Reverse Due Date Calculator

What is a Reverse Due Date Calculator?

A reverse due date calculator is the exact opposite of a due date calculator. A due date calculator works when you know the date of your last menstrual period (LMP) and it helps you predict baby’s due date from there. But a reverse due date calculator works when you know your due date, but you’re not exactly sure when you had sex or conceived the baby.

Why Use a Reverse Due Date Calculator?

• Maybe you had a dating ultrasound at your first prenatal appointment that provided your baby’s gestational age and due date.
• Maybe you want your baby to be born in September or later in the year, so you want to know when to conceive.
• Or maybe you’re feeling nostalgic and wondering when you or one of your children were conceived?

Whatever the case, you’ve come to the right place. Our reverse due date calculator can help you do the math in an instant.

• A reverse due date calculator uses your estimated due date (or the date of birth) to figure out the approximate date of conception.
• A reverse due date calculator will also tell you the window in which intercourse most likely led to conception.

What Is the Date of Conception?

The date of conception is date a baby is created, but not necessarily the date you had sex (more on that below). Conception occurs when sperm comes into contact with and fertilizes the egg that is released from a woman’s ovary during ovulation.

Can You Calculate the Exact Day of Conception?

Precisely when conception happens is a little bit more of a mystery. Pinpointing the exact date of ovulation is complex. Most women ovulate between days 11-21 of their monthly cycle, but as you can see, that’s a big window of time. What’s more, many women don’t closely track ovulation by monitoring cervical mucus and basal body temperature, and even those who do can be off by a day or more.  

To add to the confusion, conception can occur in a few hours after intercourse, or it can occur up to a week after intercourse! How is that possible? If the conditions are right, sperm can live inside a woman’s body for up to a week.

Why Is the Date of Conception Important?

Most due dates are calculated off of a woman’s last menstrual period (LMP). That’s how our due date calculator works. The problem is that this method of establishing an estimated due date isn’t always accurate, especially for women who don’t have a standard 28-day cycle. Still, it’s become the standard, since most women are more likely to know the exact date of their LMP than the exact date they ovulated.

The whole concept is a bit confusing though. According to this pregnancy dating method, your first—and sometimes even second—week of pregnancy occurs before you even did the deed! Huh?! 

Therefore, pinpointing conception—when sperm and egg actually meet to create new life—is a much more accurate representation of when your pregnancy began. And it can help you answer the question “How many weeks pregnant am I?”

If you’re currently pregnant, having a clear idea of when your pregnancy actually began is important at the end of your pregnancy when the conversation may turn to induction. There are risks for both mama and baby if induction occurs before the body is ready to go into labor, so if your healthcare provider thinks you’re further along than you really are, you could be encouraged to try induction sooner than necessary.

And that’s where the reverse due date calculator comes in.

How Do You Calculate Date of Conception?

Much like a due date calculator returns an estimated due date, a reverse due date calculator will return an estimated date of conception. Although it may seem like it, a reverse due date calculator isn’t magic—it’s just some simple math.

For a regular 28-day cycle, pregnancy is estimated to be 280 days, or 40 weeks, from your last menstrual period, and 266 days, or 38 weeks, from the date of ovulation. Therefore, to calculate the most likely date of conception, you need to subtract 266 days from your estimated due date. 

What Is My Due Date Based on Date of Conception?

If you know with relative certainty when you conceived or have used our reverse due date calculator, use that option in our due date calculator or our advanced due date calculator to get a more accurate estimated due date.

How Many Weeks Pregnant Are You?

Pregnancy is exciting, and it’s fun to use tools like the reverse due date calculator to learn as much as you can about that tiny human growing inside of you. To help, we’ve created the free Mama Natural Pregnancy Week-by-Week series.

The first thing our due date calculator tells you is, of course, your estimated due date. But the second thing it tells you is how many weeks pregnant you are. If you scroll down the page, you’ll get all kinds of information about baby’s development, plus what to expect when it comes to your changing body.

To get this information delivered straight to you each week, sign up for the whole series through Facebook Messenger or email, or visit this page to skip ahead and see all 40 weeks right now.

Ready to Use Our Reverse Due Date Calculator?