At 31 weeks pregnant you are a little over 7 months along, in your 8th month and the third trimester, with about 9 weeks to go until your estimated due date.
At 31 weeks pregnant your baby has done most of the big building work and is now gaining weight, while you juggle heartburn, a sore back and a busy bump at bedtime.
Below: development, size, symptoms, appointments, sleep and the preterm labor signs that mean you should call. Every clinical detail was read in October 2026 on the sources named.
Key Takeaways
31 weeks pregnant is a little over 7 months (your 8th month), or 29 weeks after conception (Mayo Clinic), with about 9 weeks to go.
Mayo Clinic says most major development is finished; the lungs may still be immature (Cleveland Clinic).
Mayo Clinic puts a baby at nearly 3 pounds at week 30 and about 3 3/4 pounds at week 32.
The CDC’s Tdap window is weeks 27 through 36; its RSV vaccine window opens at week 32 (September to January).
Labor before 37 weeks is preterm. Call right away for regular tightening, a change in discharge, pelvic pressure, leaking fluid or less movement.
Table of Contents
31 Weeks Pregnant at a Glance
Quick Answer
At 31 weeks pregnant you are a little over 7 months along, which puts you in your 8th month, with about 9 weeks to go. You are in the third trimester, and Mayo Clinic says your baby has finished most major development and now gains weight quickly.
31 weeks pregnant
What to know
Trimester
Third on every definition we checked (see table below).
Month
A little over 7 months, so you are in your 8th month, counting 4 weeks and a few days per month. Months are a convention, not a medical unit.
Weeks to go
About 9 (40 minus 31, counted from your last period).
Baby’s size
Between Mayo Clinic’s week 30 figure (nearly 3 pounds) and week 32 figure (about 3 3/4 pounds).
Common symptoms
Shortness of breath, heartburn, back and hip pain, Braxton Hicks contractions, trouble sleeping.
This week’s care focus
Tdap vaccine if not done yet (CDC), preterm labor signs, your baby’s movement patterns.
31 weeks pregnant: quick facts with sources.
How the weeks are counted. The Office on Women’s Health says pregnancy lasts about 40 weeks, counted from the first day of your last normal period (LMP). Conception happens about two weeks later (Cleveland Clinic), so Mayo Clinic describes week 31 as 29 weeks after conception. Mayo Clinic adds that an early ultrasound can help confirm the date, and if it differs from the period based date by more than seven days, the ultrasound date is used.
Which trimester is 31 weeks? Sources start the third trimester at different points, but week 31 is inside it on all of them:
Source
Third trimester
Where week 31 falls
ACOG, as reported by UPMC
Starts at the beginning of the 29th week (28 weeks, 0 days) and runs until delivery
Fourth week of the third trimester
MedlinePlus (NIH)
Week 28 through week 40
Third trimester
Office on Women’s Health
Week 29 to week 40
Third week of the third trimester
Trimester definitions differ by source, so all three are printed.
Your Baby’s Development at 31 Weeks
Mayo Clinic says that by week 31 your baby has finished most major development and now gains weight quickly, adding body fat that helps after birth (Cleveland Clinic).
Source and timing
What it says is happening
Cleveland Clinic, week 31
The fetus can handle more information and stimuli. You can probably notice more distinct patterns in when your baby is awake and when it is asleep.
Cleveland Clinic, week 30
The fetus can control its own body heat, and its brain is maturing and growing rapidly.
Mayo Clinic, week 30
The eyes can open wide. Red blood cells form in the bone marrow.
Office on Women’s Health, week 32
The bones are fully formed but still soft. The eyes can open, close and sense changes in light.
Fetal development around week 31, by source.
Is the baby fully developed at 31 weeks? Not completely. Cleveland Clinic says the brain develops most rapidly in month 8 (weeks 29 to 32) and the lungs may still be immature, and the Office on Women’s Health says the lungs are not fully formed at week 32.
Position. Cleveland Clinic says that around week 28 the fetus may begin turning head down, and MedlinePlus says your provider checks your baby’s position at visits. See our breech baby guide.
How Big Is Your Baby at 31 Weeks?
None of the primary sources we read gives a week 31 measurement, so here are the closest figures, printed as each source reports them.
Source
Timing
Length
Weight
Mayo Clinic
Week 30
More than 10 1/2 inches (270 mm), crown to rump
Nearly 3 pounds (1,300 g)
Mayo Clinic
Week 32
11 inches (280 mm), crown to rump
About 3 3/4 pounds (1,700 g)
Office on Women’s Health
Week 32
About 15 to 17 inches
About 4 to 4 1/2 pounds
Cleveland Clinic
Month 8 (weeks 29 to 32)
About 17 to 18 inches
As much as 5 pounds
Baby size near 31 weeks, by source.
