How it Works

The calculation of conception dates is generally performed retrospectively. While the exact moment of fertilization is difficult to pinpoint without medical intervention (such as in IVF), standard medical practice assumes a “standard” cycle of 28 days with ovulation occurring on day 14. This tool provides estimates based on two primary inputs:

• LMP Method: If the Last Menstrual Period is known, conception is estimated to have occurred roughly 14 days after the onset of menstruation.
• EDD Method: If the Estimated Due Date is known (often established via ultrasound), the tool subtracts 266 days (38 weeks) to find the likely conception date.

Formulas & Methodology

The calculator employs simple arithmetic based on the Naegele’s rule framework, adapted for reverse calculation. The mathematical relationships are defined as follows:

For a broader understanding of how your cycle affects these dates, you may refer to a Period and Menstrual Cycle Calculator.

The Fertile Window

The “Fertile Window” refers to the days during a menstrual cycle when pregnancy is possible. Sperm can survive within the female reproductive tract for up to five days, while the egg survives for only 12-24 hours after ovulation.

Consequently, the window for conception includes the five days prior to ovulation and the day of ovulation itself. The highest probability of conception occurs during intercourse 1 to 2 days before ovulation.

If you are calculating forward from your period to determine a future due date, you might use a Pregnancy Due Date Calculator (LMP Method).

Accuracy & Limitations

Users should note that these calculations are statistical estimates. They rely on the assumption of a regular 28-day cycle and a luteal phase of exactly 14 days. In reality, cycle lengths vary significantly between individuals and from cycle to cycle. Clinical dating via ultrasound remains the gold standard for determining gestational age.