The 40-Week Journey

From the moment you see two lines on a test, the most common question is: "When is the baby coming?" While babies have their own schedules, obstetricians use a few standardized methods to calculate an Estimated Due Date (EDD).

Naegele's Rule: The Standard

The most common calculation (and the one used by most online calculators) is Naegele's Rule.

It assumes a standard 28-day cycle and calculates the due date as follows:

1. Start with the first day of your last menstrual period (LMP).
2. Subtract 3 months.
3. Add 7 days.
4. Add 1 year.

Why 40 Weeks?

Pregnancy is technically calculated from the first day of your last period, even though you aren't actually pregnant for the first two weeks of that cycle. This adds up to 280 days, or exactly 40 weeks.

Factors That Can Change Your Date

• Cycle Length: If your cycle is 35 days instead of 28, you likely ovulated later, and your due date will be adjusted accordingly.
• Dating Scans: An early ultrasound (usually around 8-12 weeks) is the most accurate way to "date" a pregnancy by measuring the size of the embryo.
• Late Ovulation: If you were tracking your ovulation and know you conceived on Day 20 instead of Day 14, your "True" due date might be different from the LMP method.

Be Prepared

Remember that only about 4-5% of babies are born on their actual due date! Use our Pregnancy Due Date Calculator to get your 40-week timeline and see where you are in your trimesters.