Most fertilised eggs miscarry rather than develop into healthy babies. However, most miscarriages usually occur very early in pregnancy, without a woman even being aware she was pregnant.
A few previous studies have suggested that miscarriages are the most common outcome of conception. Now a meta-analysis claims to have 鈥渦nambiguously鈥� confirmed it.
For women in their 20s, conception is as likely to end in miscarriage as it is in a live birth, says William Rice, an evolutionary biologist at the University of California, Santa Barbara. 鈥淚t is not an abnormality,鈥� he says. 鈥淚t鈥檚 the norm.鈥�
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The rate shoots up as women age, with women in their late 40s having more than 30 miscarriages per newborn. 鈥淚t starts high and it gets extremely high,鈥� says Rice.
Miscarriage frequency
He combined data from a number of disparate sources to work out the frequency of all miscarriages, including the very early miscarriages that women are not usually aware of. For instance, IVF records reveal how many live births result per embryo implanted. Other studies have looked at how the rate of lethal chromosomal abnormalities in eggs rises as women age.
Rice gives two examples. Women in Denmark, who have free access to birth control and elective abortions up to 12 weeks, have 1.7 children on average. These women have 2.1 miscarriages in their lifetime on average, Rice calculates, along with 0.4 elective abortions.
By contrast, frontier Mormon women in the US in the 19th century, who had no birth control and an average of eight children each, had an average of 16.8 miscarriages in their lifetime, he calculates.
Whether or not these numbers are correct, there is no doubt that miscarriages are very common and also that in most cases women don鈥檛 know they have happened, says Ruth Atik, director of the Miscarriage Association in the UK.
More education
鈥淚 do think it would be enormously helpful if there were more education and thus general awareness that it can take time to conceive and that many pregnancies do end in miscarriage,鈥� she says. 鈥淭hat often comes as a shock to women and their partners.鈥�
鈥淗uman reproduction is a very inefficient process,鈥� says Sohinee Bhattacharya at the University of Aberdeen, UK, who studies pregnancy loss. But she questions the relevance of the finding. 鈥淔rom a clinical point of view, how important is it if the woman does not even know she is pregnant?鈥�
Rice argues that the high rate of miscarriages 鈥� often called 鈥渟pontaneous abortions鈥� by doctors 鈥� poses a challenge for creationists who believe in intelligent design. 鈥淭he idea that spontaneous abortion is a rare mistake is a misconception,鈥� he says.
Miscarriages can occur for many reasons but chromosomal abnormalities are the most common cause, says Rice. The eggs in women鈥檚 ovaries are paused partway through the process of duplicating their chromosomes and as they age the chances of the process going awry rises exponentially.
bioRxiv
Read more: New clues to preventing miscarriage or pre-term births
