How to calculate your due date

One of the first questions that will probably leap to mind when you find out you’re pregnant is ‘when am I due?’ Or, if you’re in the throes of early pregnancy symptoms like morning sickness, needing to pee all the time or achy boobs, it might be more like ‘when’s this thing comin’ out?!’

You might be starting to plan your mat leave, when you’ll tell work, and how heavily pregnant you’ll be at your cousin’s wedding in September (will you be able to squeeze into that dress, or will your baby bump make that a no-deal?). 

If you’re eager to know your baby’s ETA, here’s how you can get some answers. 

Ways to work it out yourself

Channel your inner mathematician

If you’ve had regular menstruation cycles, you can probably work out your due date yourself. But fair warning: it will require some maths. You can do this. 

There are two ways:

1. Calculate based on date of conception

If you know you had sex during ovulation, add 266 days to that date to work out your due date. Of course, this only works if you’ve been really closely tracking things, and aren’t a raging sex maniac who did it multiple times during your ovulation. Even if that is the case, your calculation will likely be close enough.

2. Calculate based on the date of your last menstruation cycle

Know when your last period started? Add 280 days to the date it began for an approximate due date. This one works because it takes 14 days after a period for a woman to ovulate and conceive.

Use an online due date calculator

Baby brain cannot compute? We hear you.

Bypass all that mental exertion with the help of an online calculator. To do this, you’ll type in the start date of your last period and the length of your cycle. Then it just spits out an answer, based on the same maths as above. Easy!

Ways to get an accurate pregnancy due date

See your doctor

If you want to confirm your pregnancy due date, pay a visit to your doctor. It’s good to do this at the first signs of pregnancy anyway, so you can connect with your care providers and start planning for birth. 

Your doctor will ask about your menstruation cycles and do all the working out for you. If your cycles weren’t regular or you have no idea when you and your sexual partner conceived this human, they may refer you to get a dating scan. 

If you have certain health issues or pregnancy anomalies (like TWINS!) you may even start to discuss with your birth team when you’ll deliver. You might need to plan for a particular type of birth, whether it’s a planned induction at 38 weeks (often recommended for women with diabetes), or a planned c-section to prevent complications.

Get an ultrasound

Fetal measurements are one of the most accurate ways to get a read on your due date. The technology they use today is AMAZING, and we dare you not to cry when you see your baby’s heartbeat for the first time! (Note: this appears around the 6 week mark).

By measuring your bubba’s length, head size and more, your doctor can determine the approximate age of the fetus based on average growth rates, and give you a very accurate due date. You’ll have a few scans during your pregnancy, but due date estimates are most accurate early in pregnancy as babies tend to grow at the same rate early on. 

Later in pregnancy, you’ll be able to see exciting features like the sex of the baby (from around 12 weeks), tiny fingers and toes, and even facial features! It’s incredible. But the further along you get, the less accurate ultrasounds will get for estimating the due date. 

Remember, your due date isn’t the guaranteed date of your birth. Doctors don’t wave magical wands to set a date for your baby to decide to arrive – as great as that’d be. Only 4% of women actually give birth on their due date, so you’re more likely to go into labour earlier or later than this. But at least it’s a rough date to plan towards!

You’ll find more helpful tips, high fives, and encouraging pats on the bum on Mumli, your pocket-sized personal assistant for all things motherhood – coming to your home screen soon. Join the waitlist now.

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