Pregnancy Conception Calculator
Estimate a conception date and a short fertile window from either a due date or a last menstrual period. Rules use LMP + 14 days or due date minus 266 days. Calendar sketch only, not obstetric ultrasound dating.
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How to estimate conception
- Choose whether you start from a due date or from LMP.
- Enter the date as YYYY-MM-DD in the field that appears.
- Read estimated conception, fertile window sketch, implied LMP, and due date.
- Compare with ovulation and gestational age tools when you need related calendars.
- Treat ultrasound dating from a clinician as authoritative. Nothing is uploaded.
Estimated conception date
Civil-day pregnancy calendar rules
| From LMP | Conception ≈ LMP + 14 days |
|---|---|
| From due date | Conception ≈ due - 266 days |
| Implied LMP from due | Due - 280 days |
| Fertile window sketch | Conception - 5 days through conception + 1 day |
| Inputs | Due or LMP as YYYY-MM-DD |
| Advice | Calendar only. Not ultrasound dating. |
What this conception page estimates
Many charts treat conception as about two weeks after LMP, or 266 days before a 280-day due date. This tool applies those civil-day rules and sketches a short fertile window around the conception day. It is arithmetic on dates you type.
How the dates are shifted
From due: conception = due - 266 days, LMP = due - 280 days. From LMP: conception = LMP + 14 days, due = LMP + 280 days. Fertile window sketch runs from conception - 5 days through conception + 1 day.
Pregnancy calendar entities
Entities include conception day, LMP, due date, and fertile window. Full due-date workflows sit on pregnancy due date. Cycle timing lives on ovulation. Week counting is gestational age.
Use cases
Working backward from a known due date for a journal entry. Forward from LMP for a classroom example. Aligning siblings of the due-date and ovulation pages. Not a fertility treatment plan.
Default numbers
Due date 2026-09-15 yields conception about 2026-01-23 (266 days earlier) and LMP about 2025-12-09 (280 days earlier). Starting from that LMP recovers the same due date under the +280 rule.
Limits
Irregular cycles, IVF transfer dating, and early ultrasound redating override these rules. Informational only. See the disclaimer. CZNull does not receive the dates.