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Probability you won't deliver on your due date

Date: Friday, Aug 14, 2020

You've seen the positive pregnancy test and have celebrated the good news with your partner. The next thing to do is to see your doctor to plan for the next nine months or so.

Once your pregnancy is confirmed by your gynaecologist, they will give you an estimated due date for your baby. This is done by adding 280 days – the equivalent of 40 weeks – to the first day of your last menstrual period.1

Is 40 weeks accurate?

This method of calculation is more accurate for women who have regular periods and get them every 28 days. Generally, if your cycles are longer, you'll probably deliver later than your given due date, and the opposite is applicable if you have shorter menstrual cycles.

How is the length of your cycle relevant to your baby's due date?

Conception only happens when a woman is ovulating and, the longer your cycle, the later ovulation takes place. So while a woman with a 28-day cycle would likely have conceived around 14 days after the first day of her last period, conception would have taken place a few days later for a woman with a 32-day cycle.

Your due date might also be adjusted after your first ultrasound. Your doctor will measure the length of your foetus and the size of your gestational sac and calculate a due date from this, which might vary from the estimated date of delivery you've been given.2

How much variation can there be under or over 40 weeks?

The vast majority of women give birth between weeks 38 and 42. Only around five per cent of women have their babies born exactly on the due date given. Even though a pregnancy is calculated to be 40 weeks long, a foetus is considered to have reached full term at week 37. A baby born at less than 37 weeks gestational stage is considered premature.3

All of your baby's organs are developed by week 37, so they are technically ready to be introduced to the world from this point. Actually, it’s not surprising for your baby to be born one or two weeks before or after your due date. And, if it's your first pregnancy, you're more likely to go past your due date, compared with women who have had babies before.

Also, if this isn't your first baby, your previous pregnancies will give you a better picture of when you'll give birth, whether you'll be before or after your due date. If this is your first time, you can get a clue by asking your mother about her childbirth history.4

How important is the due date, really?

So why the need for a due date if it's not an accurate reflection of when pregnant women actually give birth? Even if it's not a spot-on guide to your delivery date, it acts as a marker for the appropriate tests that have to be done during a pregnancy.

Most prenatal tests can only be done at specific times for both mother and baby’s safety; so it's good to have a due date to refer to.

However, there's no need to put too much emphasis on this one date given by your doctor, as it might cause unnecessary anxiety around what is already a stressful time for many soon-to-be-mums!

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