How the Trimester Calculator works
The calculator finds your gestational age from your last period or due date and shows which trimester you are in. It also lists the exact start and end dates of each of your trimesters, so you can see how long remains in the current one and when the next begins.
| Trimester | Weeks | Months (approx.) | Typical focus |
|---|---|---|---|
| First | Week 1 to 13 + 6 days | Months 1–3 | Confirming the pregnancy, dating, early screening |
| Second | Week 14 to 27 + 6 days | Months 4–6 | Anatomy scan, first movements, glucose screening |
| Third | Week 28 to birth (40 weeks ≈ due date) | Months 7–9 | Growth, position, birth preparation |
Why trimester boundaries matter
Trimesters are a convenient way to group pregnancy into stages with similar themes. The first is about confirming and dating the pregnancy and the formation of organs; the second about growth, the anatomy scan and the first movements; the third about maturing, positioning and preparing for birth. Some guidance — such as which screening tests are offered, or travel recommendations — refers to trimesters, so it is useful to know the exact dates for your own pregnancy.
Not every source draws the boundaries at the same place: you may see the second trimester starting at 13 weeks in some books. This calculator follows the widely used definition of 14 weeks 0 days and 28 weeks 0 days. For a week-by-week breakdown of each trimester, use the Pregnancy Timeline.