Example Data Table
| Input method | Sample input | Estimated conception | Fertile window | Estimated due date |
|---|---|---|---|---|
| Due date | Due: 2026-10-15 | 2026-01-22 | 2026-01-17 to 2026-01-23 | 2026-10-15 |
| LMP | LMP: 2026-02-01, Cycle: 30, Luteal: 14 | 2026-02-17 | 2026-02-12 to 2026-02-18 | 2026-11-10 |
| Ultrasound | Scan: 2026-04-10, GA: 10w 2d | 2026-02-14 | 2026-02-09 to 2026-02-15 | 2026-11-07 |
Formula Used
- From due date: Conception ≈ Due date − 266 days (38 weeks).
- From LMP: Ovulation ≈ LMP + (Cycle length − Luteal phase). Conception ≈ Ovulation date.
- From ultrasound: GA is counted from LMP, so Conception ≈ Scan date − (GA days − 14).
- Fertile window: 5 days before ovulation through 1 day after.
These are common educational heuristics. Real timing varies with cycle patterns and implantation.
How to Use This Calculator
- Select a method: due date, LMP, or ultrasound age.
- Enter the required dates and any cycle details shown.
- Optionally add an intercourse date for a quick window check.
- Click Calculate to view results above the form.
- Use Download CSV or Download PDF to export.
FAQs
1) How accurate is the estimated conception date?
It is a best-guess window based on common dating rules. Timing can shift due to ovulation variability, implantation timing, or measurement uncertainty.
2) Which method should I trust most?
Early ultrasound dating is often the most consistent for pregnancy age. If you have a confirmed due date from a clinician, use that. LMP-based estimates work best with regular cycles.
3) Why does the calculator subtract 266 days from the due date?
Pregnancy dating commonly assumes about 38 weeks from conception to due date. That equals 266 days, which provides a practical estimate for the likely conception timing.
4) What if my cycle is longer or shorter than 28 days?
Enter your average cycle length and luteal phase. The tool estimates ovulation from those values, then builds a fertile window around it. Irregular cycles can widen uncertainty.
5) Does intercourse on a certain day guarantee conception?
No. The fertile window indicates when pregnancy is biologically more likely, not guaranteed. Factors like sperm survival, egg timing, and overall health influence whether conception occurs.
6) What does “gestational age” mean for ultrasound inputs?
Gestational age is typically measured from the last menstrual period, not conception. The calculator approximates conception by subtracting about 14 days from the gestational age dating.
7) Can I use this for IVF or assisted reproduction?
For IVF, the true fertilization and transfer dates are known and should be used for clinical decisions. This calculator is designed for general estimation and educational planning.
8) Why do my results differ from my clinic’s dates?
Clinics use ultrasound measurements, medical history, and standardized guidelines. Small differences can come from ovulation variation, scan timing, or updated measurements during follow-up visits.