Pregnancy Due Date Calculator

Pregnancy Due Date Calculator

Knowing your estimated due date gives you a clear timeline for prenatal appointments, preparation, and developmental milestones. This calculator applies Naegele’s Rule — the standard formula used in obstetric practice worldwide — to estimate your delivery date, current gestational age in weeks and days, and the start and end dates of each trimester. All you need is the first day of your last menstrual period (LMP).

How the Due Date Is Calculated

The standard method is Naegele’s Rule:

Estimated Due Date (EDD) = First Day of LMP + 280 days (40 weeks)

The formula assumes a 28-day menstrual cycle with ovulation and conception occurring around day 14. Because gestational age counts from the LMP rather than from conception, you are considered “2 weeks pregnant” at the moment of conception — which is why a full-term pregnancy is 40 gestational weeks but only about 38 weeks of actual fetal development.

A quick mental shorthand: take the first day of your LMP, subtract 3 months, then add 7 days. For example, an LMP of April 1 gives April 1 - 3 months = January 1, then January 1 + 7 days = January 8 as the estimated due date.

Worked Examples

Example 1 — LMP January 15, 2026 January 15 + 280 days = October 22, 2026. First trimester ends March 26; second trimester ends July 2.

Example 2 — LMP April 3, 2026 April 3 + 280 days = January 8, 2027. First trimester ends June 26; second trimester ends October 2.

Example 3 — LMP July 20, 2026 July 20 + 280 days = April 26, 2027. First trimester ends October 12; second trimester ends January 18, 2027.

Trimester Milestones Reference

Due Date Reference by LMP Month

When to Use This Calculator

• Immediately after a positive pregnancy test to establish an approximate timeline before your first prenatal visit
• To calculate how many weeks pregnant you are on any given date during your pregnancy
• When planning prenatal appointments, anatomy scans (typically week 18-20), and glucose screening (week 24-28)
• To identify the boundaries of each trimester for tracking developmental milestones
• When sharing preliminary dates with a partner, family, or employer before your first clinical confirmation

Common Mistakes

1. Using the wrong start date. Naegele’s Rule requires the first day of your last menstrual period, not the last day, not the date of a missed period, and not an estimated conception date. Using the wrong date shifts the entire calculation by days or weeks.
2. Treating the EDD as a fixed delivery date. Only about 5% of babies are born on the exact estimated due date. The normal delivery window spans from 37 weeks (early term) to 42 weeks (post-term). Planning and logistical preparation should account for a 5-week window, not a single date.
3. Ignoring cycle length. The formula assumes a 28-day cycle. If your cycle consistently runs 35 days, ovulation occurs around day 21 rather than day 14, shifting your actual conception date — and the accurate EDD — by approximately 7 days. A first-trimester ultrasound corrects for this.
4. Not updating after an ultrasound. If your provider’s first-trimester ultrasound (crown-rump length measurement) yields a date more than 7 days different from your LMP-based date, your provider will typically revise your official EDD to match the ultrasound. Use the revised date for all subsequent tracking.

Understanding Your Results

The estimated due date is a statistical midpoint, not a prediction. Roughly 50% of first-time mothers deliver after their due date, with the median around 5 days past the EDD. Subsequent pregnancies tend to be shorter. Any delivery between 37 and 42 weeks is considered within the normal range and does not indicate a problem.

First-trimester ultrasound performed between 6 and 13 weeks is accurate within 5-7 days and is the gold standard for establishing gestational age. If there is a significant discrepancy between LMP-based and ultrasound-based dating, most providers defer to the ultrasound.

This calculator provides estimates for general planning and educational purposes. All decisions about your pregnancy timeline, prenatal testing schedule, and delivery planning should be made in consultation with your OB-GYN or certified midwife.

Tips

1. Write down the exact first day of your last period before entering it — using an estimated or approximate date shifts the due date calculation and all trimester boundaries
2. A first-trimester ultrasound (between 6 and 13 weeks) is the most accurate dating method and may revise your due date by up to a week; ask your provider whether your LMP-based date should be adjusted
3. Plan for a delivery window of 37-42 weeks rather than a single date — have your hospital bag packed and logistics arranged by week 36 at the latest
4. Mark your week 20 anatomy scan, week 24-28 glucose screening, and week 35-36 Group B Strep test on your calendar early — these have narrow scheduling windows
5. Track gestational age in weeks rather than months, as clinical care is organized by week (most prenatal milestones and test dates are week-specific)
6. This calculator is for educational planning only — all medical decisions about your pregnancy, including due date confirmation and delivery planning, should be directed by your healthcare provider