When Did I Conceive? (Online Conception Date Calculator)

Calculating when you conceived can be difficult, as some women do not experience any implantation symptoms or any signs of early pregnancy. It can be even more difficult to know a conception date and how how many weeks pregnant you may be if unprotected sex occurred multiple times during the fertile period. 

For this reason, conception date calculators will usually provide a result that ranges over about 10 days. This result represents the period in which conception could have occurred. 

Conception is characterized as the the moment when a sperm cell fertilizes and egg, which initiates pregnancy and embryonic development.

Conception date calculator

Enter the first day of your last period below to use our conception calculator: 

It is important to note that this calculator is most accurate for women who have a regular, 28-day cycle. You are advised to use the alternative methods below (2 or 3) if your cycle is longer, shorter or irregular. 

How to calculate conception date

There are 3 possible ways to calculate your conception date: 

1. Using your last menstrual period date 

For women with a 28-day cycle, conception generally occurs between 11 and 21 days after the first day of your last period. Therefore, if you know the first day of period, you can add 11 to 21 days to this day to have an idea of when conception occurred. 

For example, if you are pregnant and your last period started on March 5, that means you likely conceived between March 16 and 26. 

2. Using your estimated due date

This method is similar to calculating your conception date using your last period. It is especially used by women who do not remember when their last period was, and for women who have irregular periods. By knowing your estimated due date, you can calculate when your last period started, and from there, you can calculate your conception date. 

Generally, the due date is calculated 40 weeks after your last period. Therefore, if you have your due date, you can subtract 40 weeks from this date to determine your last menstrual period. Once you know the first day of your last period, you can add 11 to 21 days to figure out your conception date. 

For example, a woman who is due on November 10 can calculate the first day of her last period by subtracting 40 weeks. This means her last period started on February 3. By adding 11 to 21 days, you can calculate that the conception date would have been between the 14th and 24th of February. 

3. Using your gestational age

You can also use your gestational age to figure out when the first day of your last period was, and from there you can calculate the conception date.

For example, if you are 20 weeks pregnant and it is November 15, you should subtract 20 weeks to determine when your last period started. In this case, the first day of your last period would have been June 28. Then, by adding 11 to 21 days, you can conclude that the conception date would have been between July 9 and July 19.