Why the numbers differ. Mayo Clinic measures crown to rump, which leaves out the legs, so its lengths are shorter. The other two sources do not say how they measure, and Cleveland Clinic’s figure covers a whole month. Healthy babies vary. The Office on Women’s Health says that by week 32 your baby gains about one half pound a week.
A 31 week ultrasound? MedlinePlus says that apart from the group B strep test, there are no routine lab tests or ultrasounds for every pregnant woman in the third trimester. UPMC says a scan may be ordered for concerns about growth, amniotic fluid or the placenta.
31 Weeks Pregnant Symptoms
The Office on Women’s Health and UPMC list these third trimester changes. You may have many or few.
Shortness of breath: UPMC says to get help right away if it is sudden or severe, or comes with chest pain, a fast pulse or fever.
Heartburn: UPMC says smaller meals through the day can help.
Back and hip pain: from extra weight and stretching ligaments (UPMC).
Braxton Hicks contractions: practice contractions that are irregular and go away with rest or a change of position (UPMC). See what contractions feel like.
Needing to pee more often: from pressure on your bladder (UPMC).
Swelling: of the ankles, fingers and face (Office on Women’s Health). UPMC says sudden or extreme swelling needs checking, because it can signal preeclampsia.
Hemorrhoids: on both lists.
Leaking colostrum: a watery pre-milk (Office on Women’s Health, UPMC).
Braxton Hicks or preterm labor? March of Dimes defines preterm labor as labor before 37 weeks. Irregular tightenings that ease with rest fit Braxton Hicks (UPMC). Regular or frequent contractions that make your belly tighten like a fist, painful or not, are a preterm labor sign, and March of Dimes says to call your provider right away. If you are unsure, call.
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Your 31 weeks pregnant belly is still growing, and MedlinePlus says your provider measures your abdomen at each visit to check your baby’s growth.
Belly button: the Office on Women’s Health says it may stick out.
A high bump: UPMC says the baby drops lower into your pelvis around 36 weeks, which can ease breathing and heartburn.
Appetite: UPMC reports that ACOG suggests about 450 extra calories a day in the third trimester.
Your baby’s movements. Cleveland Clinic says kicks may feel more like pokes from around week 29 as space gets tight. UPMC says you will still feel movements and stretches. MedlinePlus says to expect active periods of rolling and squirming with a few strong kicks, alternating with quiet periods.
If movement seems to drop. MedlinePlus suggests a snack and lying down for a few minutes; if movement still seems low, call your provider. When in doubt, call. Ask your provider whether they want you to do kick counts.
Prenatal Care, Tests and Appointments Around Week 31
What
Window
Source
Prenatal visits
Every 2 weeks until week 36, then weekly
MedlinePlus
Tdap (whooping cough) vaccine
During the 27th through 36th week of each pregnancy
CDC
RSV vaccine
You can choose it during weeks 32 through 36, during September to January (opens next week)
CDC
Flu vaccine
By the end of October
CDC
Glucose testing
A three hour test may be needed if you were not screened earlier or if your screening was high
UPMC
Group B strep test
A few weeks before your due date (coming up)
MedlinePlus
Tests and vaccines near week 31, as read in October 2026.
What happens at your visits. MedlinePlus says your provider checks your weight, blood pressure, belly measurement and your baby’s position, and may discuss delivery timing and options.
Vaccines. As read in October 2026 (CDC page updated October 7, 2026): Tdap during weeks 27 through 36, with earlier in the window preferred. For RSV, either the vaccine during weeks 32 through 36 (September to January) or an RSV immunization for your baby in their first RSV season if they are 8 months or younger. Confirm every vaccine with your provider.
What to Do This Week: A Simple Checklist
Ask about Tdap if you have not had it, and note the RSV decision for next week.
Learn the preterm labor signs below and save your provider’s after hours number.
Choose a pediatrician. UPMC notes the baby’s first visit is usually 2 to 3 days after you come home.
Plan your newborn’s car seat installation. UPMC says free help is available from local programs.
Start packing your hospital bag (UPMC), including a going home outfit for your baby.
Write your birth plan if you have not already, and discuss it with your provider (UPMC).
If you have other children, arrange childcare for when you go into labor (UPMC).
Pace yourself. Sort baby clothes in short sessions and skip ladders and heavy lifting.
