Conception Calculator
Estimate when you conceived from due date, ultrasound dating, or last period. Shows conception window and fertile days.
HEALTHEstimate when you conceived based on your due date, ultrasound dating, or last menstrual period. Shows the most likely conception date plus the fertile window when intercourse may have led to pregnancy.
For a 28-day cycle, ovulation falls on day 14 of the cycle; conception happens within 24 hours of ovulation. For other cycle lengths, ovulation = cycle_length - 14 days. From a due date, conception = due_date - 266 days. From an ultrasound, conception = scan_date - (gestational_age + 14 days). The conception window spans about 5 days because viable sperm can wait in the fallopian tubes.
Conception Calculator
Estimate when you conceived based on your due date, ultrasound dating, or last menstrual period. See your most likely conception window and the fertile days that preceded it.
How Conception Dating Works
Conception (fertilization) occurs when sperm meets egg, almost always within 24 hours of ovulation. In a regular 28-day cycle, ovulation falls around day 14, so conception happens about 2 weeks after the first day of your last menstrual period (LMP). This is why gestational age is measured from LMP rather than conception - you are already 2 weeks pregnant on conception day.
For irregular cycles, adjust by (cycle_length - 14). A 32-day cycle ovulates around day 18, so conception is 18 days after LMP, not 14. An ultrasound between weeks 8 and 13 gives the most accurate dating - the embryo measurement is independent of cycle variation and is the gold standard for setting due dates.
The "conception window" spans about 5 days because viable sperm can wait in the fallopian tubes for several days. Working backwards from a due date: subtract 266 days (38 weeks of fetal age, vs 40 from LMP). Working backwards from an ultrasound: subtract the gestational age at scan + 14 days, then check against the scan date.
Estimates only. Actual conception date depends on cycle variability and timing of intercourse - it cannot be pinpointed to a single day. Ultrasound dating is more accurate than LMP for most pregnancies.