Calculator
What the Gestational Age Calculator does
Gestational Age Calculator: calculates Trimester, Gestational Weeks and Est. Due Date from First Day of Last Period. Formula: Age = the difference between two dates in whole years, then remaining months, then remaining days. Example: first day of last period 2026-01-01 gives trimester 3rd trimester.
From First Day of Last Period, this tool derives Trimester, Gestational Weeks and Est. Due Date. Edit a field and the answer updates in place.
Typical users are someone starting a new plan. A single number is most useful as a trend over time rather than a one-off judgement.
Nothing leaves the tab. You can watch your browser network panel while you use it and see for yourself.
The method, a field-by-field reference and a worked example built from the defaults are all on this page.
What do the Gestational Age Calculator fields mean?
The Gestational Age Calculator uses 1 input. Each input has a default that makes sense on its own, so a partial edit still gives a valid result.
| Field | What to enter | Default |
|---|---|---|
| First Day of Last Period | pick a calendar date | 2026-01-01 |
How does the Gestational Age Calculator work?
Month lengths vary, so this is computed calendar-wise rather than by dividing total days by 365.
Nothing is computed until the inputs pass their checks, so you will not get a confident answer built on a typo.
Worked example
These are the values the Gestational Age Calculator loads by default, and the result it produces from them. Here is the calculation as it stands the moment the page loads.
| First Day of Last Period | 2026-01-01 |
|---|
| Trimester | 3rd trimester |
|---|---|
| Gestational Weeks | 35 |
| Est. Due Date | 2026-10-08 |
| What this means | About 35 weeks pregnant → 3rd trimester. Confirm dates with your prenatal provider. |
How to use it
- Select First Day of Last Period from the date picker.
- Live output means you can search for a target value by nudging an input up and down.
- The panel reports Trimester, Gestational Weeks, Est. Due Date and What this means.
- Copy Link is the quickest way to send the exact scenario to someone else.
What this tool does not do
- Population-level formulas cannot account for medical conditions, medication or pregnancy.
- This is a screening estimate, not a diagnosis.
- The output is a number, not a judgement. Deciding what it means is still your job.
Frequently asked questions
Just First Day of Last Period. Nothing starts empty, which means a single edit is enough to get a meaningful answer.
With the values loaded when the page opens, trimester comes out as 3rd trimester. Any edit re-runs the calculation, so there is no stale number on screen.
It never leaves the tab. The page does the work itself rather than asking a server to.
Nothing at all. There is no registration, no paywall and no per-use quota.
Treat it as an estimate. It uses general population formulas that cannot account for your medical history, medication or body composition, useful for orientation, not for decisions. Talk to a clinician before acting on it.