Why do we pretend we "know" most pregnancies last 40 weeks, when most midwives – and parents – will tell you that a baby's exact exit date is the least precise science around? For decades no one has dared to challenge the idea that a pregnancy lasts precisely 40 weeks. The traditional view, here as well as in the US, is that the due date is at 40 weeks and pregnancy should last no longer than 42 weeks. Now a new study is challenging whether having a due date is helpful at all.

A study by the US National Institute of Environmental Health Sciences has found that the length of pregnancy can vary naturally by as much as five weeks. Try telling this to your midwife in the UK, where every woman is expected to give birth within a strict two-week window or face medical intervention.

Only 4% of babies are born on their due date. 80% arrive between 2 weeks before and 2 weeks after. The rest are either premature or late. For parents faced with the latter, this new study is a revelation.

The arrival of a baby is something that needs to be judged on a case-by-case basis, taking into account the health of the mother and baby, plus the mother's birth record, instead of setting arbitrary dates which are not based on sound evidence. If you fall beyond the 42-week limit, you are strongly encouraged to be induced (when labour is started artificially). One in five babies are induced in the UK, and many women choose to undergo it long before the 42-week cut-off. But it carries risks, however small. There is an increased risk of forceps or vacuum extraction, an increased risk of caesarean section and an increased risk to the baby of jaundice.

I faced the decision of whether to induce at 42-weeks with my third baby, Jack, now nearly three. Our two older children had been born naturally, 10 days and 14 days overdue. Those days of waiting are excruciating. With every day that passes, you become more and more convinced that there is something wrong with your baby.

But, despite this, I was determined not to cave in to my own anxiety and allow the baby to come in its own good time. I held out to 20 days overdue (a "10-month momma" as they call it on US pregnancy blogs), before finally consenting to induction. All was well. But I have always wondered when Jack's "real" birthday would have been.

The arguments for induction are serious. As NHS guidelines put it: "There is a higher risk of stillbirth if you go over 42 weeks pregnant, but not every pregnancy over 42 weeks is affected this way. At the moment there is no way of knowing which babies might be at risk, so induction is offered to all women who don't go into labour by 42 weeks." Just reading the word "stillbirth" is enough to make anyone get induced immediately. In reality, the research into births past 42 weeks is limited and out of date. Because, of course, very few people ever go past this date so there is no research.

It has already been acknowledged that these new results are ground-breaking, but it is a shame the study was limited to 125 women. The study found that gestation varied naturally by as much as 37 days. That's five weeks of variability. This news is relevant to every pregnancy. But it's even more relevant in a world where women are giving birth later. The study found that older women had longer pregnancies; there has been a 15% rise in births to women in their 40s in the UK over the last five years.

The study's conclusion? "Too early to make clinical recommendations" but clinicians may want to "keep the results in mind" when deciding whether to intervene in a pregnancy. This is disappointing. Just as we need to know more about whether it is safe to go past the date we've stuck to for so long, we also need far more information about this extraordinary 37-day variation.