Conception Calculator
Estimate your conception date and fertile window from your due date, last menstrual period, or ultrasound gestational age.
Conception details
The default 28-day cycle places ovulation on day 14. Adjust if your cycle differs.
Estimated conception date
Monday, March 9, 2026
Fertile window: Mar 4, 2026 to Mar 10, 2026
Gestational age
14 weeks 2 days
Trimester
2nd
Estimate only. Conception dates are approximate. See your doctor or midwife for an accurate clinical assessment.
Frequently Asked Questions about the Conception Calculator
How is the conception date calculated from a due date?
We use the reverse of Naegele's rule. A standard due date assumes 266 days (38 weeks) from conception, so conception date equals due date minus 266 days. This is an estimate, since the original due date itself assumes a 28-day cycle.
Why does the calculator assume a 28-day cycle?
Naegele's rule treats ovulation as day 14 of a 28-day cycle. If your cycle is shorter or longer, ovulation shifts accordingly. The calculator adjusts conception to LMP plus (cycle length minus 14) days when you change the cycle length input.
How accurate is the LMP method?
It is a reasonable estimate for people with regular cycles, but actual conception can vary by several days. Sperm can survive up to 5 days, so the fertile window spans the 5 days before ovulation plus the day of ovulation itself.
Why is ultrasound dating more accurate?
A first-trimester ultrasound measures the embryo directly and is typically accurate within 5 to 7 days. After 13 weeks, accuracy drops. For ultrasound mode, we work backward from today using the reported gestational age minus 14 days.
Can this calculator tell me exactly when I conceived?
No. Conception dates are always estimates. Even ultrasound dating has a margin of error. For paternity questions, legal matters, or clinical decisions, see your doctor or midwife.
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