How the Pregnancy Week Calculator works
The Pregnancy Week Calculator tells you your gestational age — how far along you are, in weeks and days. It counts the days from the start of your pregnancy (the first day of your last menstrual period) to today. If you enter a due date instead, it first works back 280 days to that start date; if you enter an ultrasound, it adds the days since the scan to the gestational age the scan measured. The result also shows the day of pregnancy, your trimester, your estimated due date, how many weeks remain, and the previous, current and next week of your timeline.
Completed weeks and the “week of pregnancy”
Pregnancy weeks are described in two ways, which can cause confusion. “20 weeks pregnant” means 20 completed weeks. During those seven days you are in the 21st week of pregnancy, just as a one-year-old child is in their second year. Ultrasound reports and healthcare professionals normally use completed weeks and days, written as 20+3 or 20w3d. This calculator shows both.
Pregnancy weeks at a glance
| Weeks pregnant | Trimester | Month (approx.) | Milestone |
|---|---|---|---|
| 4 weeks | First | 1 | Around the missed period; home tests often positive |
| 8 weeks | First | 2 | Common time for the first prenatal visit |
| 12 weeks | First | 3 | First-trimester scan window |
| 14 weeks | Second | 4 | Second trimester begins |
| 20 weeks | Second | 5 | Halfway; anatomy scan around now |
| 28 weeks | Third | 7 | Third trimester begins |
| 37 weeks | Third | 9 | Early term |
| 40 weeks | Third | 9 | Estimated due date |
Week-by-week pages
Each week from 4 to 40 has its own page explaining the stage of pregnancy, what is developing, what many people notice, and the care commonly offered around that time:
Week 4Week 5Week 6Week 7Week 8Week 9Week 10Week 11Week 12Week 13Week 14Week 15Week 16Week 17Week 18Week 19Week 20Week 21Week 22Week 23Week 24Week 25Week 26Week 27Week 28Week 29Week 30Week 31Week 32Week 33Week 34Week 35Week 36Week 37Week 38Week 39Week 40
Why weeks are counted from your last period
The first day of the last period is a date most people know, while ovulation and conception happen out of sight and are rarely certain. Counting from the period gives everyone — patients, clinics and scan reports — a shared reference point. The trade-off is that the first two “weeks of pregnancy” happen before conception. If you know your conception date, the conception method converts it to the same gestational age.
When your weeks might not match your provider’s
If your provider changed your due date after a dating scan, your weeks will follow the new date. Enter that due date in “Due date” mode and the calculator will match. Small differences of a day can also come from whether a count starts at day 0 or day 1; this calculator follows the clinical convention, where the first day of the last period is 0 weeks 0 days. For your month of pregnancy, see the Pregnancy Month Calculator; for the full week-by-week calendar, see the Pregnancy Timeline.