Pregnancy Week 31: Baby's Weight Accelerates

Week 31 is a growth week. Not much dramatic new development from here — the main story is that baby is packing on weight, fat and muscle, and things inside are getting cozier. Your body is starting to tell you clearly that this is the third trimester. And while it can feel like a long stretch, you're actually much closer to meeting your baby than you might realize.

This guide walks through what's happening at 31 weeks pregnant, the symptoms this stretch is famous for, safe nutrition and movement, kick counts, birth prep, and the signs that mean it's time to call your clinician.

This article is educational, not medical advice. Pregnancy is deeply individual, and your obstetrician or midwife is the person who knows your history. We're the team behind Vyve, the private AI pregnancy and cycle tracker, and Vyve is a tracking tool — not a diagnostic one.

What's happening this week

You're 9 weeks from full term (37 weeks) and about 9 weeks from your due date. Baby's brain is developing rapidly, weight is climbing fast, and the physical realities of the third trimester are settling in for most people. Appointments are still typically every two weeks at this point, moving to weekly around week 36.

If you'd like to see what's ahead, take a look at week 32 and week 33, when many babies settle firmly head-down.

Baby's size and development at week 31

At 31 weeks pregnant, your baby is about the size of a coconut — roughly 41 centimeters (16.2 inches) from head to heel, weighing around 1.5 kilograms (3.3 pounds). Weight gain is now about 200 grams per week.

Here's what's happening this week:

• Rapid brain development. More neural connections forming, more surface folds appearing.
• Fat continues accumulating. Skin is less transparent and more baby-like.
• Lungs are practicing. Baby inhales and exhales amniotic fluid; surfactant production is increasing to prepare for breathing air.
• All five senses functioning. Baby responds to light, sound, taste, touch, and can smell (via amniotic fluid).
• Sleep cycles are established. You may notice quiet stretches and active stretches at fairly consistent times.
• Baby can turn head from side to side.
• Bones are fully formed but still soft, especially the skull.
• Baby may be head-down in many cases, though plenty of babies haven't settled yet.

Your symptoms and body changes

Common experiences:

• Shortness of breath. Uterus is high, diaphragm has less room.
• Heartburn. Now often severe. Small frequent meals, upright after eating, and — with your clinician's ok — a pregnancy-safe antacid may help.
• Backache. Baby is heavier, ligaments looser, posture shifted.
• Braxton Hicks. More noticeable now. Still should be irregular and painless.
• Leg cramps. Often nighttime. Stretch, hydrate, and consider magnesium if your clinician approves.
• Swelling. Feet, ankles, hands. Some is normal; sudden or severe is not.
• Difficulty sleeping. Bump size + bathroom trips + racing mind. Pillow between knees, side sleeping, wind-down routines all help.
• Colostrum. Some leaking is normal.
• Nesting instincts. Sudden urge to organize, clean, prep. Enjoy it, but pace yourself.
• Emotional shifts. Excitement, anxiety about birth, feeling touched-out even before birth — all normal.

Nutrition this week

You need about 450 extra calories a day in the third trimester. Continue your prenatal vitamin. Priorities: iron (anemia risk peaks now), calcium (bones), protein (rapid growth), DHA omega-3s (brain), fiber and water (constipation, hemorrhoids), and small frequent meals to manage heartburn. Continue avoiding raw or undercooked meat and eggs, high-mercury fish, unpasteurized dairy and juice, and alcohol.

Yoga & safe exercise

Keep moving but expect to modify. Walking, swimming, prenatal yoga, and gentle strength training remain solid. Skip anything flat on your back, avoid overheating, take breaks. Pelvic floor and hip-opening work pays off in labor. If something hurts or feels wrong, stop.

What to ask your doctor

• Is baby's position what we'd expect at 31 weeks?
• How is my blood pressure and swelling — any concerns?
• Am I on track for iron and hemoglobin?
• When will we discuss the birth plan formally?
• What are your criteria for when I should come in?
• Is my medication list still appropriate?
• Any adjustments to exercise or activity?
• When should I do the Tdap vaccine if I haven't already (usually 27–36 weeks)?

Birth prep to start now

Weeks 30–34 are the sweet spot for birth prep. Things worth working on:

• Childbirth education. If you haven't finished a class, aim to.
• Birth plan. Not a script — a set of preferences to discuss with your team. Includes: pain relief preferences, positions, who's present, cord clamping, immediate skin-to-skin, feeding plan.
• Hospital bag list. Start listing; full pack comes at week 34–36.
• Car seat. Buy it and learn to install it before week 36. Most hospitals require it before discharge.
• Feeding plan. Breastfeeding, pumping, formula, or a mix — decide roughly what you want to try, and know it's all okay.
• Postpartum support. Who's helping in the first weeks? What about food, older children, pets?

Common concerns at 31 weeks

3 tips for this week

1. Elevate your legs daily. 15 minutes with legs above your heart level reduces swelling and gives your circulation a break.
2. Keep counting kicks. Same time each day, same routine. Log them somewhere — Vyve makes this effortless.
3. Sleep smart. Left side, pillow between knees, another pillow under bump, one behind your back. A pregnancy pillow is genuinely worth it now.

How Vyve helps you track your pregnancy

Vyve is a privacy-first AI pregnancy and cycle tracker built for people who don't want their body's most personal data sold to advertisers. At 31 weeks, Vyve helps you log kick counts, note new third-trimester symptoms, and prepare doctor-ready summaries.

Vyve runs its AI on your device, encrypts your data, and doesn't require an account. Your pregnancy is yours.

Frequently asked questions

How big is baby at 31 weeks pregnant?

At 31 weeks, baby is about the size of a coconut — roughly 41 centimeters (16.2 inches) from head to heel, and weighing around 1.5 kilograms (3.3 pounds). Baby is now gaining about 200 grams (7 ounces) per week.

Why am I so short of breath at 31 weeks?

The uterus has grown high enough to press against the diaphragm, giving your lungs less room to expand. It gets better in the last few weeks when baby drops lower into the pelvis. Good posture, sleeping propped up, and pausing when climbing stairs all help. Sudden severe shortness of breath, chest pain, or with a cough — call now.

Is heartburn worse in the third trimester?

For many people, yes. Progesterone relaxes the valve at the top of the stomach and the growing uterus pushes upward, making reflux more likely. Small frequent meals, avoiding lying down right after eating, staying upright for an hour or so after meals, and avoiding trigger foods can help. Talk to your clinician before taking any medication.

How much weight does baby gain per week at 31 weeks?

In the last stretch of pregnancy, baby typically gains around 200 grams (7 ounces) per week, though it can be more. The last two months account for roughly half of baby's total birth weight — which is part of why nutrition matters in the final trimester.

Is baby head-down at 31 weeks?

Many babies have settled into a head-down (cephalic) position by 31 weeks, but plenty haven't yet and it's still normal to shift positions. Most babies who aren't head-down yet turn on their own by 36 weeks. If baby stays breech or transverse later, your clinician will discuss options. Try not to worry about position at 31 weeks.

This article is educational and not medical advice. Always consult your obstetrician or midwife for personal medical guidance.