The short answer
Your due date is estimated as 40 weeks (280 days) from the first day of your last menstrual period, assuming a 28-day cycle. An early ultrasound measuring the embryo is more accurate, especially if your cycles are irregular, and may adjust the date.
A quick rule: add 9 months and 7 days to the first day of your last period. Note that Korean clinics count weeks from the last period, as most countries do. See pregnancy week by week.
Answered by Dami Women's Clinic (다미산부인과의원), Yeouido, Seoul · Dr. Ko Sang-hyun, OB/GYN. General information, not a diagnosis. Emergencies in Korea: call 119. Medical disclaimer · International Patient Center