33 weeks pregnant: practice contractions and group B strep
33 weeks pregnant is a stage at which, the NHS says, the baby is around 43.7 cm long from head to heel, about the size of a pineapple, and the brain and nervous system are fully developed (NHS, 2026). This page sets out what the NHS and ACOG say about week 33, including where they differ on the brain.
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Seek help now if you have any of these symptoms
Bleeding and cramping are common in early pregnancy and often do not mean miscarriage. The symptoms below, however, need to be assessed straight away.
- Soaking two or more sanitary pads per hour, or passing large clots
- Severe lower abdominal pain, especially on one side
- Pain at the tip of the shoulder
- Dizziness, fainting, or collapse
- Signs of shock — pale, clammy, rapid heartbeat, breathlessness
- Pain on opening bowels, or bladder or bowel changes
- Feeling very unwell, or feverish
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How the sources describe development around 33 weeks
- Size and shape, NHS
- The NHS week 33 page in its week-by-week guide (NHS, 2026) gives the baby as around 43.7 cm long from head to heel and compares it to a pineapple. It says the baby’s brain and nervous system are now fully developed.
- Bones, NHS
- The NHS says the bones are hardening up, apart from the skull bones, which stay soft and separated until the baby is around 12 to 18 months old, and that this slight flexibility makes the journey down the birth canal a bit easier (NHS, 2026).
- Development, ACOG
- ACOG groups weeks 33 to 36 together. Its list includes bones that harden while the skull remains soft and flexible, fingernails that have grown to the end of the fingers, limbs that begin to look chubby, and a fetus that may turn into a head-down position for birth (ACOG, How Your Fetus Grows During Pregnancy, 2024). The list is not broken down week by week.
Weeks are counted from the first day of the last menstrual period, so about two weeks of the count come before conception (ACOG, How Your Fetus Grows During Pregnancy, 2024). This page covers 33 weeks 0 days to 33 weeks 6 days.
A normal pregnancy lasts about 40 weeks from the first day of the last menstrual period, so at 33 weeks about 7 weeks remain to the estimated due date (ACOG, How Your Fetus Grows During Pregnancy, 2024). Your own due date depends on your last period or a scan, not on the week alone.
The brain: where the sources differ
The NHS says the brain and nervous system are now fully developed at 33 weeks (NHS, 2026). ACOG lists the lungs, brain and nervous system as finishing their development in weeks 37 to 40 (ACOG, "How Your Fetus Grows During Pregnancy", 2024). The two bodies describe the same organs as finished at different points, and this page cannot reconcile them.
Braxton Hicks contractions, as the NHS describes them
The NHS says your uterus could start preparing for the birth with Braxton Hicks contractions, sometimes called practice contractions, which can feel like a tightening over your bump for 20 to 30 seconds before the muscles relax. It says they should not hurt, and that if the contractions become painful or begin to happen at regular intervals you should contact your midwife or hospital in case you are going into labour (NHS, 2026). The guide to Braxton Hicks sets out when they start, how the sources tell them apart from labor, and when to call.
Group B strep
The NHS says group B strep is a common bacteria that up to 2 in 5 people have living in their body, that if you carry it in pregnancy there is a small risk it could make your baby seriously ill, and that most group B strep infections in newborn babies are preventable. It asks you to talk to your midwife (NHS, 2026).
How many babies arrive on the due date
The NHS says only 1 in 20 babies arrive on their due date (NHS, 2026), which matches ACOG’s figure of about 1 in 20 (ACOG, "How Your Fetus Grows During Pregnancy", 2024).
Work out which week you are in
The Pregnancy week calculator takes your last period, conception date, scan or IVF transfer and shows how many weeks and days pregnant you are today.
Sources
- NHS Best Start in Life. Week 33 of pregnancy (week-by-week guide to pregnancy). nhs.uk, 2026.
- American College of Obstetricians and Gynecologists. How Your Fetus Grows During Pregnancy (FAQ156). ACOG, 2024.
- American College of Obstetricians and Gynecologists. Routine Tests During Pregnancy (FAQ133). ACOG, 2024.
Frequently asked questions
What do Braxton Hicks contractions feel like?
The NHS says a tightening over your bump for 20 to 30 seconds before the muscles relax, which should not hurt. It says to contact your midwife or hospital if the contractions become painful or begin to happen at regular intervals (NHS, 2026).
What is group B strep in pregnancy?
The NHS says it is a common bacteria that up to 2 in 5 people have, with a small risk to a baby if the mother carries it, and that most infections in newborn babies are preventable (NHS, 2026). ACOG says the test is a swab done between 36 and 38 weeks (ACOG, Routine Tests During Pregnancy, 2024).