You’ve just found out you’re pregnant… so, naturally, one of the first things you’ll be wondering is how many weeks pregnant am I?!

It’s important to understand that a due date is calculated using a simple formula based on averages.

Not your own unique body, exactly when your baby was conceived.

So that magic, exciting due date won’t be totally accurate, but it’ll be close.

To find out how many weeks you are, use our pregnancy calculator. You’ll need to know the first day of your last menstrual period.

How many weeks pregnant am I?

It’s common to hear a pregnancy is 40 weeks long. When in fact, the length of pregnancy is more of a range.

According to the American College of Obstetricians and Gynaecologists:

Early term is from 37 weeks of pregnancy, through to 38 completed weeks of pregnancy
Full term is from 39 weeks of pregnancy, through to 40 completed weeks of pregnancy
Late term is from 41 weeks of pregnancy, through 41 completed weeks of pregnancy
Post term is from 42 weeks of pregnancy and beyond.

As little as 3-5% of babies are born on the estimated due date, with roughly 40% born in the two weeks prior, and 40% in the two weeks after.

So, once you’ve worked out the estimated due date, you might like to announce that your pregnancy due date is sometime in October, for example, instead of the 4th of October.

Why? So your friends and family don’t jump all over you and hound you with calls, text messages, Facebook messages and emails all day on the 4th of October!

It might sound fun now, but ask anyone who is about to have a baby, and they’ll wish no-one knew of their estimated due date!

Why the due date is really the ‘guess date?”

The estimated due date – or as I like to call it, the ‘guess date’ – is simply in the middle of the range of full term.

So if you prepare for the likelihood of 42 weeks of pregnancy (especially if you’re expecting your first baby, as they tend to take their sweet time), you’ll experience less disappointment and communication overload if your magic date comes and goes.

First pregnancies tend to last longer than subsequent pregnancies, but of course, not always.

Many factors come into play, so you can never be completely accurate as to a baby’s due date.

Even the experts don’t have an advantage when working out the date!

In a study on due dates, doctors, midwives and mothers had to guess what a baby’s day of birth would be.

The results?

No-one was more accurate than the other!

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So, how many weeks along is my pregnancy?

Week 1 of your pregnancy starts on the first day of your last period.

Weird huh?!

But the first day of your last menstrual period provides an exact date doctors and midwives can go by for the sake of specifics.

With assisted conception, for example IVF, you might know exactly when the egg was fertilised.

But for other pregnant women, the exact moment of ovulation and conception is usually unknown.

Due dates are notoriously inaccurate, due to the way your dates are calculated.

It’s assumed you have a 28 day menstrual cycle with ovulation on day 14.

As you likely know, every woman has a unique cycle length and ovulation day.

You may have a shorter or longer cycle than 28 days and you may have ovulated any time in between!

Pregnancy due dates are calculated based on averages, so a date can be given.

Therefore when you miss your first period, you’re already 4 weeks pregnant.

Okay, got it, but how many weeks am I, you ask…

To find out how many weeks you are, use our pregnancy calculator.

It’ll give you an estimated due date, as well as an estimated date of conception.

It will also tell you how many weeks pregnant you are, and how long you have to go until you reach the end of full term pregnancy at 42 weeks.

What if I don’t know when my last period was?

If you don’t have a regular 28 day cycle or if you can’t remember when your last period arrived, you can book an early pregnancy ultrasound to estimate how many weeks pregnant you are.

Bear in mind, the due date based on your own, unique cycle is most accurate, followed by early ultrasound, then late ultrasound.

The reason for this is when a baby first forms, they follow a predictable growth pattern, as they develop all the initial building blocks. But as your baby gets bigger, he or she will develop at their own rate – genetics come into play.

Ultimately, it’s best to avoid ultrasounds if you can, because ultrasound heats tissue. Especially early ultrasounds (in the first trimester of pregnancy), which are often internal. Your developing baby will have the internal ultrasound wand placed very close to their brain.

Have an ultrasound if you must, but try to keep the time to a minimum and avoid any that are unnecessary.