Estimates a due date from the first day of the last menstrual period (LMP) using Naegele's rule (LMP + 280 days), and works out how far along the pregnancy is today.
This estimate assumes a regular 28-day cycle. Cycles that are longer, shorter or irregular shift the date, and an early ultrasound is usually more accurate — your midwife or doctor may revise the due date after the first scan.
How is a due date calculated?
The standard method (Naegele's rule) adds 280 days — 40 weeks — to the first day of the last menstrual period (LMP). It assumes a 28-day cycle with ovulation on day 14, so it is an estimate, not a prediction.
How accurate is a due date?
Only about 1 in 20 babies arrives on the exact due date. Most arrive within about two weeks either side. An early ultrasound measurement is generally considered more accurate than LMP dating, and your care team may adjust the date based on it.
What do the trimesters mean?
Pregnancy is divided into three trimesters: the first runs to 13 weeks, the second from 14 to 27 weeks, and the third from 28 weeks until birth. Full term is 39–40 weeks.