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Third trimester

31 weeks pregnant

At 31 weeks your baby is growing quickly and adding fat, and makes rhythmic breathing movements even though the lungs are not fully mature. Braxton Hicks tightenings are common. Knowing the signs of preterm labor, and when to call, is useful from here on.

This page covers 31 weeks 0 days to 31 weeks 6 days (31+0 to 31+6), counted from the first day of the last period. Check how far along you are

Enter it to see the calendar dates of your week 31.

Botanical illustration of a napa cabbage

About the size of a napa cabbage

Typical estimated weight 1.75 kg (3 lb 14 oz) at 31+0, about 1.92 kg (4 lb 4 oz) by 31+6. This is the 50th centile; healthy babies commonly vary by about 12.7% either way.1

A reference value, not a measurement of your baby.

How your baby is developing

  • This is a stretch of fast growth and substantial fat gain.2
  • Your baby makes rhythmic breathing movements, but the lungs are not yet fully mature.2
  • The bones are fully formed but still soft, and the body is starting to store iron, calcium and phosphorus.2
  • Your baby is very active, moving, turning and sucking their fingers. Sucking is a skill they are practicing now.3
  • Sleep and wake cycles are becoming easier to notice, and your baby is starting to recognize voices from outside the womb.3,4
  • Some babies are already lying head down. A baby lying another way still has time to turn.3

Changes you may notice

Everyone’s experience is different. You may notice some of these, or none.

  • You may notice Braxton Hicks contractions, when your bump tightens for around 20 to 30 seconds and then relaxes. They are normal and should not be painful.3
  • If your baby has moved head down, you may notice your bump has dropped. Some babies do not settle into the pelvis until labor starts.3
  • Sleep problems, heartburn, backache, hemorrhoids, leg cramps, dizziness and swollen hands and feet are all common in the third trimester. Get medical advice about any vaginal bleeding.3
  • Your baby's movements should not slow down as you get closer to birth. If they slow down, stop or change, contact your provider or maternity unit straight away, even at night.5

Care around this week

  • Labor that starts before 37 completed weeks is preterm labor. Signs can include contractions every 10 minutes or more often, leaking fluid or bleeding from the vagina, pressure in the pelvis, a low dull backache, period-like cramps, or abdominal cramps with or without diarrhea. If you think it might be happening, or Braxton Hicks become painful or regular, contact your provider or maternity unit immediately, even in the middle of the night.3,6,7
  • US: prenatal visits often become more frequent in the third trimester, moving to every two weeks and later to weekly. UK: if this is your first baby, you will usually be offered an appointment at 31 weeks to check your blood pressure, test your urine and measure your bump, and to talk about preparing for birth, your birth preferences, recognizing labor, your baby's movements and your mental health.4,8
  • US: Tdap is recommended during weeks 27 through 36, preferably early in that window, and the maternal RSV vaccine is given at 32 to 36 weeks during September through January in most of the country. UK: the whooping cough vaccine is ideally given before 32 weeks, and the RSV vaccine is offered from 28 weeks and can be given up until labor.9,10,11

Questions to ask your provider

  • Which signs of preterm labor should make me call you, and what number do I use after hours?
  • How can I tell Braxton Hicks from real contractions?
  • Do you know which way my baby is lying yet?
  • When would I be due for the RSV vaccine, or will my baby be offered an antibody after birth?
When to contact your provider straight away

Call 911 (US) or 999 (UK) in an emergency, such as heavy bleeding or bleeding with severe pain, a seizure, chest pain, or severe trouble breathing.13,14,15,16

These symptoms are not always serious, but each one needs prompt advice from your provider, midwife or maternity unit (or NHS 111 in the UK if you cannot reach them) rather than waiting for your next appointment, and it helps to tell them you are pregnant.

  • Vaginal bleeding or spotting at any stage of pregnancy. Call 911 or 999 if the bleeding is heavy or comes with severe belly pain, shoulder pain, or feeling faint or dizzy.15,17
  • Fluid leaking or gushing from your vagina, or discharge that is unusual for you. Get checked straight away if the fluid is greenish, colored or smells bad.7,15,16,18
  • Your baby moving less than usual, not moving, or a change in their usual pattern of movements. Call immediately, even in the middle of the night, and do not use a home Doppler to check yourself.5,15
  • A severe headache that does not go away with simple painkillers or gets worse, or sudden changes in your vision such as blurring or seeing flashing lights.13,15,16
  • Sudden swelling of your face, hands or feet, or swelling, redness or pain in a leg.13,15,16
  • Belly pain that is severe or does not go away after resting, or pain just below the ribs.13,15,19
  • Regular tightenings or contractions, period-type pains, or backache that is unusual for you before 37 weeks, which can be signs of preterm labor.18,19
  • A fever of 100.4°F (38°C) or higher, or feeling hot, cold or shivery.15,16,18
  • Shortness of breath that comes on suddenly or happens when you lie flat, chest pain, a racing or pounding heart, or dizziness or fainting.15,16
  • Very itchy skin, especially on the palms of your hands or soles of your feet, or itching that is worse at night.20
  • Being sick so often that you cannot keep food or drink down, or signs of dehydration such as dark urine, peeing very little, or feeling dizzy.15,21
  • Thoughts of harming yourself or your baby. Get help right away, and call 911 or 999 if you think you might act on them.15,22

Questions about 31 weeks

How do I tell Braxton Hicks from preterm labor at 31 weeks?

Braxton Hicks tighten the bump for around 20 to 30 seconds and should not hurt. If tightenings become painful or regular, or come every 10 minutes or more often, contact your provider or maternity unit straight away.3,6

Should my baby be head down at 31 weeks?

Not necessarily. Some babies are already head down, but a baby lying another way still has time to turn.3

Is a baby born at 31 weeks preterm?

Yes. Labor that starts before 37 completed weeks is preterm labor, and ACOG's term categories begin at 37 weeks 0 days.6,12

Sources

  1. In utero analysis of fetal growth: a sonographic weight standardRadiology 181(1):129–133, 1991
  2. Fetal developmentUS National Library of Medicine
  3. Week 31 of pregnancyNHS Best Start in Life
  4. Fetal development: stages of growthCleveland Clinic
  5. Your baby's movementsNHS (England)
  6. Preterm laborUS National Library of Medicine
  7. Signs of going into labourNHS Best Start in Life
  8. Your antenatal appointmentsNHS (England)
  9. Tdap Vaccination for Pregnant WomenUS Centers for Disease Control and Prevention
  10. Protecting Infants from RSVUS Centers for Disease Control and Prevention
  11. Vaccinations in pregnancyNHS (England)
  12. Committee Opinion No. 579: Definition of Term PregnancyAmerican College of Obstetricians and Gynecologists, 2013
  13. Pre-eclampsiaNHS (England)
  14. Epilepsy and pregnancyNHS (England)
  15. Urgent Maternal Warning Signs and Symptoms (Hear Her campaign)US Centers for Disease Control and Prevention
  16. Pregnancy symptoms you need to get help forNHS (England)
  17. Vaginal bleeding in pregnancyNHS (England)
  18. Premature labour and birthNHS (England)
  19. Stomach pain in pregnancyNHS (England)
  20. Itching in pregnancyNHS (England)
  21. Severe vomiting in pregnancyNHS (England)
  22. Depression in pregnancyNHS (England)

Links checked November 12, 2026. Citing a source does not mean it endorses this site.

Written from the sources above and checked against them on 2026-11-12. Not reviewed by a clinician. Our editorial policy