Both calculators answer pregnancy-dating questions, but from different centers. The gestational age calculator reports how far along a pregnancy is — weeks and days — as of a date you choose, using last menstrual period, conception date, or ultrasound dating. The pregnancy due date calculator centers on the estimated due date itself, computed from the last menstrual period or a conception date, with trimester and term milestones around it. Both are educational date arithmetic, not medical advice.
What each calculator does
The gestational age calculator takes a dating basis and a calculate-as-of date. In the last-menstrual-period path it counts calendar days from the LMP date to the as-of date; the conception path subtracts 14 days from the conception date to create a dating start; and the ultrasound path converts the scan’s weeks and days to a gestational age and adds the days elapsed since the scan. Current week is whole gestational days divided by 7, current day is the remainder, and the result displays as “N weeks, M days” with a trimester hint. The same arithmetic estimates a due date (280 days from the dating start), a conception date, milestone dates, and suggested appointment timing, and it rejects results beyond 294 days (42 weeks).
The pregnancy due date calculator centers on the estimated due date. In last-menstrual-period mode it adds 280 calendar days to the entered date; in conception mode it adds 266 days to the entered conception date. It then derives a timeline from an internal start date: first trimester ends at start plus 84 days, second trimester at start plus 182 days, and the full term date at start plus 259 days, with current week from the difference between the start date and a selected current date. The article flags one quirk of its current behavior: in conception mode the timeline start is conception plus 14 days, which places trimester endpoints and current week later than typical gestational-age counting would. Both modes keep the 280-day and 266-day conventions of the ACOG Committee Opinion on estimating the due date.
Side-by-side
| Gestational age calculator | Pregnancy due date calculator | |
|---|---|---|
| Center of the calculation | How far along: weeks and days as of a chosen date | The estimated due date (EDD) |
| Dating inputs | LMP, conception, or ultrasound dating | LMP (due date +280 days) or conception (due date +266 days) |
| Primary output | ”N weeks, M days” with trimester hint | EDD plus current week, trimester ends, full term date |
| Timeline basis | Dating start: LMP, conception −14 days, or ultrasound-derived | Internal start: LMP, or conception +14 days in conception mode |
| Extra outputs | Due date, conception date, milestones, appointment windows | Term ranges and milestone dates from the start date |
| Range guard | Rejects results beyond 42 weeks (294 days) | None stated; conception-mode timeline quirk flagged in the article |
When to use which
Use the gestational age calculator when the question is “how far along right now”: it reports completed weeks and days as of today or any past date, works from an ultrasound report as well as from dates, and attaches milestones and suggested appointment timing to that age. It is the tool for reproducing what a chart or scan should show on a specific date.
Use the pregnancy due date calculator when the question is “when is the baby due”: it produces the EDD from the last period or a conception date and shows the trimester and full-term milestones around that date, with ACOG’s term definitions (early, full, late, and postterm) for interpretation.
When the two disagree — for example the conception-mode timeline in the due date calculator versus the gestational age calculator’s dating-start convention — treat the dates as estimates and rely on the due date your clinician assigns. Both pages link to the same related tools, including the reverse due date and pregnancy conception calculators, when you need to work backward from a due date or find a conception window.
Limits and disclaimer
Both pages are educational only and are not medical advice. A due date is an estimate, not a prediction of the delivery day; actual birth timing varies with cycle length, ovulation timing, uncertain dates, and individual pregnancy factors. The gestational age calculator cannot decide which clinical date should be authoritative — ACOG emphasizes using the best obstetric estimate, and early ultrasound may revise dating. The due date calculator’s conception mode places trimester and current-week dates later than typical gestational-age counting, which its article explicitly flags. Neither calculator evaluates bleeding, pain, fetal growth, ectopic pregnancy risk, or medication safety. Confirm dates, growth, and care timing with a qualified obstetric, midwifery, or other healthcare professional.