Reverse Due Date Calculator
Calculate your conception date and intercourse window from your due date or baby's birthday. Free reverse due date calculator with pregnancy timeline, gestational age tracking, and trimester chart.
Standard pregnancy is 280 days (40 weeks). Adjust if you know your exact pregnancy duration.
About This Calculator
The Reverse Due Date Calculator helps you determine when conception occurred and when intercourse most likely took place based on your due date or your baby's actual birthday. This is the reverse of a standard due date calculator — instead of calculating the due date from your last menstrual period, it works backwards from the due date to find the conception date and fertile window.
The calculation uses Naegele's rule, the standard obstetric formula that estimates a pregnancy length of 280 days (40 weeks) from the first day of the last menstrual period. Since ovulation typically occurs around day 14 of a 28-day cycle, conception occurs approximately 266 days before the due date. The calculator subtracts the pregnancy length from your due date to estimate the conception date, then shows a 7-day intercourse window (sperm can survive up to 7 days in the female reproductive tract).
This tool is useful for:
- Determining when you conceived if you know your due date
- Finding the likely intercourse date window that led to pregnancy
- Calculating your current gestational age and trimester from the due date
- Planning future pregnancies by understanding the relationship between conception and due dates
Regional Notes
India: Due dates are calculated using Naegele's rule with standard 280-day pregnancy. Most Indian hospitals use ultrasound dating for accuracy. The due date is typically expressed as EDD (Estimated Date of Delivery).
United States: The standard 280-day pregnancy length is used. Due dates are confirmed by first-trimester ultrasound. The ACOG recommends ultrasound dating for the most accurate gestational age assessment.
United Kingdom: NHS guidelines recommend a dating scan between 10-14 weeks for accurate due date estimation. The reverse due date calculation is useful for conceiving parents who want to understand their pregnancy timeline.
Note: This calculator provides estimates only. Every pregnancy is unique, and due dates can vary by up to two weeks. Always consult your healthcare provider for medical advice and pregnancy management.
Frequently Asked Questions
How does the Reverse Due Date calculator work?
Enter your due date or baby's birthday and the calculator subtracts the standard pregnancy length (280 days / 40 weeks) to estimate the conception date. It then calculates a 7-day intercourse window before conception when fertilization most likely occurred.
What is the formula for calculating reverse due date?
Conception date = Due date − 280 days (40 weeks). Estimated intercourse window = Conception date − 7 days to Conception date. These calculations are based on Naegele's rule, which assumes a 28-day menstrual cycle and ovulation on day 14.
How accurate is the reverse due date calculation?
The calculation is accurate within a range since human pregnancy length varies and not all women have 28-day cycles. The intercourse window accounts for this by showing a 7-day range. For the most accurate results, use an early ultrasound dating scan rather than relying solely on dates.
What is the difference between conception date and intercourse date?
Conception (fertilization) occurs when a sperm fertilizes an egg, which happens within 24 hours of ovulation. Intercourse must occur in the days leading up to and including ovulation day. Since sperm can survive up to 5-7 days in the reproductive tract, the intercourse window is wider than the conception date.
Is this calculator free?
Yes, it is completely free to use with no registration or hidden charges.
Can I share my results?
Yes, the URL saves your inputs so you can bookmark or share the exact calculation with your healthcare provider, partner, or family.
Does this calculator work for IVF pregnancies?
This calculator uses Naegele's rule for natural conception. For IVF pregnancies, the conception date is known precisely (egg retrieval + fertilization date), and the due date is calculated from embryo transfer date plus 266 days minus the age of the embryo. Use our IVF Due Date Calculator for more accurate IVF-specific results.