What Is a Due Date?
Your due date — also called the Estimated Due Date or EDD — is one of the most important numbers in pregnancy. Here is exactly what it means and how it is calculated.
The Definition of a Due Date
A due date, formally known as the Estimated Due Date (EDD) or Estimated Date of Delivery, is the date on which a pregnant person is estimated to give birth. It is calculated as approximately 40 weeks (280 days) from the first day of the last menstrual period (LMP).
The term "estimated" is key: the EDD is a statistical midpoint, not a precise prediction. Human gestation naturally varies, with most pregnancies completing anywhere between 37 and 42 weeks.
Why 40 Weeks?
The 40-week figure comes from Naegele’s Rule, published in 1812. It is based on the observation that human pregnancy lasts approximately 10 lunar months (10 × 28 days = 280 days) from the LMP. The 40-week count starts 2 weeks before conception, because the LMP precedes ovulation by about 14 days in a 28-day cycle.
How Your Due Date Is Determined
At your first prenatal appointment, your provider will ask for the date of your LMP and use Naegele’s Rule to set an initial EDD. They will then confirm and potentially adjust this date at your first ultrasound.
ACOG recommends that a first-trimester ultrasound (8–14 weeks) be used to confirm dating. If the ultrasound differs from the LMP-based EDD by more than 7 days, the ultrasound date is typically used. For IVF pregnancies, the transfer date and embryo age give an even more precise EDD.
What Your Due Date Is Used For
- Scheduling prenatal appointments and screenings at the right gestational age
- Determining when to perform specific tests (e.g., nuchal translucency at 11–14 weeks)
- Defining "full term," "preterm," and "post-term" for clinical decisions
- Planning leave from work, childcare arrangements, and birth preparation
- Monitoring fetal wellbeing as you approach and pass 40 weeks
Frequently Asked Questions
Common questions answered by our medical team