Conception Date Calculator
Estimate conception dates based on medical due dates. Includes a fertility window analysis based on the standard 266-day gestation model.
About
Determining the exact date of conception helps in understanding fetal development milestones. Most medical professionals date a pregnancy from the first day of the last period. However, conception actually occurs roughly 14 days later during ovulation. This calculator reverses the calculation from an established Due Date or Ultrasound Date. Sperm can survive in the reproductive tract for up to 5 days. This creates a "Fertile Window" rather than a single point in time. Accuracy depends on the regularity of the ovulation cycle. Standard calculations assume a 28-day cycle.
Formulas
The naive calculation subtracts the gestation period from the due date. The standard Naegele's rule adds one year, subtracts three months, and adds seven days to the LMP.
Since ovulation varies, we calculate a window of high probability:
Reference Data
| Trimester | Weeks (Approx) | Developmental Stage | Avg Fetal Weight |
|---|---|---|---|
| First | 1 to 12 | Embryogenesis & Organ Formation | 1 g − 14 g |
| Second | 13 to 26 | Growth & Movement | 100 g − 900 g |
| Third | 27 to 40 | Maturation & Fat Accumulation | 1 kg − 3.5 kg |
| Term | 37+ | Ready for Birth | 2.5 kg+ |
| Post-Term | 42+ | Overdue | Risk of Placental Decline |