How trimesters are defined
Pregnancy is divided into three trimesters of roughly three months each. Different sources draw the lines slightly differently; we use the widely adopted clinical definitions:
| Trimester | Gestational age | Approx. months |
|---|---|---|
| First | 0w 0d – 13w 6d | Months 1–3 |
| Second | 14w 0d – 27w 6d | Months 4–6 |
| Third | 28w 0d – birth | Months 7–9 |
First trimester (weeks 1–13)
The most dramatic changes happen when you look least pregnant. By the end of week 13 the baby has every major organ in place. For you, rising hCG and progesterone can bring nausea, fatigue, breast tenderness and frequent urination. Key tasks: confirm the pregnancy, start folic acid, first antenatal visit and blood tests, dating scan, and the NT scan with first-trimester screening at 11–13+6 weeks.
Second trimester (weeks 14–27)
Often called the “honeymoon” trimester. Nausea usually eases, energy returns and the bump becomes visible. Most women feel the first flutters of movement between 16 and 22 weeks. Key tasks: start iron-folic acid and calcium tablets as prescribed, the detailed anomaly scan at 18–22 weeks, and the glucose tolerance test at 24–28 weeks.
Third trimester (week 28 to birth)
The baby gains most of their birth weight and their lungs and brain mature rapidly. You may notice back ache, heartburn, breathlessness, swollen feet and Braxton Hicks tightenings. Key tasks: more frequent check-ups, a growth scan, Tdap/Td vaccination as advised, learning the signs of labour, and packing your hospital bag.
Go deeper with our month-by-month guide or explore any week in the week-by-week guide.
Frequently asked questions
How many weeks are in each trimester?
The first trimester is weeks 1–13 (about 13 weeks), the second is weeks 14–27 (14 weeks) and the third is week 28 until birth (about 12–14 weeks).
When does the second trimester start?
At 14 weeks 0 days of pregnancy, counted from your last period.
Which trimester is the most important?
Every trimester matters. The first is when organs form and the baby is most sensitive to medicines, infections and alcohol; the third is when most growth and brain development happen.
Is 13 weeks first or second trimester?
13 weeks (up to 13 weeks 6 days) is still the first trimester. The second trimester begins at 14 weeks.
When does the third trimester start?
At 28 weeks 0 days. That’s roughly the start of the seventh month.
Medical disclaimer: The results and information on this page are estimates for general education only. They are not a diagnosis and do not replace advice from your gynaecologist, obstetrician or midwife. If you have pain, bleeding, reduced baby movements or feel unwell, contact your doctor or call 108 immediately. Read the full disclaimer.
Sources & further reading
- ACOG. Committee Opinion No. 579: Definition of Term Pregnancy.
- World Health Organization. WHO recommendations on antenatal care for a positive pregnancy experience (2016). who.int
- Ministry of Health & Family Welfare, Government of India. Maternal health guidelines, PMSMA and Anemia Mukt Bharat. nhm.gov.in