Estimate your conception date and fertile window from your due date or last menstrual period using reverse Naegele's rule.
This calculator provides estimates only. Actual conception dates can vary based on individual cycle patterns and ovulation timing. Consult your GP or midwife for accurate pregnancy dating.
Working out when conception occurred can be helpful for understanding your pregnancy timeline, confirming gestational age, or simply satisfying curiosity. Conception typically happens around ovulation, which occurs approximately 14 days before your next period in a standard 28-day cycle. This calculator estimates your conception date using two methods: from a known due date (reverse Naegele's rule) or from your last menstrual period (LMP). It also provides a conception window, accounting for the fact that sperm can survive up to 5 days in the reproductive tract, making the exact moment of fertilisation difficult to pinpoint. Whether you are working backwards from a due date given by your midwife or forwards from your last period, this tool provides a clear estimate along with your likely fertile window.
To estimate your conception date: 1. Choose your calculation method. Select "Due Date" if you already know your expected due date (from a scan or midwife), or select "Last Menstrual Period" if you want to calculate from your LMP. 2. Enter your date. For the due date method, enter the due date given to you. For the LMP method, enter the first day of your last period. 3. If using the LMP method, adjust your cycle length. The default is 28 days, but if your cycle is longer or shorter, changing this will give a more accurate ovulation and conception estimate. A longer cycle means ovulation occurs later, pushing the conception date further from the LMP. 4. Review your results, which include the estimated conception date, the conception window (a 6-day range), your estimated due date, and weeks pregnant from LMP.
The conception date calculator uses established obstetric formulas: From a due date, the calculator applies reverse Naegele's rule. A standard pregnancy lasts 280 days (40 weeks) from the LMP, or 266 days from conception. Subtracting 266 days from the due date gives the estimated conception date. From the LMP, the calculator adds the number of days until ovulation: (cycle length - 14). For a 28-day cycle, ovulation and likely conception occur on day 14. For a 35-day cycle, ovulation occurs on day 21. The due date is then estimated as LMP + 280 days (Naegele's rule). The conception window spans from 5 days before the estimated conception date to 1 day after. This reflects the biological reality that sperm can survive up to 5 days, and the egg is viable for approximately 24 hours after ovulation. Weeks pregnant is calculated from the estimated LMP (conception date minus 14 days) to today's date, following the standard gestational age convention used by the NHS.
Inputs: Method: Due Date, Date: 25 December 2026, Cycle length: 28 days
Inputs: Method: LMP, Date: 1 March 2026, Cycle length: 28 days
Inputs: Method: LMP, Date: 1 March 2026, Cycle length: 32 days
This calculator provides estimates only. The NHS notes that only about 5% of babies are born on their actual due date. Dating scans performed between 8 and 14 weeks of pregnancy are considered more accurate than LMP-based estimates for determining gestational age. If you have concerns about your pregnancy dates, speak to your midwife or GP.