New research shows most human pregnancies end in miscarriage

It's treated as a taboo subject, but miscarriages of pregnancy happen a lot.

Well according to a new paper, they happen a lot more than any of us may realise - even the women having them. The research has found that more than half of successful fertilisations will end in miscarriage.

The research, which has yet to be peer reviewed, was penned by evolutionary geneticist William Richard Rice of the University of California, Santa Barbara, and draws upon many previously conducted studies and health databases for a meta-analysis.

Previous research has found that somewhere between 10 and 20 percent, or as many as 1 in 4 known pregnancies end in miscarriage, also known medically as spontaneous abortion.

One study in Denmark, which included 1,221,546 pregnancies between 1978 and 1992, found the overall miscarriage rate was 13.5 percent.

Obviously that's going to vary by region and socioeconomic status, and the risk sharply increases by age.

But there's one other very important qualifier to note: Many women don't even know they're pregnant initially; and, since most miscarriages happen in the first 12 weeks of pregnancy, many miscarry without even knowing it's happening.

It's these unknown miscarriages that make up the majority, according to Rice. In fact, he found that a woman in her 20s is just as likely to miscarry as carry the foetus to term.

And, because the miscarriage rate only rises with age, the number of miscarriages far outnumber live births, his analysis asserts.

"It is not an abnormality," he told New Scientist. "It's the norm."

It's not just known pregnancy statistics that provide this information. We can also learn a lot from IVF statistics.

A 2014 IVF study found that, of 284 successfully karyotyped embryos from young women, 151 had abnormalities in the number of chromosomes - a rate of 53.2 percent.

This sort of abnormality, called aneuploidy, is the most common cause of miscarriages, accounting for 50 percent, and the risk of it rises with age.

Using information such as this, Rice calculated that women in Denmark, based on the aforementioned study, have 1.7 live births on average in their lifetimes - but 2.1 miscarriages. And this is a country with access to free birth control and elective abortions up to 12 weeks.

By contrast, Mormon women in America in the 19th century had an average of 8 live births, and 16.8 miscarriages.

It's a finding that suggests two things. Firstly, that miscarriage is "the predominant outcome of fertilisation" and "a natural and inevitable part of human reproduction at all ages," Rice wrote in his paper.

Secondly, that access to birth control can reduce the number of miscarriages a woman will have in her lifetime.

While the actual numbers will likely never be known for certain, it's common sense that the use of birth control vastly reduces the risk of miscarriage while in use - because it vastly reduces the risk of pregnancy in the first place.

So it's unsurprising that this is also what Rice's numbers reveal.

"To reproduce, a human female cannot forgo a high risk of [spontaneous] abortions, and to have a large family it is virtually impossible to avoid multiple [spontaneous] abortions," he wrote in his paper.

"Modern birth control with access to elective abortions, markedly reduces - rather than increases - the lifetime number of abortions a woman produces."

You can read the paper for yourself on preprint resource bioRxiv.