Guide
How Pregnancy Due Dates Are Calculated
Understand how due dates are estimated from LMP or conception, what the weeks mean, and the limits of estimation.
A pregnancy due date calculator estimates the delivery date from the first day of the last menstrual period (LMP) or the conception date, and shows the current pregnancy week. It is an estimate, not a guarantee.
How the due date is calculated
The standard Naegele's rule adds 280 days (40 weeks) to the first day of the LMP. If you know the conception date, the due date is about 266 days (38 weeks) later. The calculator also counts the weeks and days elapsed to show current pregnancy progress.
What the weeks mean
Pregnancy is counted in weeks from the LMP, so "12 weeks" means 12 weeks since the last period. Full term is 37–42 weeks; the estimated due date at 40 weeks is a midpoint, and most births occur within a couple of weeks of it.
How to use the calculator
- Enter the first day of your last period, or the conception date.
- Read the estimated due date and current pregnancy week.
- Use the date as a guide and confirm with your healthcare provider.
Examples
An LMP of 1 January gives an estimated due date around 8 October (280 days later). At 1 April, that is about 13 weeks pregnant. An ultrasound in the first trimester can refine this date, especially for irregular cycles.
Limitations
Only about 5% of babies arrive on their exact estimated due date; most are born within two weeks of it. Irregular cycles, unknown LMP, or variation in ovulation all affect accuracy, and a first-trimester ultrasound gives the most reliable dating. Always confirm dates and care plans with your healthcare provider — this tool is for general information only.
Put it into practice
Use the Pregnancy Due Date Calculator right now — free, in your browser, no sign-up required.
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