“When did I conceive” is a common question during early pregnancy, especially when menstrual cycles are irregular or early symptoms are unclear. Pinpointing the exact date of conception can be challenging, but several methods can help estimate it with reasonable accuracy.
The conception date typically falls between 11 and 21 days after the first day of the last menstrual period. Knowing when conception occurred helps determine gestational age and estimate the due date, which is essential for prenatal care planning. 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.
According to the American College of Obstetricians and Gynecologists, estimating gestational age by the last menstrual period assumes a regular 28-day cycle with ovulation occurring around day 14, which may not apply to everyone.
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 February 14 and 24.
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.
When did I conceive if I have irregular periods?
Determining when did I conceive can be more difficult with irregular menstrual cycles because ovulation does not occur at a predictable time. According to the ACOG, dating a pregnancy based only on the last menstrual period is less reliable in people with irregular cycles, making early ultrasound the preferred method.
First-trimester ultrasound measurements, particularly crown-rump length, are considered the most accurate way to establish gestational age in these cases. The ACOG notes that pregnancies without an ultrasound before 22 weeks are considered suboptimally dated, highlighting the importance of early imaging when cycle length varies.
Is conception date the same as the day I had sex?
The conception date is not always the same as the day intercourse occurred. Sperm can survive in the female reproductive tract for up to five days under optimal conditions, while the egg remains viable for about 12 to 24 hours after ovulation.
Because of this, conception may occur several days after intercourse if sperm were already present when ovulation happened. This explains why the estimated conception date often falls after, rather than on, the day of sexual activity.
How accurate is an ultrasound for conception dates?
Ultrasound is considered the most accurate method for determining gestational age, especially in early pregnancy. According to the ACOG, first-trimester ultrasound dating using crown-rump length has an accuracy of approximately plus or minus 5 to 7 days.
Accuracy decreases later in pregnancy, with second-trimester ultrasounds being less precise and third-trimester measurements showing the widest variation. For this reason, ACOG guidelines emphasize early ultrasound as the gold standard for confirming gestational age and estimating when conception occurred.