Loona Calculators
Working backwards from a due date is the standard way pregnancies are dated. Enter the expected due date and get the estimated conception date, the estimated first day of the last period, and when the first trimester ends.
A pregnancy averages 280 days from the last period, and conception typically occurs about 14 days after it — 266 days before the due date. These are the standard averages medicine uses.
It is an estimate within several days either way; sperm survival and cycle variation mean the true date can differ. Only ultrasound dating narrows it further.
For the forward direction — last period to due date — use our Pregnancy Due Date Calculator.