Most healthcare professionals use “Naegele’s Rule” to calculate pregnancy due dates, which is a simple mathematical formula that adds 280 days (40 weeks) to the date of the first day of a woman’s last normal menstrual cycle. If your healthcare professional has given you your due date and you want to figure out your possible conception date, you can use an equation similar to Naegele’s Rule, only in reverse. Naegele’s Rule is based on an average 28-day menstrual cycle, so you’ll need to adjust your calculations if your last menstrual cycle was longer or shorter than 28 days.
Determine how many days your menstrual cycle lasted during your last normal period. Most women have a 28-day menstrual cycle, but you’ll need to adjust your calculations if your previous menstrual cycle was shorter or longer than 28 days.
Figure out when you last ovulated. If your month of menstruation was a 28-day cycle, you ovulated on day 14 of that month. If your cycle was longer than 28 days, you likely ovulated after day 14. For example, if you had a 32-day menstruation cycle, add four days, and estimate that you ovulated on day 18.
Subtract 266 days (or 38 weeks) from your due date by counting back on a current calendar. A normal human pregnancy is approximately 40 weeks. If you subtract 280 days (40 weeks) from your due date, the result will give you the first day of your last menstrual cycle. To calculate the time when you last ovulated and likely conceived, subtract 266 days.
Add or subtract the number of days your last normal menstrual cycle varied from 28 days. For example, if your cycle was 32 days, add four days to the result.
Look on your calendar at five days prior to and one day after the ovulation date you just determined. You can become pregnant during the five days leading up to ovulation, the ovulation date itself or the day after ovulation. Any of these seven days could be your conception date.
Keep in mind that pinpointing an exact conception date is very difficult. Even the most precise calculation will usually give you a range of a few days in which conception likely occurred.
Your due date and conception date may be difficult to determine if you experienced a false menstruation right after you conceived, which is called an “implantation bleed” and is caused by an egg implanting into the uterine lining five to seven days after it is fertilized. Some women can also have light periods or spotty bleeding during pregnancy that can be confused with normal menstruation.