28 weeks pregnant: the third trimester and the RSV vaccine
28 weeks pregnant is the start of the third trimester, the NHS and ACOG say, and the NHS gives the baby as around 37.6 cm long from head to heel, about the size of an aubergine (NHS, 2026). This page sets out what the NHS, NICE, ACOG and the CDC say about week 28, including the appointment, the RSV vaccine and sleeping position.
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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 28 weeks
- Size and shape, NHS
- The NHS week 28 page in its week-by-week guide (NHS, 2026) gives the baby as around 37.6 cm long from head to heel and compares it to an aubergine.
- Heart rate, NHS
- The NHS says the baby’s heart rate changes all the time: around 110 beats a minute when the heart is first detectable at week 5 or 6, around 170 in weeks 9 and 10, around 140 now, and around 130 at birth. It gives your own heart rate as around 80 to 85 and says the baby’s heart can be heard through a stethoscope (NHS, 2026). Its week 10 page gives 180 beats a minute, so the NHS pages give two figures for that week.
- 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, eyelids that can open and close, lungs that begin to make surfactant, 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 28 weeks 0 days to 28 weeks 6 days.
A normal pregnancy lasts about 40 weeks from the first day of the last menstrual period, so at 28 weeks about 12 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 28-week appointment
The NHS page on antenatal appointments says that at 28 weeks the midwife gives you information about labour and birth, including newborn screening, offers an injection if your baby is at risk of haemolytic disease of the fetus and newborn (rhesus disease), asks to do a blood test to check your health, iron levels and blood group, and asks to measure your bump (NHS, "Antenatal (pregnancy) care and appointments", 2026). NICE guideline NG201 says that at the 28-week appointment 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 sensitised (NICE, 2021).
RSV vaccination in the UK and the US
The NHS recommends the RSV vaccination at around week 28 and says it can be given later, right up until labour. It says the vaccination can reduce the risk of severe bronchiolitis by 70% in the baby’s first 6 months of life (NHS, "Week 28 of pregnancy", 2026). The CDC recommends a single dose of the maternal RSV vaccine at weeks 32 through 36 of pregnancy, 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). The UK and US timing differ, so ask your midwife or doctor which applies to you. The guide to the third trimester sets out the other appointments and tests of the final weeks.
Sleeping position
NICE guideline NG201 advises women to avoid going to sleep on their back after 28 weeks of pregnancy and to consider using pillows to maintain their position (NICE, 2021). The NHS says sleeping on either side is better than sleeping on your back (NHS, "Tiredness and sleep problems in pregnancy", 2024).
Swelling and pre-eclampsia
The NHS says your ankles, feet and face could puff up a bit, particularly when it is hot, that this is probably water retention, but that you should get it checked in case it is pre-eclampsia, a condition in which you may feel well but blood pressure can rise very quickly. It asks you to talk to your midwife or doctor, or call NHS 111, if you are worried about anything at all (NHS, "Week 28 of pregnancy", 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 28 of pregnancy (week-by-week guide to pregnancy). nhs.uk, 2026.
- NHS. Antenatal (pregnancy) care and appointments. nhs.uk, 2026.
- National Institute for Health and Care Excellence. Antenatal care (NG201). NICE, 2021.
- 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. RSV Vaccine Guidance for Pregnant Women. CDC, 2024.
- NHS. Tiredness and sleep problems in pregnancy. nhs.uk, 2024.
Frequently asked questions
Is 28 weeks pregnant the third trimester?
The NHS and ACOG both start the third trimester at week 28 (NHS, 2026; ACOG, 2024). The Office on Women’s Health defines it as week 29 to week 40 (Office on Women’s Health, Stages of pregnancy, 2025).
When is the RSV vaccine given in pregnancy?
The NHS recommends it at around week 28, and it can be given later up until labour (NHS, 2026). The CDC recommends weeks 32 through 36, from September through January (CDC, 2024).