EDD & Gestational Age
Naegele assumes 28 days; a different cycle shifts the EDD by the difference.
Enter the first day of the LMP to see the EDD.
A dating scan beats the LMP. Where first-trimester ultrasound dating differs materially from the LMP, the scan date is normally used — LMP dating assumes a regular cycle and accurate recall, and both fail often.
Trimesters are split here at 14+0 and 28+0 weeks. Other texts divide at 12 or 27 weeks; it is a convention rather than a physiological boundary.
Preterm is below 37+0 weeks and post-term from 42+0 weeks.
Show formula
EDD = LMP + 280 days + (cycle length − 28)
Gestational age = days since the LMP, as completed weeks + days
A dating scan is converted to an equivalent LMP first, so both routes share the same arithmetic.
Source: Naegele's rule — EDD = first day of the LMP + 280 days, adjusted for cycle length.