Mombymomby
Third trimester · Week 31

Week 31 of pregnancy

Updated 25 Oct 2026

Your baby this week

About the size of a pineapple

About 41.8 cm long
About 1,568 g

Source: Vinmec – Bảng cân nặng và chiều dài thai nhi theo tiêu chuẩn của WHO

Your baby's development

Your baby weighs about 1.5 kg and is about 41 cm long, the size of a coconut. The fine body hair (lanugo) is gradually shed and replaced by hair. Your baby's eyes are gradually sensing light, though not clearly yet.

The connections between your baby's brain cells are developing very quickly, forming billions of nerve cells (neurons). Your baby can now receive and process information from all 5 senses, and the first smell your baby can sense is your smell – baby's favourite smell later on.

Your baby is now a big fan of sleep, sometimes sleeping for several hours. Your baby is now breathing, hiccuping, kicking and even sucking their fingers. Interestingly, some babies are born with a slightly callused thumb because they sucked it so hard and so often while still in the womb.

Your body

At week 31 varicose veins and leg swelling may get worse. This is caused by rising pregnancy hormones and the uterus pressing on the vena cava, putting pressure on the peripheral veins. You may also feel short of breath because the growing uterus pushes your organs upwards and presses on your diaphragm. In addition, as your bump grows your body gets heavier and you may have back pain. Sometimes there are occasional headaches and mild sleep problems.

You will need to wee more often. Your memory may get clumsier too; this is because brain cell volume shrinks slightly in the third trimester, but it returns to normal a few months after birth. Don't worry. It's actually rather endearing anyway – like a superhero saving the world who forgot to brush her teeth in the morning.

Advice for you

Lying on your left side while resting eases breathlessness. Practising deep breathing can also help you sleep better, so practise deep breathing regularly with Doti. If your legs swell a lot, you need to see a doctor because this is one of the symptoms of pre-eclampsia.

Doing yoga and drinking plenty of water bring many benefits for both you and your baby. You can still talk to your baby during this time as a way to feel better and forget about those symptoms. Or think about what colour your baby's first outfit will be.

Watch the video for this article

Signs that need a doctor now

If you notice any of the signs below, please go to the nearest health facility to be checked by a doctor.

Sources

Content advised by our doctors.

  1. Vinmec (accessed 22 Oct 2026)
  2. ACOG (accessed 22 Oct 2026)
  3. NHS (accessed 22 Oct 2026)
  4. Source for fetal size: Vinmec – Bảng cân nặng và chiều dài thai nhi theo tiêu chuẩn của WHO
  5. Source for the signs that need a doctor: Bệnh viện Đa khoa Tâm Anh – Các dấu hiệu nguy hiểm trong thai kỳ

This article helps you understand and prepare. It does not replace a check-up. For an answer about your own situation, please ask the doctor who is caring for you.