Third trimester: when it starts and what happens

The third trimester brings more frequent appointments and the last scheduled tests and vaccinations of pregnancy. If something worries you at any point, do not wait for the next appointment: the box above shows the emergency and urgent numbers for where you are, and get help now lists the symptoms that need checking straight away.

When does the third trimester start?

Is it week 27 or 28? Week 28 is the line ACOG and the NHS share, and week 29 is the Office on Women's Health's. ACOG defines the third trimester as 28 weeks 0 days to 40 weeks 6 days (ACOG, "How Your Fetus Grows During Pregnancy", 2024). The NHS week-by-week guide opens its third trimester section at week 28 (NHS, "Week-by-week guide to pregnancy", 2023), and Tommy's, the UK pregnancy charity, gives it as the start of week 28, 28 weeks and 0 days, to week 40 (Tommy's, "the third trimester", 2025). Cleveland Clinic says most providers recognize 28 weeks as the start, while noting slight variations depending on who you ask (Cleveland Clinic, "Third trimester", 2024). The Office on Women's Health defines the third trimester as week 29 to week 40 (Office on Women's Health, "Stages of pregnancy", 2025).

So when does the 3rd trimester start? At 28 weeks for most sources, and when the third trimester starts for you is the line your clinic uses. Some hospital pages start it at week 27, which is why both numbers turn up in searches. The full comparison of all three trimesters is on the hub for pregnancy trimesters, and the weeks before are in the guide to the second trimester.

Third trimester weeks, 28 to 40

This site's week pages follow the NHS grouping, so the third trimester weeks run from 28 to 40. The NHS guide adds a page for week 41, the week after the due date (NHS, "Week-by-week guide to pregnancy", 2023).

28 weeks pregnant, 29 weeks pregnant, 30 weeks pregnant, 31 weeks pregnant, 32 weeks pregnant, 33 weeks pregnant, 34 weeks pregnant, 35 weeks pregnant, 36 weeks pregnant, 37 weeks pregnant, 38 weeks pregnant, 39 weeks pregnant, 40 weeks pregnant.

How the baby grows in the third trimester

ACOG describes third trimester pregnancy as the time when the fetus gains weight and the organs mature so that they are ready to work after birth (ACOG, "How Your Fetus Grows During Pregnancy", 2024). The Office on Women's Health gives these markers (Office on Women's Health, "Stages of pregnancy", 2025):

• At 32 weeks: the bones are fully formed but still soft; kicks and jabs are forceful; the eyes open and close and sense light; the lungs are not fully formed, but practice breathing movements occur; and the baby gains weight quickly, about half a pound a week, at about 15 to 17 inches long and 4 to 4½ pounds.
• At 36 weeks: body fat increases and the baby has less room to move, so movements are less forceful but you will feel stretches and wiggles; the baby is about 16 to 19 inches long and weighs about 6 to 6½ pounds.
• Weeks 37 to 40: at 39 weeks the baby is considered full term, with organs ready to function on their own; as the due date nears, most babies turn head down for birth.

The same page gives a typical birth weight of 6 pounds 2 ounces to 9 pounds 2 ounces and a length of 19 to 21 inches, and adds that healthy babies come in many sizes.

Appointments and tests usually offered

Appointments become more frequent in the final weeks, and the checks offered differ between the UK and the US.

In the UK

• The 28-week appointment. NICE guideline NG201 says care teams should offer a blood test for full blood count, blood group and antibodies, and anti-D prophylaxis to rhesus D-negative women who are not known to be sensitized (NICE, "Antenatal care (NG201)", 2021). 28 weeks pregnant sets out what the NHS says the midwife covers.
• Sleeping position. NICE advises women to avoid going to sleep on their back after 28 weeks (NICE, 2021).
• The baby's position. NICE says care teams should feel the abdomen at every appointment after 36 weeks 0 days to identify a possible breech presentation, and offer an ultrasound if breech is suspected (NICE, 2021). 36 weeks pregnant covers what happens next.
• Going past the due date. NICE says care teams should discuss prolonged pregnancy, and how it is managed, from 38 weeks (NICE, 2021).

In the US

• Group B strep test. ACOG says the test is a swab of the vagina and rectum, done between 36 and 38 weeks (ACOG, "Routine Tests During Pregnancy", 2024).
• Tdap vaccine. The CDC recommends Tdap during the 27th through 36th week of each pregnancy (CDC, "Tdap Vaccination for Pregnant Women", 2025).
• RSV vaccine. The CDC recommends a single dose of the maternal RSV vaccine at weeks 32 through 36, given from September through January in most of the continental United States, as an alternative to an RSV antibody given to the infant (CDC, "RSV Vaccine Guidance for Pregnant Women", 2024).
• More frequent visits. Cleveland Clinic describes visits about every two weeks until week 36 and then weekly for the last four weeks, unless the pregnancy is high risk (Cleveland Clinic, 2024).

The UK and US windows for the same vaccine are not the same, so ask your midwife or doctor which schedule applies to you. If testing for gestational diabetes has not happened yet, the gestational diabetes test guide explains the usual 24 to 28 week window and later testing.

Third trimester symptoms, as the Office on Women's Health lists them

The Office on Women's Health says some discomforts of the second trimester continue, and lists these new body changes in the third (Office on Women's Health, "Stages of pregnancy", 2025):

• shortness of breath
• heartburn
• swelling of the ankles, fingers and face
• hemorrhoids
• tender breasts, which may leak a watery pre-milk called colostrum
• a belly button that may stick out
• trouble sleeping
• the baby dropping, or moving lower in the abdomen
• contractions, which can be a sign of real or false labor

It gives one instruction to act on, repeated here as it gives it: if you notice any sudden or extreme swelling, or gain a lot of weight very quickly, call your doctor right away, because it could be a sign of preeclampsia (Office on Women's Health, "Stages of pregnancy", 2025). This page lists what the source lists and cannot say what any symptom means for you. The box above shows who to call.

Many people feel tightenings before labor begins. The NHS describes Braxton Hicks contractions, or practice contractions, as a tightening over the bump for 20 to 30 seconds that should not hurt, and asks you to contact your midwife or hospital if they become painful or regular (NHS, "Week 33 of pregnancy", 2026). The guide to Braxton Hicks contractions covers them in full, and 33 weeks pregnant covers the week the NHS mentions them.

Weight gain continues to the end. The pregnancy weight gain calculator works out a range from your weight before pregnancy.

When the third trimester ends

The third trimester ends with the birth. ACOG's definition runs to 40 weeks and 6 days, and ACOG says only about 1 in 20 women give birth on their due date (ACOG, "How Your Fetus Grows During Pregnancy", 2024). The due date calculator works out the date and the range around it, and 40 weeks pregnant covers the week of the due date and what the NHS says about going past it.

Planning a name is the other job of these weeks; our sister site BabyNames.io has name lists and meanings.

This guide is information, not a diagnosis or a recommendation of treatment, and it is no substitute for your own clinician. See the medical disclaimer.