How to figure out a conception date

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Most pregnancies last around 38 weeks from conception. Typically women ovulate about two weeks after their menstrual cycle starts, so the best way to estimate your due date is to count 40 weeks, or 280 days, from the first day of your last menstrual period.

Another way to do it is to subtract three months from the first day of your last period and add seven days. So if your last period started on April 11, you’d count back three months to January 11 and then add seven days, which means your due date would be January 18.

This is how your doctor will estimate your due date — and it’s a pretty solid target. But remember: It’s just as normal to deliver a week or two before or after.

You have several options for determining your conception date:

An ultrasound is the most accurate way to see how far along your pregnancy is. Ultrasounds provide a detailed look at your baby’s development and will give you a good estimate of how far along your pregnancy is.
An online pregnancy calculator can give an estimate of when you conceived based on when your last period was. This method is not as accurate as a confirmation ultrasound.

We offer free ultrasounds at our pregnancy center. Our medical professionals are trained in limited obstetrical ultrasound and provide medical services under the direction and supervision of a licensed physician.

How to Know When You Conceived

In general, women can only conceive when they are ovulating – that is, when their ovaries release a new egg into their fallopian tubes for fertilization. Ovulation usually occurs about two weeks after your menstrual period and lasts for two or three days.

However, not all women have the same cycles of menstruation and ovulation. Some women’s menstrual cycles are irregular. Others have ovulation periods closer to or further from their menstrual period. Some women simply aren’t sure when their last period was. In other words, calculating conception based just on your body cycles can be difficult.

You can also simply try to determine the last time you had sexual intercourse, but this method isn’t totally accurate either. Conception is a process and takes time, and a man’s sperm can actually stay alive inside your body for up to a week after sex. All of these factors can make it hard to know when exactly you conceived.

This is why a pregnancy confirmation ultrasound is the best way to know how far along your pregnancy is and to determine when you conceived. Pregnancy ultrasounds look directly at the development of your growing baby to determine its age and when you likely conceived. This method is much more accurate than date-based calculation.

Use the drop-down menu to calculate your due date or current week of pregnancy.

This calculator allows you to calculate the following:

Last menstrual period (LMP): The LMP calculation uses the standard method of the first day of your last menstrual period plus 280 days to calculate the due date.

Ultrasound due date: If a due date has been calculated from an ultrasound in the first trimester, the reverse calculation method is preferred. This calculates the current gestational age and gives an estimated date of LMP.

Conception date: If conception is known (eg, date of insemination, ovulation or egg retrieval), then the due date can be calculated by adding 266 days.

Gestation at a future or past date: Use to calculate gestation in the future or past. First, calculate current gestation using one of the other methods, then go to this section and choose past or future date.

Date at future or past gestation: Use to calculate the date at a specific gestation (enter in weeks). First, calculate current gestation using one of the other methods, then go to this section and choose a gestational age.

To estimate your conception date based on your menstrual cycle, simply add approximately two weeks from the first day of the last menstrual period you had before you became pregnant. This should give you the start of your ovulation period, when your body is most fertile and likely to become pregnant. Ovulation is a normal part of a woman’s monthly cycle, and it usually happens about two weeks after your menstrual period ends.

Keep in mind that this method isn’t 100% accurate. It is possible to become pregnant at any time during the month, not just during your ovulation period. You can even become pregnant during your menstrual period. Also, remember that these estimates are based on the average woman’s regular 28-day menstrual cycle. If your cycles are irregular, this method may not work for you.

You should also know that a man’s sperm can stay alive inside your uterus and fallopian tubes for several days after intercourse. In other words, you might not actually conceive until days after having intercourse.