Pregnancy Week Calculator
"How Far Along Am I" Has an Exact Answer
Gestational age is simply elapsed time counted in completed weeks and days, but the reference point people have on hand varies — some know their last period date, others only have a due date from a clinician. This calculator accepts either and converts it to the same result: your current week and day, your trimester, and how many days remain until your due date.
The Formula
Weeks Pregnant = Days Pregnant ÷ 7 (remainder = extra days)
Due Date = Last Period Start Date + 280 days
When you start from a due date instead, the calculator simply works backward — Last Period Start Date = Due Date − 280 days — and then runs the same forward calculation from there.
Trimester Boundaries Used Here
| Completed weeks | Trimester |
|---|---|
| 0 to 12 | 1st Trimester |
| 13 to 26 | 2nd Trimester |
| 27 onward | 3rd Trimester |
These cutoffs are conventions, not biological thresholds — other sources sometimes draw the line a week either way. These are the specific boundaries this calculator applies.
Why Two Starting Points
- From last period — the default if you're tracking your own cycle and haven't yet had a dating scan.
- From due date — useful once a clinician has given you a due date, particularly if it was adjusted by ultrasound and no longer matches simple LMP arithmetic.
Both modes converge on the same output format, so switching between them mid-pregnancy — for instance after a dating scan revises your due date — is straightforward.
How to Use This Calculator
- Select a mode: From Last Period Date or From Due Date.
- Enter the relevant date — your Last Period Start Date, or your Due Date.
- Select Calculate for your current week and day, trimester, due date, and days remaining.
Related Calculations
For the original due-date calculation with a cycle-length adjustment, see the Due Date Calculator. To check your weight gain against a pacing curve for your current week, use the Pregnancy Weight Gain Calculator.