How many weeks are in an average pregnancy

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.

The length of a normal pregnancy can be anything from 37 weeks to 42 weeks. So you have a five-week period when your baby could arrive at any time and it would be considered normal.

If you give birth before 37 weeks of pregnancy, your baby is defined as premature and is likely to need extra care. A pregnancy that lasts longer than 42 weeks is called a prolonged pregnancy and may carry a higher risk of complications.

Even though all pregnant women are given a due date or estimated date of delivery (EDD), only about one in 25 babies are born on their exact due date.

Most babies arrive between 37 weeks and 41 weeks of pregnancy, usually within a week either side of their due date. Twins and particularly triplets are more likely to be born premature.

The due date you are given at your dating scan will be more accurate than using your last period to calculate it. The day of ovulation in the menstrual cycle can vary from woman to woman depending on their cycle length, while a scan can pinpoint the exact week and day of pregnancy based on the size of the embryo.

The length of your pregnancy may also be influenced by how long your baby took to implant when she was conceived. Fertilised eggs that take longer to implant tend to have a longer gestation from implantation to birth.

Mothers with naturally longer pregnancies also tend to be older, have had longer pregnancies in previous births and have been heavier at birth than mothers with shorter pregnancies.

Tall women tend to have longer pregnancies than short women, and white European women tend to have naturally longer pregnancies than African-Caribbean or Asian women.

It's common for babies to arrive a week or so after their due date. About one in five babies are born at 41 weeks of pregnancy or more.

It's thought that between one in 10 and one in 20 women has a pregnancy that would naturally last longer than 42 weeks. Although most women in the UK have their labour induced before they get to this point.

Chess Thomas is BabyCentre's research editor and a qualified antenatal teacher.

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