How to Use a Conception Date Calculator

If you are interested in knowing the date you most likely conceived your baby, a conception calculator might make it easier for you. However, any calculation of the date is only an estimate of the actual day a sperm met and fused with your egg. Conception (egg fertilization) usually occurs on the day of ovulation if there is sperm around but may be within a day or two later if intercourse is delayed until then.

How to Use the Calculator

Once you know what your conception date means, you may want to figure it out. The calculator assumes that ovulation occurs 14 days before your next expected period, no matter the length of your cycle. It only works if your cycle lengths are 21 to 38 days long.

To use the calculator to help predict when you may have conceived, do the following:

1. Select the average length of your menstrual cycles in the first box.
2. Select the month, day, and year of the first day of your last period in the next boxes.
3. Click the "Calculate" button.

The calculator will display the date you likely ovulated and therefore conceived.

The Principles Behind Conception Dates and the Calculator

The principles behind the date of conception and construction of the calculator are as follows:

• In a normal, regular cycle, the date of your ovulation determines the date of your next period.
• No matter the length of your cycle, your next period will come 12 to 14 days after you ovulate - this is the most fixed part of your cycle.
• In an idealized, average 28-day menstrual cycle, you ovulate and conceive between days 12 to 14 of your cycle, or start your period between days 26 to 28.
• As mentioned above, for purposes of ease of calculation, the conception calculator assumes your next period is due 14 days after you ovulate.
• If your cycles are regular, you can count back 14 days from when you expected your next period, or subtract 14 days from the length of your usual cycle to come up with the same conception date the calculator does.

Variations in Cycle Lengths

If you have intercourse around days 12 to 14 of a 28-day cycle, those are your potential conception days. However, if your cycles are longer or shorter than 28 days, your ovulation and conception date will be sooner or later than days 12 to 14. It will, however, occur around 12 to 14 days before the expected date of your next period, which is the fixed part of your cycle as explained above. Note that if your cycle lengths vary widely, this calculator will not be useful to you.

BBTs, LH Kits, and IVF

If you are doing basal temperature charts (BBTs), ovulation predictor (LH) kits, or tracking other signs of fertility, these tools can also help you narrow down your time of conception. Women who conceive using in-vitro fertilization (IVF) or other assisted reproductive technology will have other information from the procedures to figure out their date of conception.