Trimester: what each of the three pregnancy trimesters covers, week by week
In this article
A trimester is one of the three stages pregnancy is split into, each about three months long. HealthHub counts the first trimester as weeks 1 to 12, the second as weeks 13 to 28 and the third as weeks 29 to 40. Some sources move the lines by a week, so do not worry if your gynae says third trimester at 28 weeks.
| First trimester | Weeks 1 to 12, HealthHub says |
|---|---|
| Second trimester | Weeks 13 to 28 |
| Third trimester | Weeks 29 to 40, or until birth |
| Full term | From 37 weeks |
Why the week lines differ between sources
There is no single official cut-off. HealthHub's pages use weeks 1 to 12, 13 to 28 and 29 to 40. Cleveland Clinic says the first trimester lasts until the end of week 13, and that most major health organisations put the start of the third trimester at week 28.
Either way, the weeks are counted from the first day of your last period, which is your gestational age. Enter that date in our due date calculator to see which trimester you are in today.
First trimester: dating and early screening
SingHealth's KKH timeline puts your first doctor's visit at weeks 8 to 12, with a dating scan to estimate your due date and blood tests for anaemia, thalassaemia, blood group, antibodies and infections. Between 11 and 14 weeks you can choose an optional screen for Down syndrome, and NIPT is a blood test option from 10 weeks.
HealthHub says that by week 12 most of the baby's major organ systems have formed. Cleveland Clinic lists nausea, tender breasts and tiredness among the usual first trimester symptoms.
Second trimester: the anomaly scan and the sugar test
SingHealth lists the fetal anomaly scan at 18 to 21 weeks, a head to toe look at your baby for structural problems, and a blood test at 24 to 28 weeks for anaemia and gestational diabetes. SingHealth's timeline also suggests the whooping cough vaccine here.
HealthHub says this is when you may start to feel your baby move, and that after 24 weeks a baby is considered potentially viable. By the end of 28 weeks, it says, your baby should weigh about 1.2 kg.
Third trimester: growth scans and getting ready
SingHealth says a growth scan is offered at 28 to 32 weeks, mums with a Rhesus negative blood group get an anti-D (Rhogam) injection at 28 weeks, and a swab for Group B Streptococcus is taken at 35 to 37 weeks. Cleveland Clinic says check-ups also become more frequent.
Cleveland Clinic lists shortness of breath, false contractions and needing to pee more often as common third trimester symptoms. Those false contractions are Braxton Hicks. HealthHub says your baby is considered full term from 37 weeks, and labour can begin any time after that.
Frequently asked questions
How many weeks is each trimester?
About 13 weeks, or three months. HealthHub splits pregnancy into weeks 1 to 12, 13 to 28 and 29 to 40, so the second and third trimesters are slightly longer on its count.
When does the second trimester start?
At week 13 on HealthHub's count. Cleveland Clinic counts week 13 as the last week of the first trimester, so you may see either.
When does the third trimester start?
At 28 weeks by most international bodies, Cleveland Clinic says, or at week 29 on HealthHub's pages. It runs until your baby is born.
Which trimester am I in?
Count the weeks from the first day of your last period, or use the date from your dating scan. Our due date calculator works it out for you.
Related terms
- Gestational Age: How far along your pregnancy is, in weeks and days, counted from the first day of your last period.
- LMP (Last Menstrual Period): The first day of your last period before pregnancy. Your week count and due date are worked out from it.
- EDD (Estimated Due Date): The date your baby is expected: 40 weeks from the first day of your last period, checked by a dating scan.
- Morning Sickness: Nausea and vomiting in early pregnancy, at any time of day. It usually settles by 14 to 16 weeks.
- Braxton Hicks Contractions: Irregular, usually painless tightenings of the womb before labour. They do not open the cervix or mean labour has started.
- NIPT (Non-Invasive Prenatal Testing): A blood test from the mother, from 10 weeks, that screens the baby for Down syndrome and trisomies 13 and 18.
- HealthHub - What happens in the first trimester? (read 9 November 2026)
- HealthHub - What happens in the second trimester (read 9 November 2026)
- HealthHub - What happens in the third trimester (read 9 November 2026)
- Cleveland Clinic - Third trimester (read 9 November 2026)
- Cleveland Clinic - First trimester (read 9 November 2026)
- SingHealth - Your pregnancy journey in KKH (read 9 November 2026)
