How to calculate your due date

One of the most exciting parts about finding out you’re pregnant is calculating your due date.

The big day.

The day your baby (potentially) arrives.

Pop your details in below to find out when your baby is due to make his or her debut:

menstrual cycle calculator

Why Calculate Due Date?

Ok, I know. Who wouldn’t want to calculate their baby’s due date? If only for daydreaming purposes!

But there are actually quite a few reasons behind why it gets calculated:

  • for your medical team to make sure that everything is progressing well.
  • to book in your hospital date.
  • so you can plan for maternity leave.
  • so you can think about what you’ll need for baby, taking into account seasons.
  • so you can count down the days to see your baby / not be pregnant anymore.

How is Due Date Calculated?

Your due date is calculated as 40 weeks after the first day of your last period. 

There are a lot of assumptions going on in there, for example:

  • it assumes your period is a textbook 28 days long. 
  • it assumes you ovulated on textbook cycle day 14.
  • it assumes you have a textbook luteal phase (between ovulation and your period starting) of 14 days.
  • it assumes that your pregnancy will be 40 weeks long, but that’s just an average. It’s not unusual for your pregnancy to last up to or even longer than 41 weeks (though you should stay in touch with your medical team if you’re overdue.)

How Else Can Due Date Be Calculated?

If you don’t have a textbook period… and I sure don’t, your due date calculated using your last menstrual period could be inaccurate. At your initial appointment with your doctor, you may be offered a dating scan. This is an ultrasound scan that will gauge the age of your baby in weeks and will be more likely correct than using last menstrual period for those of us with irregular cycles.

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