27 weeks pregnant: the end of the second trimester
27 weeks pregnant is a stage at which, the NHS says, the baby is around 36.6 cm long from head to heel, about the size of a head of cauliflower, and you are two-thirds of the way through your pregnancy (NHS, 2026). This page sets out what the NHS, ACOG and the CDC say about week 27, including where the second trimester ends.
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Medically reviewed by Dr Mary Davidson, Reproductive endocrinologist, checked on
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 27 weeks
- Size and shape, NHS
- The NHS week 27 page in its week-by-week guide (NHS, 2026) gives the baby as around 36.6 cm long from head to heel and compares it to a head of cauliflower. It says the baby’s lungs are now capable of breathing and the baby is getting plumper by the day.
- Skin and organs, NHS
- The NHS says that a few weeks ago the baby looked a bit like a wrinkled prune, that the folds of skin are now being filled out by fat, and that all the organs are maturing as the baby prepares for life outside the uterus (NHS, 2026).
- Development, ACOG
- ACOG’s list for weeks 25 to 28 includes lungs that are fully formed but not yet ready to function outside the uterus, lungs that begin to make surfactant, a developing nervous system and skin that looks smoother as more fat is added (ACOG, How Your Fetus Grows During Pregnancy, 2024). It does not date these to a single 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 27 weeks 0 days to 27 weeks 6 days.
A normal pregnancy lasts about 40 weeks from the first day of the last menstrual period, so at 27 weeks about 13 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.
Where the second trimester ends
The NHS says week 27 is the final week of the second trimester and that next week you start the third and final trimester (NHS, 2026). ACOG defines the second trimester as 14 weeks and 0 days to 27 weeks and 6 days and the third as starting at 28 weeks and 0 days (ACOG, 2024). The Office on Women’s Health defines the second trimester as week 13 to week 28 and the third as week 29 to week 40 (Office on Women’s Health, Stages of pregnancy, 2025). The NHS and ACOG agree that the third trimester starts at week 28. The Office on Women’s Health starts it a week later. The guide to the third trimester covers the weeks ahead, and the hub for pregnancy trimesters compares all three.
Whooping cough vaccination in the UK and the US
The NHS says the whooping cough vaccine can be had from week 16 to week 32 and is usually offered around week 20 (NHS, 2026). The CDC recommends Tdap during the 27th through 36th week of each pregnancy, preferably in the earlier part of that time, and says the timing does not depend on when the last tetanus or diphtheria vaccine was given (CDC, Tdap Vaccination for Pregnant Women, 2025). Week 27 is the first week of the CDC window. Ask your midwife or doctor which schedule applies to you.
Your body at 27 weeks, as the NHS describes it
The NHS says you are probably putting on a few pounds now and may be feeling bloated and constipated, partly because your stomach is being squeezed by your growing baby and partly because of the pregnancy hormone progesterone (NHS, 2026).
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 27 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.
- Office on Women's Health, US Department of Health and Human Services. Stages of pregnancy. womenshealth.gov, 2025.
- Centers for Disease Control and Prevention. Tdap Vaccination for Pregnant Women. CDC, 2025.
Frequently asked questions
Is 27 weeks pregnant the second or third trimester?
The NHS, ACOG and the Office on Women’s Health all treat week 27 as the second trimester (NHS, 2026; ACOG, 2024; Office on Women’s Health, 2025). The NHS and ACOG start the third trimester at week 28, the Office on Women’s Health at week 29.
When is the whooping cough vaccine given in the US?
The CDC recommends Tdap during the 27th through 36th week of each pregnancy, preferably in the earlier part of that time (CDC, 2025). The NHS gives a different window, from week 16 to week 32 (NHS, 2026).