Reverse Due Date Calculator

How to Use This Calculator

You need to input just one date to get started: your due date, your baby's birth date, or the date of intercourse. The calculator works bidirectionally, so you can enter data in any field and receive estimates for the others.

By default, the tool assumes a 280-day pregnancy (40 weeks), which aligns with standard obstetric practice. If your pregnancy duration differs—perhaps due to early induction or other clinical factors—adjust this setting for more precise results.

The output gives you a date range rather than a single point because human conception windows span several days around ovulation. This range accounts for sperm survival and the unpredictable nature of ovulation timing.

Conception and Intercourse Date Formulas

The reverse due date calculation relies on two straightforward relationships. First, we subtract the standard pregnancy length from the due date to find estimated conception. Second, we account for the fertile window by calculating backward from conception.

• Due Date — The expected date of birth or the actual delivery date
• Conception Date — The estimated date when fertilisation occurred
• Pregnancy Length — Duration of pregnancy in days; typically 280 days (40 weeks) from conception to delivery
• Intercourse Date — The earliest possible date sexual intercourse could have resulted in conception

Working Backwards from a Known Due Date

If you know your due date, subtract 280 days to estimate your conception date. This gives you a single point estimate based on average pregnancy length.

Next, subtract 7 additional days from the conception date to find the earliest plausible day intercourse occurred. Why seven days? Sperm can survive up to 5–7 days in the female reproductive tract, and ovulation itself is variable. This 7-day window represents the start of the fertile period before expected ovulation.

For example, a due date of August 1st, 2024 suggests conception around October 25th, 2023, with intercourse potentially as early as October 18th, 2023. Keep in mind these are estimates; individual variation in cycle length and ovulation timing means actual dates may differ.

Planning Conception Around Your Desired Birth Date

If you want a baby born in a specific month, reverse the calculation. Decide your target due date, then subtract 280 days to find when conception should occur.

From there, add roughly 7 days to establish your intercourse window—the dates when sexual intercourse is most likely to result in pregnancy for your target due date. For instance, to conceive for a July 2024 due date, aim for intercourse between mid-September and mid-October 2023.

Ovulation tracking enhances accuracy: monitor cervical mucus changes, basal body temperature, or use an ovulation test kit. These methods help pinpoint your actual ovulation date rather than relying solely on assumed 28-day cycles.

Important Limitations and Considerations

Due dates are estimates, not guarantees, because pregnancy duration varies naturally among individuals.

1. Irregular menstrual cycles shift everything — A 28-day cycle is the standard assumption, but many people have cycles of 21–35 days or longer. Irregular periods make the 280-day estimate less reliable. If your cycle is shorter or longer, adjust the pregnancy length in the calculator for better accuracy.
2. Ultrasound dating may override calculations — Early pregnancy ultrasounds (8–14 weeks) are more accurate for dating than due date calculators. If you've had an ultrasound, use the date your clinician provided rather than assuming the backward calculation is correct.
3. Due dates can shift by up to two weeks — Pregnancy duration naturally varies ±2 weeks around the 280-day mark. Factors like maternal age, genetics, and fetal development rate all influence when labour begins. Your actual delivery date may differ from the estimate despite correct calculation.
4. Intercourse date window is approximate — Sperm survival varies; conception can occur up to 5 days after intercourse in optimal conditions. The 7-day window is conservative but not absolute. If timing is critical for medical or personal reasons, professional fertility counselling is advisable.

Medical Accuracy and When to Verify

This calculator uses obstetric conventions—specifically, Naegele's rule—which assumes a 28-day cycle and 280-day pregnancy. These are population averages, not universal truths.

For medical decision-making, your healthcare provider's dating based on ultrasound, last menstrual period, and clinical examination takes precedence over calculator estimates. Discrepancies between calculated and clinically determined dates warrant discussion with your midwife or obstetrician, as they may indicate a different conception date or pregnancy duration.

If you're investigating paternity, genetic testing provides definitive answers that date calculations cannot offer. Use this tool for planning and curiosity, but rely on clinical assessment for prenatal care.