Calculator
What the Pregnancy Week Calculator does
Pregnancy Week Calculator: calculates Gestational weeks from First day of last period. Example: first day of last period 2026-01-01 gives gestational weeks 35.
The Pregnancy Week Calculator turns First day of last period into Gestational weeks. Values update as you type, so comparing options is a matter of editing one field.
Typical users are planners scheduling around holidays. Time zones and daylight saving turn a simple subtraction into a trap.
Because it is all local, the tool is as fast as your device and as private as your own notes.
Everything needed to verify a result is further down: the fields, the method and one fully worked example.
What do the Pregnancy Week Calculator fields mean?
The Pregnancy Week Calculator uses 1 input. Nothing is blank on load, so you can see the shape of the answer before supplying your own figures.
| Field | What to enter | Default |
|---|---|---|
| First day of last period | pick a calendar date | 2026-01-01 |
How does the Pregnancy Week Calculator work?
Each output is derived from the inputs above in a single pass; there is no hidden state carried between runs, so the same inputs always give the same calculation.
Bad input produces a message, not a silently incorrect figure.
Worked example
These are the values the Pregnancy Week Calculator loads by default, and the result it produces from them. These are the exact values loaded when the page opens, and the answer they produce.
| First day of last period | 2026-01-01 |
|---|
| Gestational weeks | 35 |
|---|
Worth knowing about gestational age and due date
Pregnancy dating by "weeks" is conventionally measured from the first day of the last menstrual period, not from the estimated date of conception, meaning gestational age is typically about two weeks ahead of actual fetal age, a distinction that surprises many first-time expectant parents.
Conventions and limitations
- The estimated due date from Naegele’s rule (last period plus 280 days, exactly 40 weeks) is a statistical average, not a prediction for any individual pregnancy. Only a small minority of babies are born on their exact estimated due date; a range of a couple of weeks either side is entirely normal.
- Naegele’s rule assumes a standard 28-day cycle with ovulation on day 14. For people with longer or shorter cycles, actual conception and true gestational age can differ from the last-period-based estimate, which is one reason clinicians often revise the due date after an early ultrasound.
- This is a scheduling estimate only and does not substitute for a clinician’s dating, typically confirmed by ultrasound, which is more accurate than a last-period-based calculation alone, especially for irregular cycles.
How to use it
- Select First day of last period from the date picker.
- No confirmation step: what you type is what gets calculated.
- Read Gestational weeks in the results panel.
- Print gives a tidy version without the navigation, which is handy for attaching to a file.
What this tool does not do
- Time zone and daylight-saving rules change by jurisdiction and by year.
- Calendar months have unequal lengths, so "one month" is not a fixed number of days.
- Precision beyond what the inputs justify is not meaningful, however many digits appear.
Frequently asked questions
The form asks for First day of last period. Nothing starts empty, so nothing is mandatory before you see a result.
With the values loaded when the page opens, gestational weeks comes out as 35. Overwrite any of the starting values and the result follows.
Not at all: the whole thing runs offline once the page has loaded, which tells you nothing is being sent.
No payment and no login. Advertising covers the running costs.
As accurate as the inputs allow. The arithmetic itself introduces no error beyond display rounding.