Free family guide · Pregnancy week by week · Third trimester

31 weeks pregnant

Week 31 is in the third trimester in both the US Office on Women's Health (OWH) system and the UK's NHS week-by-week guide.[1][2]

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Sources on this page: OWH; NHS; ACOG; NICHD; Safe to Sleep; CDC; NIMH; HRSA; MedlinePlus

This page shares what health organizations say. It is not medical advice. It is general information. Talk to your doctor, midwife or pediatrician about your own situation.

In an emergency, call 911.

At a glance

  • The NHS says the baby is around 41.1 cm long.[3]
  • The NHS says the baby recognizes voices from outside the womb.[3]
  • The NHS says the baby's sleep space can be a crib, carry cot or Moses basket, with a new mattress that fits snugly and no pillows or duvets.[3]

Which trimester is week 31?

OWH counts the trimesters as weeks 1 to 12, 13 to 28 and 29 to 40.[1] The NHS guide counts them as weeks 4 to 12, 13 to 27 and 28 to 41.[2] ACOG counts the first trimester to 13 weeks 6 days, the second from 14 weeks to 27 weeks 6 days, and the third from 28 weeks to 40 weeks 6 days.[4]

All three place week 31 in the third trimester.[1][2][4]

Your body this week

The NHS says your midwife may measure your bump and feel the baby's position.[3] It says a baby lying bottom first, called breech, may affect the type of birth.[3]

Your baby this week

The NHS says the baby is around 41.1 cm long.[3]

The NHS says the baby recognizes voices from outside the womb.[3] It says a birth in 6 weeks would not be considered early.[3]

Visits and tests around week 31

OWH lists a nonstress test after week 28, when a provider orders one.[5]

NICHD describes visits every 2 weeks from 28 to 36 weeks.[6]

Things to do this week

  • The NHS says the baby's sleep space can be a crib, carry cot or Moses basket, with a new mattress that fits snugly and no pillows or duvets.[3]
  • Safe to Sleep says to use a firm, flat, level surface with only a fitted sheet.[7]
  • The NHS says the safest position to go to sleep is on your side, left or right.[8]
  • CDC says to avoid lying flat on your back during exercise after the first trimester.[9]
  • The NHS suggests packing your hospital bag at least 3 weeks before your due date.[10]

Feelings check-in

The National Institute of Mental Health (NIMH) says perinatal depression includes depression during pregnancy, and its signs last at least 2 weeks.[11]

HRSA says the National Maternal Mental Health Hotline, 1-833-852-6262, takes calls and texts 24/7 in English and Spanish.[12]

When to call your provider or get help

MedlinePlus says to call your provider in the third trimester for:[13]

  • bleeding, or discharge that smells bad;[13]
  • fever, chills, or pain when you urinate;[13]
  • headaches or changes in vision;[13]
  • the water breaking, or regular painful contractions;[13]
  • the baby moving less;[13]
  • a lot of swelling, chest pain or trouble breathing.[13]

The NHS says you should still feel the baby move at the same rate until you give birth, and to contact your midwife or hospital as soon as possible if the pattern changes or the baby stops moving.[14]

The NHS says Braxton Hicks tightenings last 20 to 30 seconds and should not hurt.[3] It says to contact your midwife or doctor if they are painful or come at regular intervals, because this could be early labor.[3]

Questions families ask

Which trimester is week 31?

OWH places week 31 in the third trimester.[1] The NHS guide places it in the third trimester.[2] ACOG places it in the third trimester.[4]

How big is the baby at 31 weeks?

The NHS says the baby is around 41.1 cm long.[3]

What are Braxton Hicks contractions?

The NHS says they are tightenings that last 20 to 30 seconds and should not hurt.[3] MedlinePlus says they are irregular and do not change the cervix.[15]

Free from UpOnly Learning. Written for families of every faith. No outside ads and no affiliate links. Every fact on this page comes from the sources listed below. How we source and review these pages

Updated 2026-10-24: first published.

Sources

All sources were opened and checked on the date shown.

  1. Office on Women's Health (HHS). Stages of pregnancy. Checked October 24, 2026.
  2. NHS (United Kingdom). Week-by-week guide to pregnancy. Checked October 24, 2026.
  3. NHS (United Kingdom). 31 weeks pregnant guide. Checked October 24, 2026.
  4. American College of Obstetricians and Gynecologists. How Your Fetus Grows During Pregnancy. Checked October 24, 2026.
  5. Office on Women's Health (HHS). Prenatal care and tests. Checked October 24, 2026.
  6. Eunice Kennedy Shriver National Institute of Child Health and Human Development (NIH). What happens during prenatal visits?. Checked October 24, 2026.
  7. NICHD Safe to Sleep campaign (NIH). Ways to Reduce Baby's Risk. Checked October 24, 2026.
  8. NHS (United Kingdom). Tiredness and sleep problems in pregnancy. Checked October 24, 2026.
  9. Centers for Disease Control and Prevention. Pregnant & Postpartum Activity: An Overview. Checked October 24, 2026.
  10. NHS (United Kingdom). Hospital bag checklist – what to pack for you and your baby. Checked October 24, 2026.
  11. National Institute of Mental Health (NIH). Perinatal Depression. Checked October 24, 2026.
  12. Health Resources and Services Administration (HRSA). National Maternal Mental Health Hotline. Checked October 24, 2026.
  13. MedlinePlus (National Library of Medicine). Prenatal care in your third trimester. Checked October 24, 2026.
  14. NHS (United Kingdom). 32 weeks pregnant guide. Checked October 24, 2026.
  15. MedlinePlus (National Library of Medicine). Am I in labor?. Checked October 24, 2026.