Sleep at 31 Weeks Pregnant
Trouble sleeping is on the Office on Women’s Health third trimester list: breathlessness, heartburn, bathroom trips and a big belly all compete with rest.
Sleep position. UPMC says that as your belly grows you should avoid sleeping on your back, because the uterus may compress a blood vessel, which can make you dizzy and reduce blood flow to your baby. If you roll onto your back, the discomfort usually wakes you, so just roll back to your side (UPMC).
Use a body pillow: UPMC suggests one for support. Our guide on how to use a pregnancy pillow shows setups for side sleeping.
Breathe slowly: UPMC suggests practicing deep breathing to help you fall asleep.
Get outside in the day: UPMC says getting outside and walking during the day can also help with sleep.
Ask before any sleep aid: MedlinePlus says to call your provider if you plan to start any new medicines, vitamins or herbs. Read can you take melatonin while pregnant before trying a supplement.
Your baby has a sleep schedule too. Cleveland Clinic says that around week 31 you can probably notice more distinct patterns in when your baby is awake and asleep.
For Partners: How to Help at 31 Weeks
Learn the preterm labor signs below, so you can make the call if she is unsure or hesitant.
Own one checklist item start to finish, like the car seat.
Do the lifting: laundry baskets, grocery bags and anything that strains a sore back.
Take over evening chores so she can go to bed early.
Talk or read to the bump. Cleveland Clinic says the fetus can hear most stimuli now.
Looking Ahead: What Comes Next
Week 32: toenails are visible and lanugo starts to fall off (Mayo Clinic), and the CDC’s RSV window opens. See 32 weeks pregnant.
Week 33: the bones are hardening while the skull stays soft (Mayo Clinic). See 33 weeks pregnant.
Week 36: visits become weekly (MedlinePlus), and the CDC’s Tdap and RSV windows close.
Week 37: labor is no longer preterm (March of Dimes).
Signs of preterm labor. March of Dimes says that before 37 weeks you may be in preterm labor if you have any of these signs, and to call your provider right away if you have even one:
Regular or frequent contractions that make your belly tighten like a fist. They may or may not be painful.
A change in your vaginal discharge (watery, mucus or bloody), or more discharge than usual.
Pressure in your pelvis or lower belly, like your baby is pushing down.
A constant low, dull backache.
Belly cramps, with or without diarrhea.
Your water breaks.
Seek care right away. UPMC and MedlinePlus list these warning signs for the third trimester:
A decrease in your baby’s movement.
Any vaginal bleeding, or discharge with an odor.
A severe headache that does not go away, or vision changes or blind spots.
Sudden or severe swelling of your hands or face.
Chest pain, a fast heartbeat, or trouble breathing.
A fever above 100.4 F, or pain when you pass urine.
One leg that is swollen, red and warm.
Thoughts of harming yourself.
UPMC notes that sudden or extreme swelling can signal preeclampsia. If you cannot reach your provider quickly, go to labor and delivery or the emergency room. For heavy bleeding, severe chest pain or trouble breathing, call 911, and say you are 31 weeks pregnant. This article is general education, not medical advice.
Frequently Asked Questions
How many months is 31 weeks pregnant?
A little over 7 months, which puts you in your 8th month, counting 4 weeks and a few days per month. Cleveland Clinic, which uses 4 week months, also places week 31 in month 8. Months are only a convention, so providers count in weeks.
How big is a baby at 31 weeks?
Our sources give no week 31 figure. Mayo Clinic says nearly 3 pounds and more than 10 1/2 inches crown to rump at week 30, and about 3 3/4 pounds and 11 inches crown to rump at week 32. Healthy babies vary.
Is a baby fully developed at 31 weeks?
Not completely. Mayo Clinic says most major development is finished by week 31 and the baby now gains weight quickly, but the Office on Women’s Health says the lungs are not fully formed at week 32, and Cleveland Clinic says they may still be immature.
What symptoms should you not ignore at 31 weeks pregnant?
March of Dimes lists regular or frequent tightening, a change in discharge, pelvic pressure, a constant low backache, cramps and your water breaking as preterm labor signs. Also seek care for less movement, bleeding, a severe headache, vision changes, sudden swelling or chest pain.
Can you give birth at 31 weeks pregnant?
Labor can start early. March of Dimes defines labor before 37 weeks as preterm, says to call your provider right away with even one sign, and says your provider may offer treatment to stop labor or improve your baby’s health before birth.
What position is the baby in at 31 weeks?
It varies. Cleveland Clinic says that around week 28 the fetus may begin turning head down as it gets ready for birth. MedlinePlus says your provider checks your baby’s position at third trimester visits, so ask at your next appointment.
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