How to Calculate Due Date
What is Due Date?
The estimated due date (EDD) is the predicted date of a baby's birth, typically calculated as 40 weeks (280 days) from the first day of the last menstrual period (LMP) — known as Naegele's rule. The actual birth normally occurs between 37 and 42 weeks gestation. Only about 4% of babies are born on their exact due date.
Step-by-Step Guide
- 1Naegele's rule: EDD = LMP + 280 days (40 weeks)
- 2Alternatively: EDD = LMP + 9 months + 7 days
- 3Gestational age is counted from LMP, not conception
- 4Conception occurs approximately at week 2 of pregnancy
- 5Trimesters: 1st = weeks 1–12, 2nd = 13–26, 3rd = 27–40
Worked Examples
Frequently Asked Questions
What is Due Date?
The estimated due date (EDD) is the predicted date of a baby's birth, typically calculated as 40 weeks (280 days) from the first day of the last menstrual period (LMP) — known as Naegele's rule. The actual birth normally occurs between 37 and 42 weeks gestation
How accurate is the Due Date calculator?
The calculator uses the standard published formula for due date. Results are accurate to the precision of the inputs you provide. For financial, medical, or legal decisions, always verify with a qualified professional.
What units does the Due Date calculator use?
This calculator works with percentages. You can enter values in the units shown — the calculator handles all conversions internally.
What formula does the Due Date calculator use?
The core formula is: Naegele's rule: EDD = LMP + 280 days (40 weeks). Each step in the calculation is shown so you can verify the result manually.
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