Due Date Calculator: How Many Weeks Pregnant Am I?

Okay, so you got your positive pregnancy test (insert whatever emotion you are currently feeling here). Naturally one of the first things you’re going to want to do - after washing your hands - is figure out when this little babe is going to make their grand entrance.

When it comes to calculating your due date, there are a few key factors that will help give you an idea of when the big day might be.

Things that are helpful to know:

- First day of your last menstrual period

- Your average cycle length

- Date of conception or embryo transfer

- If you’re carrying a single embryo, or if there are twins (or triplets!) in there

Of course even not knowing any of these things, your medical practitioner should be able to give you a pretty good idea of how far along you are either by performing a first trimester ultrasound, or a blood test at your first prenatal appointment. But if you’re a ways out from that appointment and want to get a general idea, a due date calculator can help!

How a Due Date Calculator Works

Assuming your cycle is 28 days, that means your luteal phase - the time after ovulation, where an egg travels down the fallopian tube chancing an encounter with a sperm - typically happens around day 12-14 of your cycle. This is your window of opportunity, and is the time frame where fertilization generally occurs. But for those who have longer or shorter luteal phases, conception can happen several days in either direction of this window, moving the due date accordingly. A due date calculator uses these numbers and gives you a rough estimate that doesn’t take into account any other factors that can influence your due date.

Fortunately, this is just one calculation method, with several others that can produce more accurate estimates.

Naegele’s Rule

There are a few schools of thought about how to get the most accurate due date, but the most common method, Naegele’s Rule, is made by counting out 280 days from the start of your last menstrual period. Just like any due date calculator, this one isn’t exactly perfect since it makes the assumption that you have a 28-day cycle, but it’s a fairly good place to start.

A Slightly More Comprehensive Method

The Mittendorf-Williams rule predicts the average due date to be 288 days from your LMP, and uses 16 factors that may have an influence, like age, medical history, and ethicity to name a few. Studies suggest it is twice as accurate as Naegel’s Rule, but again, all due dates are a rough estimate of when the baby will be here, rather than a hard and fast rule.

Are Ultrasounds the Most Accurate Method?

According to ACOG (the American College of Obstetrics and Gynecology), the most accurate way to establish or confirm gestational age is an ultrasound where precise measurements are taken of the embryo or fetus. Ideally this occurs at or before 13.85 weeks of gestation. That said, while knowing gestational age is a huge help in predicting when your baby will be done cooking, it’s still just an estimate, and your baby may come before or after the provided date.

What If I’ve Had IVF or Was Otherwise Medically Assisted?

Pregnancies that require a little help from your friendly gynecologist can complicate using a due date calculator because of a few factors, even if you know your exact conception date. The age of the embryo when they were transferred, if it was a day-three transfer or a day-five transfer, if it’s multiples - all of these things can make determining an accurate due date with a generic calculator tricky. Fortunately, your medical provider can give you a much more accurate date at (or possibly even before) your first prenatal appointment.

But Still - It’s Just a Guess

No matter what method you use for calculating your due date, only about 4% of babies are born on their actual due dates, and designating a birth month is a more accurate (albeit more annoying) way to think about things.

Babies. Unpredictable and teaching us patience and flexibility literally from minute one. Let the games begin, right?!