The third trimester, weeks 28 to 40
The home stretch. Your baby is putting on weight and maturing, and you are getting ready to meet them.
Updated October 13, 2026 · sources checked October 13, 2026
In short
The third trimester runs from 28 weeks to birth. Your baby gains weight while the lungs, brain and nervous system mature. Three habits matter most: go to sleep on your side, know your baby's usual pattern of movements and call at once if it changes, and keep your hospital bag and birth plan ready from around 36 weeks.
The dates
ACOG defines the third trimester as 28 weeks and 0 days to 40 weeks and 6 days, the time when the baby's weight increases and the organs mature for life outside the womb. ACOG also notes that only about 1 in 20 women give birth on their due date, so think of it as a window rather than a day. Not sure of your date? Use our due date calculator.
Your baby's development
| Weeks | What ACOG describes |
|---|---|
| 29 to 32 | Stretching, kicking and grasping; eyes sense changes in light; bone marrow makes red blood cells |
| 33 to 36 | Bones harden but the skull stays soft and flexible; limbs look chubby; the baby may turn head-down |
| 37 to 40 | Circulation and the musculoskeletal system finish developing; lungs, brain and nervous system complete their development; fat builds up to keep the baby warm |
Go to sleep on your side
The NHS advises not going to sleep on your back after 28 weeks. If you wake up on your back, do not worry: just turn onto your side before going back to sleep. The evidence comes from studies of late stillbirth: a 2019 analysis pooling individual data from several studies found that going to sleep on the back was linked with about 2.6 times the odds of late stillbirth, while the right side had similar odds to the left. Our guide to sleeping positions in pregnancy and our pregnancy pillow guide help you get comfortable on your side.
Baby movements: know the pattern
The NHS says you should feel your baby move right up to and during labour. It also says you do not need to count kicks, but you should know your baby's usual pattern.
Call your midwife or maternity unit immediately if your baby is moving less than usual or the pattern changes. Do not wait until the next day, even in the middle of the night.
Tests and vaccines in the third trimester
| What | When | Source |
|---|---|---|
| RSV vaccine (UK) | Offered around your 28-week appointment; as soon as possible from 28 weeks | NHS |
| Whooping cough vaccine (UK), if not yet given | Ideally before 32 weeks, but still possible later | NHS |
| Group B strep culture (US) | 36 to 38 weeks | ACOG |
ACOG notes that about 1 in 4 pregnant women carry group B strep, and that treatment with antibiotics during labour lowers the chance of the newborn becoming ill. The full visit schedules are in our guide to prenatal appointments.
Getting ready for birth
| Around | To do |
|---|---|
| 28 to 32 weeks | Finish antenatal classes; plan maternity leave; choose where you want to give birth |
| 32 to 36 weeks | Write your birth plan; install the car seat; prepare the baby's sleep space |
| 36 weeks | Pack your hospital bag; save your maternity unit's number in your phone |
| 37 weeks onwards | Learn the signs of labour and how to time contractions |
Printable checklist: ready by 36 weeks
For your partner or birth partner
Whoever will be by her side, partner, wife, mother, friend or doula: learn the warning signs below, save the maternity unit number, know the route, and pack your own bag. If she notices fewer movements, help her call straight away rather than waiting to see.
Is this normal? When to call
Breathlessness, heartburn, broken sleep, practice tightenings and feeling heavy are all common in the last weeks. The CDC's urgent maternal warning signs are different, and need a call straight away:
- A headache that will not go away or gets worse, changes in vision, dizziness or fainting.
- Extreme swelling of the hands or face.
- Trouble breathing, chest pain or a fast-beating heart.
- Severe belly pain that does not go away.
- Your baby's movements stopping or slowing.
- Vaginal bleeding or fluid leaking.
- A fever of 100.4°F (38°C) or higher.
- Thoughts about harming yourself or your baby.
US: call 911 in an emergency, otherwise your OB-GYN or labor and delivery unit. UK: call 999 in an emergency, your maternity unit otherwise, or NHS 111 if unsure.
Questions parents ask
Does it matter which side I sleep on?
No. The NHS advice is to go to sleep on your side after 28 weeks, and the 2019 analysis found the right side had similar odds to the left. Choose whichever is comfortable.
Should I count kicks?
The NHS says you do not need to count kicks. Get to know your baby's usual pattern and call your midwife or maternity unit straight away if movements slow down or change.
When should my hospital bag be ready?
Many parents pack it from around 36 weeks so it is ready if labour starts early. Our hospital bag checklist lists what to pack for you, your baby and your birth partner.
What is the group B strep test?
In the US, ACOG advises a swab of the vagina and rectum between 36 and 38 weeks. If GBS is found, antibiotics are given through a drip once labour starts to protect your baby.
Sources
- ACOG — How Your Fetus Grows During Pregnancy checked October 13, 2026
- NHS — Stillbirth (reducing the risk) checked October 13, 2026
- NHS — Your baby's movements checked October 13, 2026
- ACOG — Group B Strep and Pregnancy checked October 13, 2026
- NHS — Vaccinations in pregnancy (RSV) checked October 13, 2026
- CDC — Urgent Maternal Warning Signs (Hear Her) checked October 13, 2026
- Cronin RS et al. (2019), EClinicalMedicine — Going-to-sleep position and late stillbirth, IPD meta-analysis (PMID 31193832) checked October 13, 2026
Mamavela explains; it does not replace your midwife, doctor or pediatrician, who know your situation. If something worries you, call them. In an emergency, call 911 (US), 999 (UK) or 112.