickly now, filling out beneath skin that’s becoming less translucent by the week. That fat layer serves two purposes: it helps regulate body temperature after birth and gives your baby the rounder, smoother appearance of a newborn. The brain is also in a period of intense growth, forming billions of neural connections that support the senses, movement, and eventually learning.
By 31 weeks, your baby’s eyes can detect light filtering through your body. While the womb is dim, it’s not pitch black. Researchers at the University of Lancaster describe the light level inside the uterus as similar to being in a room with the lights off and the curtains drawn. Fetuses at this stage react to patterns of light shone through the abdomen, which means your baby is already processing visual information. Hearing is also well developed. Your baby recognizes your voice and may startle at loud sounds.
Sleep-wake cycles are more defined now, and you’ll probably notice predictable windows when your baby is active versus quiet. Those kicks, rolls, and jabs can feel strong because there’s less room to move around in.
Common Symptoms at 31 Weeks
Shortness of breath is one of the most noticeable changes around this time. Your expanding uterus pushes upward on the diaphragm, compressing your lungs and reducing the space they have to expand. On top of that, progesterone speeds up your breathing rate, and your heart is pumping a significantly larger volume of blood to supply the placenta. The result is that climbing stairs or even talking for a while can leave you winded. This is normal, though sudden or severe breathing difficulty is not.
Braxton Hicks contractions often become more frequent around 31 weeks. These are irregular tightenings of your uterus that come and go without a pattern. They don’t get progressively stronger, they don’t get closer together, and they typically ease up if you change positions or take a short walk. The key distinction from real labor contractions: Braxton Hicks never follow a rhythm. If you experience strong contractions less than five minutes apart for an hour, or contractions so intense you can’t walk or talk through them, that’s a reason to call your provider.
Other symptoms you might be dealing with:
- Heartburn and indigestion, as your uterus crowds your stomach
- Frequent urination, from pressure on your bladder
- Swelling in your feet and ankles, especially by the end of the day
- Lower back pain, as your center of gravity shifts forward
- Trouble sleeping, from a combination of discomfort, bathroom trips, and difficulty finding a good position
Pelvic Pain and What Causes It
Some women develop pain at the front of the pelvis that can range from a mild ache to sharp, shooting sensations. This is often caused by the joint at the front of the pelvis loosening more than it should, a condition sometimes called symphysis pubis dysfunction. Hormones that soften your ligaments in preparation for birth can make this joint feel unstable.
The pain may radiate into your lower back, groin, or inner thighs. Everyday movements like walking, climbing stairs, getting out of a car, or even rolling over in bed can make it worse. If pelvic pain is interfering with your daily life, let your provider know. Physical therapy, support belts, and specific movement modifications can help manage it through the remaining weeks.
Sleep Positioning in the Third Trimester
Sleeping on your side is the recommended position from here on out. Lying flat on your back puts the full weight of your uterus on the inferior vena cava, the large vein that returns blood from your lower body to your heart. That pressure can reduce blood flow to both you and your baby. Sleeping on your left side is considered ideal because it maximizes blood flow to the placenta, though either side is fine.
A pillow between your knees and another tucked behind your back can help you stay comfortable and keep you from rolling onto your back during the night. If you wake up on your back, don’t panic. Just shift to your side and settle back in.
What Happens at Your Prenatal Visits
During the third trimester, prenatal visits typically happen every two to four weeks. At a 31-week appointment, your provider will check your blood pressure and weight, listen to your baby’s heartbeat, and ask about symptoms like contractions, fluid leaks, or bleeding. You may also have your belly measured to track your baby’s growth.
This is a good time to ask about kick counts if your provider hasn’t brought them up yet. Tracking your baby’s movements is a simple way to monitor well-being at home. Methods vary, but the general idea is to pick a time when your baby is usually active, sit or lie down, and note how long it takes to feel a certain number of movements. Your provider will give you specific instructions for what to track and when to call.
Nutrition and Energy Needs
Your body needs about 300 extra calories a day during pregnancy, which is less than you might expect. That’s roughly an extra snack or a slightly larger meal, not eating for two in any literal sense. What matters more than quantity is quality. Your baby’s brain development depends on adequate intake of omega-3 fatty acids, iron, and choline. Iron needs are especially high in the third trimester because your blood volume has expanded so much.
If you’re feeling fatigued beyond what seems reasonable, it’s worth mentioning at your next visit. Iron-deficiency anemia is common in the third trimester and easy to identify with a blood test. Small, frequent meals can help with both energy levels and the heartburn that comes from a compressed stomach.
Preparing for the Final Stretch
At 31 weeks, you still have a comfortable window to handle practical preparations. If you haven’t installed a car seat, packed a hospital bag, or discussed your birth preferences with your provider, now is a good time. Babies born at 31 weeks, while premature, have high survival rates with medical support, so having basics in place is a reasonable precaution rather than a worry.
This is also when many women start feeling the emotional weight of what’s coming. The combination of physical discomfort, broken sleep, and the reality of a major life change can feel overwhelming. That’s not a sign of weakness or unreadiness. It’s a normal part of the transition, and talking about it with your partner, friends, or provider can make a real difference.