Breakdown
How it works
Pregnancy is dated from the first day of the last period: due date = LMP + 280 days, adjusted for cycles longer or shorter than 28 days. From conception, add 266 days; from a day-5 IVF transfer, add 261. Only about 4% of babies arrive on the due date; 37–42 weeks is normal.
Formula
Due = LMP + 280 days + (cycle − 28)
Due = conception + 266 days
Due = day-5 transfer + 261 days
Worked example
LMP 1 August 2026, 28-day cycle: due 8 May 2027; conception around 15 August; full-term window 17 April – 22 May 2027.
Frequently asked questions
Why did my scan change the date?
Ultrasound measurement in the first trimester is accurate to ±5 days and is used when it differs from LMP by more than 7 days.
How are weeks counted?
From LMP, so at "4 weeks pregnant" conception was about 2 weeks ago. Trimesters: 1–12, 13–26, 27–40 weeks.
When is maternity leave in the UAE/UK?
UAE: 60 days (45 full pay, 15 half). UK: up to 52 weeks, from 11 weeks before the due date.