When Did I Conceive? How to Calculate Your Conception Date

By JustMommies staff

A ring of plastic keys against a calendar.

You’re pregnant! Now that you’ve received the wonderful news, one of the first things you’ll want to know is the due date. There are several key dates at the beginning of your pregnancy that can help you calculate your due date, including the first day of your last menstrual period and the date of conception. But sometimes at the beginning of pregnancy all of the timing seems a bit confusing. So, here’s a quick rundown on how the calculations are made.

How to Calculate Conception

A doctor meeting with a man and a woman.

Many women assume that the date of conception is the same date that they had intercourse. Since sperm can live inside your body for as long as five days, however, conception could happen up to five days after intercourse, depending on when ovulation occurred.

Unless you’ve had in vitro fertilization or some other fertility treatment, you probably will not know the exact date of conception. In fact, most fertility calculators will begin with the assumption that your date of conception is two weeks after the first date of your last menstrual period. That’s because ovulation and conception generally comes in the middle of your menstrual cycle, and the calculations assume a normal 28-day cycle.

Doctors count the length of a pregnancy as 280 days, which equals 40 weeks or 10 months. Why not nine months? The counting begins with the first day of your last normal menstrual period before you got pregnant. In other words, the counting begins about two weeks before you’ve even conceived. Why does the counting begin that early? It starts at that point primarily because it’s difficult to pinpoint the exact date a woman conceives, and the first date of her last menstrual cycle is a much easier marker to identify. Once you have that date down, you can establish your estimated due date by adding 40 weeks, or 280 days.

Chart Your Ovulation for a Precise Date

An ovulation chart.

But let’s say you want to be more precise based on your specific circumstances. If you’ve been charting your ovulation date, you can pinpoint your conception date more precisely because conception would have to occur within 24 hours of your ovulation. So how do you identify your ovulation date? No matter how long or short your menstrual cycle, the luteal phase of your cycle (the second half after ovulation) is almost always 14 days. If your menstrual cycle is 30 days long, you probably ovulated on day 16. If your menstrual cycle is 24 days long, you probably ovulated on day 10, and so on, subtracting 14 days from the length of your cycle to pinpoint the ovulation date. It’s safe to say that the date of conception would either be the day of or the day after ovulation.

Utilize Online Tools

Of course, the primary reason that most women seek this information is to learn what the due date will be. For that, you can turn to Just Mommies’ due date calculator, which will give you the projected due date based on either your date of conception or the date of your last menstrual period.

Alternatively, if you already have the due date and you’re looking to pinpoint the date of conception (did it happen on that romantic getaway weekend?), you can use an online conception calculator, or you can take your due date and subtract 266 days. That reflects a total of 280 days for the pregnancy, minus 14 days for the first two weeks before you ovulated and conceived. Got that? Good, because now it’s time to focus on what’s really important—the healthy development of your new little one.