How we calculate your due date, and why it is 9 months and not 10

By Jenny, from Midwife and Life

When you get pregnant, it’s one of the first things you want to know, when is my due date?  Nowadays there are lots of online calculators you can use.  Then, people start questioning, why 40 weeks?  Isn’t that 10 months?  And why am I 4 weeks pregnant when 4 weeks ago was the start of my period?  What happens in a leap year?

Digital tests and due dates

The introduction of the digital pregnancy test hasn’t helped this confusion, when you take that test it either says ‘Pregnat 1-2 weeks,’ ‘Pregnant 3-4 weeks,’  or ‘Pregnant 4+ weeks.’ These tests count pregnancy from conception (ovulation and fertilisation of the embryo).  A lot of women come to their booking with this in mind and then we change it for them!  The pregnancy tests are looking at the strength of the pregnancy hormone HCG in the urine, and according to how much it finds, gives you a corresponding reading.

How your due date is calculated

What all the due date calculators do, is they assume every woman has a 28 day cycle, so that day 1 is the first day of your last period, 14 days later ovulation occurred and fertilisation happened, then on day 28 would be the day of your missed period and a positive pregnancy test.  You would then be 4 weeks pregnant.  So for the first 2 weeks you’re not actually pregnant, you’re only pregnant for 38 weeks – remember a month is slightly longer than 28 days, spread that out over the 40 weeks, minus the 2 weeks at the beginning and you roughly get 9 months.  Phew!

The thing is, women aren’t all identical, and cycles can vary from 21 days to very irregular cycles, some get pregnant after having their last baby without having a period in between.  The most accurate way to ascertain your due date is from an ultrasound scan between 8 and 14 weeks.  Scans done after this aren’t as reliable at dating due to familial aspects coming into play like long legs, bigger heads etc.

As midwives you may have seen us use a magical wheel which tells all, this is essentially the same as the online calculators, we point the arrow at either your due date or first day of last period, and then look at today’s date to tell us how many weeks you are or at the due date arrow to see your due date.  It’s not infallible!  The way I was taught when I was a student midwife by a lovely old school midwife was to take the first day of the last period, add 7 days and minus 3 months.  So if your period date was 25th Feb, the due date would be 4th December.  This also gives you a rough idea.

How do we calculate your due date?

But I know exactly when I conceived!

You may know the exact time you had sex to make the baby (maybe it was the only time that month or the only time without contraception), but sperm can live inside the woman’s body for up to five days,  so we never really know when fertilisation happens or when the embryo actually implants, so there may always be a discrepancy.

How your due date is calculated.

Remember, the due date is just a guide, once it’s confirmed, try and treat it as a window of time rather than a date set in stone.  It can feel so disheartening when it comes and goes and you feel ‘late.’  You’re not late for anything, your baby or your body isn’t ready yet.  In other countries, they count the due date differently, but the end result is the same.  Try as we might, we don’t have an awful lot of control over the situation, you may have complications which mean baby comes earlier, or they decide to come when it’s most inconvenient, like mine did!

Oh, and leap years?  Well, they didn’t really think of that, so factor that in!  Clear as mud?  Good.  Any questions, comment below.

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