Pregnancy Conception Calculator

Pregnancy dating uses gestational age measured from the first day of the last menstrual period (LMP), not the actual conception day. Full-term pregnancy spans approximately 280 days (40 weeks) from LMP, or 266 days (38 weeks) from estimated conception — the difference accounts for roughly two weeks before ovulation when the egg was not yet fertilized.

Mode 1 — From due date: Estimated conception date ≈ due date − 266 days. The fertile window around conception spans approximately ±5 days — sperm survival and ovulation timing create this range. Current pregnancy week is calculated from an estimated pregnancy start (conception − 14 days, aligning with LMP-based dating).

Mode 2 — From last period: Ovulation ≈ LMP + (cycle length − 14 days). Conception most likely occurred within ovulation ± 2 days. Due date = LMP + 280 days. Current pregnancy week counts from LMP to today. A 28-day cycle places ovulation around day 14; shorter cycles ovulate earlier, longer cycles later.

Ultrasound dating in the first trimester provides the most accurate gestational age, especially with irregular cycles. LMP-based estimates assume regular 28-day cycles and known LMP date. Implantation occurs 6–12 days after conception; pregnancy tests detect hCG after implantation.

Conception date estimates help understand pregnancy timeline and fetal development stages but cannot pinpoint the exact fertilization moment. Use results for general planning and pair with prenatal care for official dating.