How it is calculated
Due date = conception date + 266 days (38 weeks)Gestational age adds 14 days to the baby’s age, since doctors count from the first day of the last period.
Sperm can live up to 5 days, so conception may differ from the day of intercourse by several days.
Worked example
Conception on 15 March 2026: due date = 15 March + 266 days = 6 December 2026.
Frequently asked questions
Is intercourse day the conception day?
Not always; conception can happen one to five days later, when ovulation occurs.
Conception date or last period — which is more accurate?
Conception date, if known for certain, as with IVF or tracked ovulation.
When can I test?
About 14 days after conception, around when your period is due.