Pregnancy Due Date Calculator

Pregnancy due date — formally Estimated Date of Delivery (EDD) — is calculated using Naegele’s rule: take the first day of your Last Menstrual Period (LMP), add 280 days (40 weeks), and that is your due date. The rule assumes a textbook 28-day menstrual cycle with ovulation on day 14. For people with shorter cycles (e.g. 24 days) the EDD shifts earlier; for longer cycles (e.g. 32 days) it shifts later. This calculator adjusts automatically when you enter your typical cycle length. The most accurate dating comes from a first-trimester ultrasound scan, which the NHS offers as a "dating scan" between 8 and 14 weeks (typically around 12 weeks). Crown-rump length on the scan dates the pregnancy to within 3-5 days at that gestation. NHS guidance is that if the scan-dated EDD differs from the LMP-dated EDD by more than 5-7 days, the scan date is used and the LMP-dated estimate is replaced. Only about 4-5% of babies are born on their exact due date; roughly 80% arrive within the two weeks either side; "term" is officially 37-42 weeks. Anything before 37 weeks is preterm; after 42 weeks is post-term and induction is usually offered around 41-42 weeks. The calculator shows your due date, current gestational week and day, the date you reach 12 weeks (typical first scan), 20 weeks (anomaly scan), 24 weeks (legal viability threshold in England, Scotland and Wales), 28 weeks (third trimester begins), 37 weeks (full term) and 40 weeks (EDD). It also estimates conception date (LMP + 14 days adjusted for cycle length) and the start of the safer-to-deliver window. For IVF pregnancies, the calculator can also use the embryo transfer date with day-3 or day-5 adjustment for far more accurate dating. UK maternity entitlements pin to gestation week. Statutory Maternity Leave can start 11 weeks before EDD; Statutory Maternity Pay (90% of average earnings for 6 weeks then £187.18/week or 90%, whichever is lower, for 33 weeks) requires you to have been employed continuously for 26 weeks by the qualifying week (15 weeks before EDD). The pregnancy notification (MAT B1 form, issued from 20 weeks by the midwife) is needed for SMP claims. For maternity pay modelling use the Maternity Pay calculator; for paternity leave use the Paternity Pay calculator; for child-related benefits use the Child Benefit calculator. The calculator does not handle multiple pregnancies (twins often deliver at 36-37 weeks, triplets earlier), surrogate-arranged due dates, or fertility-treatment specific protocols beyond the basic IVF transfer mode.