Third Trimester: When It Starts, Symptoms and Signs of Labour

When does the third trimester start?

At 28 weeks, according to the NHS week-by-week guide, the American College of Obstetricians and Gynecologists (ACOG) in the US and Ireland's Health Service Executive (HSE). It lasts until your baby is born. Australia's Pregnancy, Birth and Baby is the exception, starting it at week 27.

The NHS guide opens week 28 with "Welcome to the 3rd and final trimester!" In months, it is roughly months 7 to 9. Why the sources differ by a week is covered in pregnancy trimesters by week.

What happens to your baby in the third trimester?

ACOG describes the third trimester as "the time when the fetus's weight increases and the organs mature so they will be ready to function after birth." Its guide to fetal growth lists:

• Weeks 29 to 32: your baby can stretch, kick and make grasping movements, and the eyes can sense changes in light.
• Weeks 33 to 36: bones harden but the skull stays soft and flexible, limbs begin to look chubby, and your baby may turn head-down for birth.
• Weeks 37 to 40: the circulatory and musculoskeletal systems finish developing, the lungs, brain and nervous system are finishing theirs, and fat continues to be added to keep your baby warm after birth.

Pregnancy, Birth and Baby says that by week 31 your baby's lungs are more developed but they would probably need help to breathe if born then, and that around 1 in 25 babies are in the breech (bottom-first) position when labour begins.

Common third trimester symptoms

The NHS says you will probably start to feel more uncomfortable and tired over these weeks. Its week-by-week guide lists heartburn and indigestion, backache as your back takes the extra weight, looser joints and ligaments, puffy ankles, feet and face, particularly when it is hot (which the NHS says to get checked in case it is pre-eclampsia), nosebleeds, piles, leg cramps and sleeping problems.

• Braxton Hicks contractions. The HSE says these practice tightenings are usually irregular, do not happen as often as labour contractions, usually last less than a minute and do not build in intensity. See Braxton Hicks vs real contractions.
• Breathlessness. Pregnancy, Birth and Baby says your growing baby puts more pressure on your lungs and other organs.
• Your baby dropping. In a first pregnancy, Pregnancy, Birth and Baby says you may notice your baby move lower into your pelvis around 36 weeks, which eases your breathing but means more trips to the toilet.
• Stretch marks. The HSE says almost 8 out of 10 women get them.
• Tiredness and mood. Pregnancy, Birth and Baby says 1 in 5 people will experience antenatal anxiety or depression. The HSE says to tell your midwife or doctor if anxiety is severe or prolonged.

The NHS says you may need an extra 200 calories a day in the third trimester, about two slices of wholemeal toast with margarine.

Your baby's movements in the third trimester

Movements should continue right up to and during labour, says the NHS. The HSE puts it directly: "It is not true that babies move less towards the end of pregnancy."

The NHS says there is no set number of movements to feel each day and you do not need to count kicks; what matters is knowing your own baby's usual pattern. If movements slow down, stop or change, call your midwife or maternity unit straight away. Reduced fetal movements explains what happens next.

Appointments, tests and vaccines in the third trimester

Appointments come closer together. In England, at 28 weeks you are offered a blood test for iron levels and blood group, and an injection to protect your baby if they are at risk of rhesus disease. At 36 weeks the midwife feels your bump to check your baby's position.

If you are offered a growth scan, our page explains what it measures and why.

Signs of labour, and when full term starts

The NHS lists these signs that labour might be starting: contractions or tightenings, a "show" (the mucus plug coming away), backache, an urge to go to the toilet as your baby's head presses on your bowel, and your waters breaking. Call your midwife or maternity unit if you think you are in labour or your contractions are coming every 5 minutes or more often. The early latent phase can take hours or sometimes days. See signs of labour.

A birth before 37 weeks is premature. ACOG calls 37 weeks 0 days to 38 weeks 6 days early term, 39 weeks 0 days to 40 weeks 6 days full term and 41 weeks late term. Pregnancy, Birth and Baby says that if your baby has not arrived by week 42, an induction may be recommended. And plan loosely: ACOG says only about 1 in 20 women give birth on their due date.

When to call your midwife or doctor

The NHS says to call your midwife or maternity unit urgently, even in the middle of the night, if:

• your waters break
• you have vaginal bleeding
• your baby is moving less than usual
• you are less than 37 weeks pregnant and think you might be in labour (see preterm labour)
• any contraction lasts longer than 2 minutes, or you are having 6 or more contractions every 10 minutes

Call 999 and ask for an ambulance if you think your baby is coming now and you have a strong urge to push. Pregnancy, Birth and Baby also says to contact your midwife, doctor or hospital immediately for a constant, clear watery discharge, severe nausea, vomiting or tummy pain, sudden blurry vision or a severe headache, sudden swelling of your face, fingers or ankles, or a fever. The NHS says swelling should be checked in case it is pre-eclampsia, a condition in which you may feel perfectly well while your blood pressure rises dangerously high, very quickly.

Frequently asked questions

When does the third trimester start?

At 28 weeks for the NHS, for ACOG in the US and for the HSE in Ireland. Australia's Pregnancy, Birth and Baby starts it at week 27. At 27 weeks you are still in the second trimester by every source except the Australian one, which counts week 27 as the third.

How many weeks is the third trimester?

About 13 weeks. ACOG's definition, from 28 weeks 0 days to 40 weeks 6 days, is exactly 13. In practice it ends whenever your baby is born, which can be earlier, or later: the NHS offers an appointment at 41 weeks, and ACOG's late term runs to 41 weeks 6 days.

Is nausea normal in the third trimester?

It can be. The NHS week-by-week guide lists morning sickness among the earlier symptoms that may carry on, and indigestion and heartburn among third-trimester symptoms. Pregnancy, Birth and Baby says to contact your midwife or doctor immediately for severe nausea, vomiting or tummy pain.

Is spotting without pain normal in the third trimester?

Treat it as a reason to call. The NHS says to call your midwife or maternity unit urgently for vaginal bleeding. Near labour, a show is pink mucus with a small amount of blood, but the NHS says losing more blood than that may be a sign something is wrong.