Why do they say pregnancy is 9 months and not 10?

It’s commonly held that human pregnancy lasts nine months, but then you fall pregnant and you do the maths and suddenly find yourself very confused.

Well, obviously every pregnancy is different and babies can be born well before (and after) their due date...

Typically, pregnancy is measured as 40 weeks (280 days) and broken into three trimesters of approximately 13 weeks each, so it’s not quite as simple as saying 9 months. 

So what are the three trimesters?

• 1st trimester = weeks 1-13
• 2nd trimester = weeks 14-27
• 3rd trimester = weeks 28 until 41 (or whenever your baby arrives). 

‘Full term’ is when the baby is considered fully formed and would survive in the outside world and is generally counted from week 39 (weeks 37-38 are called ‘early term’). Beyond week 42 the placenta starts to become unviable and poses a risk to the mother and baby. 

So when you factor in a month(ish) of being pregnant before you even realise it, and then the potential to go overdue it’s more like 10 months in the end. 

So how do you work out your due date?

The first week of your pregnancy is actually counted as your last menstrual cycle (so before conception even actually takes place – just to confuse you even more!) and so your expected due date will be counted as 40 weeks from the first day of your last period. 

Alternatively, if you know the exact date you conceived you can then add 38 weeks on this. 

But don’t worry if you can’t remember when any of these dates were, when you have a scan your doctor will be able to gauge your due date. This tends to be more accurate as every woman’s cycle is different and the day in which she ovulates – and the egg is fertilised – will vary. 

Wait, what is the fourth trimester?

The fourth trimester you ask? Don’t worry its not another hidden trimester that you’ll be pregnant for, this is actually what is referred to as the period from when your baby is born to when they are three months old.

While it might seem odd to still refer to it as a ‘trimester’ when the baby is no longer in the whom it is a way to understand the transition that the baby is going through now on the outside and acknowledge that it is still a period of major growth and development. 

When my wife became pregnant with our first child, I believed women were supposed to carry a baby for nine months before giving birth.

But I soon learned this was a myth.

No matter how you estimate the duration of a "typical" pregnancy — using the latest scientific data on pregnancies or converting months into weeks or days — nine months misses the mark.

Just 4% of pregnant women deliver a baby right at 40 weeks, which is a number used interchangeably with nine months.

But even nine months does not equal 40 weeks, or 280 days — another number you'll hear from doctors, government agencies, nonprofit organizations, and other sources of information you encounter during a pregnancy.

That's because months vary in days. Averaging 365 days over 12 months, your average month lasts 30 days and 10 hours. So 280 days (or 40 weeks) is actually nine months plus nearly a week.

While this is a minor quibble, the core figure of 280 days is also suspect.

The number first appeared in a German manual for midwives published in 1836, and it's an estimate from a woman's last period all the way through to birth. That's because ovulation can be notoriously difficult to detect, and fertilization even more so.

So researchers at the NIH in 2013 — equipped with the latest tools of science — decided to check this number by instead measuring a pregnancy from the moment of ovulation (fertilization usually occurs within 12 to 24 hours after an egg is released).

The NIH researchers followed 125 women, starting with a sensitive chemical test for ovulation all the way through to the birth of one child. (Another mark against "nine months": A vast majority of twins and other multiples are born in the seventh or eighth month of pregnancy.)

The researchers discovered that the median pregnancy was not 280 days, but 268 days — and that was after excluding pre-term or post-term babies and accounting for a woman's weight, alcohol use, sex during pregnancy, and other factors.

This means a "typical" pregnancy likely lasts 38 weeks and 2 days — or 8 months, 24 days, and 16 hours. — if you're counting from fertilization of an egg. (Note: If you're counting from the last menstrual period, which is a very imprecise way to measure pregnancy duration, it's more like 40 weeks and 5 days.)

The NIH study's real kicker, however, is how much the length of a traditional pregnancy can vary: by a whopping 37 days, or 5 weeks, spanning before and after the 268-day mark.

The reality is that every woman's pregnancy is different, as science writers Tara Haelle and Emily Willingham make abundantly clear in their meticulously researched book, "The Informed Parent: A Science-Based Resource for Your Child's First Four Years".

And if you aren't convinced this actuarial nitpicking matters, consider the fact that some doctors can and do rush pregnant women into cesarean sections because of timing concerns, and the procedure — while typically very safe — not only costs thousands of dollars more than a vaginal birth, but also increases many kinds of risks to babies and their moms.

All of that said, the NIH researchers did not go so far as to make a clinical recommendation; after all, 125 women is a small sample size.

Yet it stands to reason that freely passing around a nice, round, and imprecise number could have some real and unfortunate consequences.

This story has been updated.

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