Calculator
Choose a method, then calculate.Example Data Table
| Scenario | Input | Output (high level) |
|---|---|---|
| Known due date, 28-day cycle | Due date: 2026-10-15 Cycle length: 28 |
LMP ≈ 2026-01-08 Conception ≈ 2026-01-22 |
| Known due date, 32-day cycle | Due date: 2026-10-15 Cycle length: 32 |
LMP shifts earlier by ~4 days Conception shifts later by ~4 days |
| IVF, day-5 transfer | Transfer date: 2026-02-20 Embryo age: 5 |
Due date ≈ 2026-11-08 LMP ≈ 2026-01-31 |
Example outputs are rounded estimates; actual clinical dating may differ.
Formula Used
- Cycle-adjusted due date model: LMP = Due Date − [280 + (Cycle Length − 28)] days. This keeps the standard 280-day pregnancy length, then adjusts for longer or shorter cycles.
- Estimated conception: Conception ≈ LMP + (Cycle Length − 14) days. Ovulation is often approximated as 14 days before the next period.
- IVF transfer model: Due Date = Transfer Date + (266 − Embryo Age) days, then LMP = Due Date − 280 days.
These are planning estimates. Ultrasound dating and clinical context can change the final timeline.
How to Use This Calculator
- Select Known due date or IVF embryo transfer.
- Enter the required date and (if applicable) your cycle length or embryo age.
- Click Calculate to see LMP, conception, and milestone windows.
- Use Download CSV for spreadsheets, or Download PDF for sharing.
- If your provider gives different dating, follow that timeline.
Insights
Cycle length shifts the starting point
The calculator starts from an estimated due date and works backward. A standard pregnancy length is modeled as 280 days from the last menstrual period. If your cycle is longer than 28 days, the estimated LMP moves earlier; shorter cycles move it later. For example, a 32‑day cycle shifts LMP about 4 days earlier.
What “reverse due date” means in days
With a 28‑day cycle, the tool subtracts 280 days to estimate LMP and then estimates conception near ovulation. The ovulation anchor is set as cycle length minus 14 days after LMP. This gives a practical conception estimate for planning, not a guarantee of the exact day.
IVF transfer dating uses embryo age
For IVF, due date is derived from transfer date using a 266‑day fertilization‑to‑due interval, then adjusted by embryo age. A day‑5 transfer uses Due Date = Transfer + 261 days, while a day‑3 transfer uses Transfer + 263 days. LMP is then back‑calculated by subtracting 280 days.
Trimester landmarks are standardized anchors
The calculator maps trimesters to week ranges commonly used in care planning: first trimester ends at 13w6d, second ends at 27w6d, and the third begins at 28w0d. These anchors help organize appointments and expectations across the 40‑week timeline.
Screening windows are shown as ranges
Time windows matter more than a single day for many checkups. This tool provides a nuchal translucency window (11–13w6d) and a typical anatomy scan window (18–22w). It also highlights a 24w0d viability milestone often discussed in counseling and planning.
Use results as a planning layer, not a diagnosis
Date estimates can change with ultrasound findings, irregular cycles, or clinical history. Use this output to organize calendars, compare app dates, and prepare questions. If your provider gives a different due date, prioritize that timeline for medical decisions.
FAQs
Is this the same as a due date calculator?
It works in reverse. You enter a due date (or IVF transfer date), and it estimates LMP, conception, and milestone windows for planning and scheduling.
How accurate is the estimated conception date?
It is an approximation based on an ovulation estimate (cycle length minus 14 days). Real ovulation timing varies, so use it as a planning reference, not a definitive date.
What cycle length should I enter?
Use your typical cycle length from tracking history. If you are unsure, keep 28 days. The calculator supports 20–45 days to cover many patterns.
Why does a longer cycle move LMP earlier?
The model adjusts the 280‑day standard to reflect longer follicular phases. Longer cycles typically delay ovulation, so the estimated LMP shifts earlier to preserve the due date anchor.
Which IVF option should I choose?
Select day‑5 for blastocyst transfers and day‑3 for cleavage‑stage transfers. The calculator adjusts the due date by embryo age to match common clinic dating conventions.
Can I use this output for medical decisions?
No. This tool is for education and planning. Always follow your clinician’s dating, scan schedule, and guidance for testing and care decisions.