Clinical tool

EDD

Estimated date of delivery from LMP or scan.

First day of last menstrual periodCycle lengthdaysAdjusts for ovulation later or earlier than day 14

LMP dating assumes a regular cycle and accurate recall, and is unreliable with irregular cycles, recent hormonal contraception, or bleeding in early pregnancy. Where an early ultrasound (before 14 weeks) differs from LMP dating by more than 5–7 days, scan dating should be used — it is substantially more accurate. Dating by scan becomes progressively less reliable later in pregnancy. Only about 4% of babies arrive on the EDD itself; term spans 37–42 weeks.

For practising medics only. Clinical tools support judgement, they do not replace it. Always verify results against current guidelines and the patient in front of you.