Conception Calculator

First Day of Your Last Period

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A conception calculator estimates when an existing or previous pregnancy was conceived. To achieve this, calculators use formulas based on the average timing of ovulation and the average length of pregnancy to estimate the date of conception. Conception calculators can also estimate the days when intercourse could have led to conception.

Our conception calculator uses your last menstrual period to estimate the date of conception and the days when intercourse could have resulted in conception.

If you are not currently pregnant and are trying to conceive, then go to our ovulation calculator to find your predicted best days for conceiving.

Understanding Conception

You might be surprised to find that conception has its own timetable and doesn't always happen the day intercourse takes place.

For example, if you had intercourse once during your cycle and became pregnant, you would naturally assume the day of intercourse was the day you conceived. Yet the date of conception can be as much as five days after intercourse.

The date of conception can be as much as five days after intercourse

Conception occurs after ovulation but intercourse needs to take place on or before the day of ovulation for conception to occur. Conception occurs after a woman ovulates yet research shows that conception is likely only if intercourse takes place during the five days before ovulation or on the day of ovulation.[1]

Sperm are good at waiting: Sperm released during intercourse can remain in a woman's reproductive tract for up to five days. Then, when the woman ovulates, sperm are ready and waiting to fertilize the egg.[1]

However, the reverse does not occur and eggs do not hang around waiting for sperm to arrive. For reasons that are not fully understood, the chances of conception fall extremely fast once a woman has ovulated.[1]

Conception Date = Ovulation Date*

A conception calculator estimates your conception date by calculating your possible ovulation date. Since conception occurs soon after ovulation, the date of ovulation is considered an indicator of the date of conception.

*There may be a slight variation in this, depending on the exact timing of ovulation and fertilization, but for the purposes of estimation, it is reasonable to assume that ovulation and conception occur the same day. [2]

Keep in mind that the actual intercourse which led to baby's conception could have occurred on any of the five days before ovulation or on the day of ovulation.

Conception Calculator Assumptions

Unfortunately, when calculating ovulation dates, conception calculators must rely on assumptions and these assumptions do not always fit individuals. The calculator works on averages and your ovulation pattern might not match that predicted by a calculator.

Chances are that you are not an outlier (far from average) and the date calculated by the calculator will hopefully be close to the day you conceived.  We hope you enjoy this calculator but do not rely on the dates produced, especially if you are trying to work out paternity.

How Conception Calculators Work

A conception calculator calculates your possible conception date by counting back from your estimated due date (EDD) or counting forward from your last menstrual period (LMP).

Counting back from EDD: A conception calculator counts back 38 weeks, or 266 days, to find your estimated conception date.
Counting forward from LMP: A conception calculator counts forward 14 days from LMP to find your estimated conception date.

Your Results Page

Once you have entered your estimated due date or the first day of your last menstrual period, the calculator takes you to the results page where you will see the estimated date of conception. You will also see the range of dates on which intercourse could have led to conception on the estimated conception date.

Calculate Estimated Conception Date

Example: Calculate Estimated Conception Date
LMP = January 1, 2017
EDD = October 8, 2017
Estimated conception date = LMP + 14 Days
Estimated conception date = January 1 + 14 Days
Estimated conception date = January 15, 2017

Alternatively, count back from the estimated due date:
Estimated conception date = October 8 - 266 Days
Estimated conception date = January 15, 2017

The Fertile Window: The Days on Which Intercourse May Have led to Conception

Research shows that almost all pregnancies result from intercourse during the fertile window - the five days before ovulation and the day of ovulation.[1] The day of ovulation is the day conception is considered to take place.

Calculate Your Estimated Fertile Window

Example: Calculate Your Estimated Fertile Window:

Once you have your estimated conception date (that you have calculated yourself or the conception calculator has calculated), you can calculate the days on which intercourse was likely to have resulted in pregnancy.
LMP = January 1, 2016
EDD = October 7, 2016
Estimated conception date = January 15

Fertile window starts 5 Days before Ovulation = January 10, 2015
Fertile window ends on the Day of Ovulation = January 15, 2015

Your fertile window began five days before ovulation, on January 10, and ended on January 15. Intercourse on any of these days could have resulted in conception on January 15.

Note: this calculation assumes that you did, in fact, ovulate on January 15.

Highest Fertility Days

Of the six days when intercourse is likely to lead to pregnancy, the two days before ovulation and the day of ovulation have the highest chance of pregnancy.[1] The calculator also shows these days.

LMP = January 1, 2017
EDD = October 8, 2017
Estimated conception date = January 15, 2017
Your days of highest fertility: January 13, 14, 15, 2017

Note: Again, this calculation is based on the assumption that ovulation occurred on January 15.


1. Wilcox, A, C. Weinberg, and D. Baird, A. “Timing of sexual intercourse in relation to ovulation: Effects on the probability of conception, survival of the pregnancy, and sex of the baby.” The New England Journal of Medicine 333 (1995): 1517 1521. doi: 10.1056/NEJM199512073332301
2. A. Jukic et al. Length of human pregnancy and contributors to its natural variation. Human Reproduction 28 (2013): 2848 - 2855. doi: 10.1093/humrep/det297