Due Date Calculator
Estimate your due date (EDD) in seconds. Simply enter the first day of your last menstrual period and get your due date, pregnancy week, trimester, and a progress bar.
How is the due date calculated?
The estimated due date calculation is based on Naegele's rule, named after the German obstetrician Franz Naegele. This method adds 280 days (or 40 weeks of gestation) to the first day of your last menstrual period (LMP).
This calculation assumes a regular 28-day menstrual cycle with ovulation occurring on day 14. It is the method used by most healthcare professionals at the first prenatal visit, before a first-trimester ultrasound refines the estimate.
How accurate is the calculation?
The estimated due date is a statistical estimate, not a certainty. In reality, only 5% of babies are born on their exact due date. The vast majority of births occur within a window of 38 to 42 weeks of gestation.
A dating ultrasound, ideally performed between 11 and 13 weeks, measures the crown-rump length (CRL) of the embryo and can refine the EDD with an accuracy of approximately 5 days. If the difference between the date calculated by Naegele's rule and the ultrasound exceeds one week, the ultrasound date is generally retained.
What is gestational age?
Gestational age is counted from the first day of the last menstrual period. It is the reference system used by healthcare professionals to date the pregnancy and plan required examinations.
Fetal age (or conceptional age) is counted from the presumed date of conception (approximately 14 days after the start of the last period for a 28-day cycle). The conversion is simple: gestational age = fetal age + 2 weeks. So a woman at 12 weeks gestational age is at 10 weeks fetal age.
For example, the first-trimester ultrasound is done between 11 and 13 weeks gestational age (9 to 11 weeks fetal age), and a full-term delivery occurs between 39 and 41 weeks gestational age (37 to 39 weeks fetal age).