This site splits a pregnancy into three blocks of gestational weeks, counted from the first day of the last menstrual period:
- First trimester: week 0 through week 12 (the day before you reach 13 weeks 0 days)
- Second trimester: week 13 through week 27
- Third trimester: week 28 through the due date and after
Some clinic handouts move a boundary by about a week. The pregnancy week calculator follows the cuts above so the page and the tool agree. The labels are a calendar convention. They are not a health assessment.
Example from 1 January
Last period on Thursday 1 January 2026, counted as day 0:
| Boundary | Gestational age | Date |
|---|---|---|
| Start | 0 weeks 0 days | 1 January 2026 |
| End of week 12 | 12 weeks 6 days | 1 April 2026 |
| Second trimester starts | 13 weeks 0 days | 2 April 2026 |
| End of week 27 | 27 weeks 6 days | 15 July 2026 |
| Third trimester starts | 28 weeks 0 days | 16 July 2026 |
| Due date | 40 weeks 0 days | 8 October 2026 |
Week 28 is 196 days after the period (28 × 7). The due date is 280 days after it. A different period date shifts every row by the same number of days. A cycle that is not 28 days shifts the due date, not the raw week count from the period you entered.
Months are a second label
Weeks to months uses unequal months. Week 13 is month 3 and also the first week of the second trimester. Week 28 is month 7 and the first week of the third. Quoting “month 7” and “third trimester” can describe the same day.
How many weeks pregnant am I is the division that feeds this table. How many days a pregnancy is is the 280-day length. If an ultrasound has already set the due date, count weeks backward from that date rather than from a conflicting period.