Free family guide · Pregnancy week by week · Third trimester

40 weeks pregnant

Week 40 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; MedlinePlus; CDC; NIMH; HRSA

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 51.2 cm long.[3]
  • ACOG says that in weeks 37 to 40 the baby's circulation and musculoskeletal systems are complete, and fat is added for warmth.[4]
  • The NHS says there are no proven ways to safely bring on labor at home.[3]

Which trimester is week 40?

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 40 in the third trimester.[1][2][4]

Your body this week

The NHS says real contractions hurt as your belly tightens and ease as the muscles relax.[3] It says the first stage of labor lasts 6 to 12 hours, and less if you have had a baby before.[3] Its tips for early labor include sleeping if it starts at night, staying upright and active by day, a warm bath, breathing and light snacks.[3]

Your baby this week

The NHS says the baby is around 51.2 cm long.[3]

ACOG says that in weeks 37 to 40 the baby's circulation and musculoskeletal systems are complete, and fat is added for warmth.[4]

Visits and tests around week 40

The NHS says there is an antenatal appointment this week in a first pregnancy.[3]

NICHD describes visits every week from 36 weeks until birth.[5]

Things to do this week

  • The NHS says there are no proven ways to safely bring on labor at home.[3]
  • MedlinePlus says induction may be used after the due date or if there is a problem.[6]
  • The NHS says the safest position to go to sleep is on your side, left or right.[7]
  • CDC says to avoid lying flat on your back during exercise after the first trimester.[8]

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.[9]

HRSA says the National Maternal Mental Health Hotline, 1-833-852-6262, takes calls and texts 24/7 in English and Spanish.[10]

When to call your provider or get help

MedlinePlus says to call your provider in the third trimester for:[11]

  • bleeding, or discharge that smells bad;[11]
  • fever, chills, or pain when you urinate;[11]
  • headaches or changes in vision;[11]
  • the water breaking, or regular painful contractions;[11]
  • the baby moving less;[11]
  • a lot of swelling, chest pain or trouble breathing.[11]

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.[12]

The NHS says movements should not slow down or stop, and to contact your midwife or maternity unit straight away if they do.[3]

Questions families ask

Which trimester is week 40?

OWH places week 40 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 40 weeks?

The NHS says the baby is around 51.2 cm long.[3]

Can I bring on labor at home?

The NHS says there are no proven ways to safely bring on labor at home.[3]

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). 40 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. Eunice Kennedy Shriver National Institute of Child Health and Human Development (NIH). What happens during prenatal visits?. Checked October 24, 2026.
  6. MedlinePlus (National Library of Medicine). Childbirth. Checked October 24, 2026.
  7. NHS (United Kingdom). Tiredness and sleep problems in pregnancy. Checked October 24, 2026.
  8. Centers for Disease Control and Prevention. Pregnant & Postpartum Activity: An Overview. Checked October 24, 2026.
  9. National Institute of Mental Health (NIH). Perinatal Depression. Checked October 24, 2026.
  10. Health Resources and Services Administration (HRSA). National Maternal Mental Health Hotline. Checked October 24, 2026.
  11. MedlinePlus (National Library of Medicine). Prenatal care in your third trimester. Checked October 24, 2026.
  12. NHS (United Kingdom). 32 weeks pregnant guide. Checked October 24, 2026.