Methodology
How the dates are calculated
Common obstetric and IVF calendar conventions for planning estimates. Date-only math in your browser (Worker when available) so timezone wall-clock quirks do not slide a due date by accident.
Last updated: July 17, 2026
Summary table
Ultrasound redating, clinic policy, and real physiology override any website. Only about 5% of births land on the EDD — treat it as a planning center, not a promise.
| Mode | Rule of thumb |
|---|---|
| LMP → EDD | EDD ≈ LMP + 280 + (cycleLength − 28); default cycle 28. |
| Conception / ovulation → EDD | EDD ≈ date + 266. LMP-equivalent ≈ EDD − 280. |
| IVF / FET transfer | EDD ≈ transfer + (266 − embryo day age). Day 3 → +263 · Day 5 → +261 · Day 6 → +260. FET uses the same day-age offset. |
| Reverse from EDD | Conception midpoint ≈ EDD − 266; LMP-equivalent ≈ EDD − 280; window, not an exact hour. |
| Gestational age | From LMP: asOf − LMP. From EDD: 280 − (EDD − asOf). Shown as completed weeks + leftover days. |
| Trimesters (disclosed) | 1st through 13w6d · 2nd 14w0d–27w6d · 3rd from 28w0d. Hospitals sometimes cut differently. |
| Implantation / test timing | Implantation ≈ ovulation +6 through +12 (highlight +8–+10). Test markers ≈ +10 / +12 / +14 DPO — windows, not guarantees. |
| Cycle support | Next period ≈ LMP + cycleLength. Rough ovulation ≈ LMP + (cycle − luteal), default luteal 14. |
IVF / FET due date
Clinics usually date from embryo age on transfer day, not from a last period you never had. Freezer storage months do not become pregnancy weeks. Day labels mean embryology age, not the fifth day of the calendar month.
EDD ≈ transfer + (266 − age)
Gestational age & trimesters
Primary display is “X weeks Y days,” not a decimal week. Fetal/embryonic age is often about two weeks behind gestational age in the classic LMP model — shown as an educational note when relevant.
Trimester cutoffs vary by source; we disclose ours on the trimester page and in the table above.
Reverse, implantation, test timing
Reverse dating answers with a conception window, not a fake exact hour for a group chat. Implantation and home-test pages use ovulation-anchored day ranges disclosed on each tool — planning markers, not guarantees you implanted that morning.
Cycle support
Period and luteal tools are arithmetic sketches for the stretch between appointments — not Flo, not a wearable, not contraception advice. Irregular cycles need wider uncertainty than a single average length can show.
Chinese gender calendar
Entertainment only. A deterministic folk-table toy from Western age and Gregorian birth month. Not science, not ultrasound, not medical advice — and not part of the site’s clinical-trust path.
Estimate only — not medical advice. Ultrasound and clinic dating policy override any browser estimate. Confirm medical decisions with your clinician.