When Did I Conceive? Reverse Calculator
Use this calculator to estimate your conception date based on your Estimated Due Date (EDD) or your Last Menstrual Period (LMP).
Enter your doctor-provided Estimated Due Date. This is often confirmed by ultrasound.
Enter the first day of your last menstrual period. (If EDD is also provided, EDD will take precedence).
Understanding Your Conception Date
Determining the exact date of conception can be a fascinating and sometimes crucial piece of information for expectant parents. While it's impossible to pinpoint the precise moment of fertilization, we can estimate it with reasonable accuracy using a few key dates related to your pregnancy.
How Pregnancy Dates Are Calculated
Most pregnancies are dated from the first day of the woman's Last Menstrual Period (LMP), even though conception typically occurs about two weeks later. This method assumes a standard 28-day menstrual cycle with ovulation occurring around day 14. A full-term pregnancy is generally considered to be 40 weeks (280 days) from the LMP.
- From LMP: If you know the start date of your LMP, your conception date is estimated to be approximately 14 days after your LMP. Your Estimated Due Date (EDD) would then be 280 days (40 weeks) from your LMP.
- From EDD: If you have an Estimated Due Date (EDD) provided by your doctor (often confirmed by early ultrasound), you can work backward. Since the EDD is 280 days from the LMP, and conception is 14 days after the LMP, conception is approximately 266 days (38 weeks) before your EDD.
Why Estimate Conception Date?
Knowing your estimated conception date can be helpful for several reasons:
- Curiosity: Many parents are simply curious about when their baby was conceived.
- Medical Dating: While doctors primarily use LMP and ultrasound for official dating, understanding the conception date can help align expectations.
- Planning: It can assist in understanding the timeline of early pregnancy symptoms, prenatal appointments, and developmental milestones.
- Legal/Paternity Reasons: In some rare cases, it might be relevant for legal or paternity considerations.
Accuracy and Variability
It's important to remember that these calculations provide an estimate. Several factors can influence the actual conception date:
- Irregular Cycles: Women with irregular menstrual cycles may ovulate earlier or later than day 14, making LMP-based calculations less accurate.
- Ovulation Timing: Even with regular cycles, the exact day of ovulation can vary.
- Sperm Viability: Sperm can live inside the female reproductive tract for up to 5 days, meaning conception can occur several days after intercourse.
Early ultrasounds are often considered the most accurate method for dating a pregnancy, especially if there's uncertainty about the LMP or cycle length.
Examples
Example 1: Calculating from Estimated Due Date (EDD)
Sarah's doctor told her that her Estimated Due Date (EDD) is October 20, 2024.
To find her conception date, we subtract 266 days from her EDD:
October 20, 2024 – 266 days = January 27, 2024
Her estimated Last Menstrual Period (LMP) would be 280 days before her EDD:
October 20, 2024 – 280 days = January 13, 2024
Example 2: Calculating from Last Menstrual Period (LMP)
Maria remembers that the first day of her Last Menstrual Period (LMP) was March 5, 2024.
To find her conception date, we add 14 days to her LMP:
March 5, 2024 + 14 days = March 19, 2024
Her estimated Due Date (EDD) would be 280 days after her LMP:
March 5, 2024 + 280 days = December 10, 2024