Clinical tool
EDD
Estimated date of delivery and gestational age from LMP or a dating scan.
Dating method
Enter the first day of the last menstrual period to see the estimated date of delivery.
What this calculates
Estimated date of delivery and current gestational age, calculated from the last menstrual period or from a dating scan.
When to use it
At the first antenatal contact, and whenever gestational age is needed for a decision or a record.
How to interpret it
Naegele's rule adds 280 days to the first day of the LMP, adjusted for cycle length. Scan dating counts forward from the gestation measured on the scan date. Term is 37 to 41+6 weeks; the EDD is the midpoint, not a deadline.
Important limitations
- LMP dating assumes a regular cycle and reliable recall.
- Where an early scan (before 14 weeks) differs from LMP dating by more than the locally agreed threshold, scan dating is generally used.
- Ultrasound dating becomes less accurate as pregnancy advances. Follow local policy for re-dating and for assisted conception.
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.