More than half of all pregnancy losses are caused by randomly occurring chromosomal abnormalities in embryos and fetuses, according to the biggest study of its kind.
Having a miscarriage is often highly traumatic and can leave people wondering why it happened. One US survey found that 41 per cent of those who lost a pregnancy , like lifting heavy objects or being too stressed.
In the past, there was no way of knowing what caused some pregnancies to end, but advances in genetic testing mean tissue from miscarriages can now be analysed for possible causes.
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“Clinicians are increasingly sending us these samples so they can provide patients with answers and help them try to overcome the self-blame that often happens,” says at Invitae, a genetic testing company based in California.
Out of the 24,900 samples analysed by the company – of which 98 per cent were from embryos and fetuses that had been naturally conceived – 56 per cent had chromosomal abnormalities. These included extra or missing chromosomes, or chromosomes with duplicated or deleted segments.
The abnormalities probably prevented the pregnancies from progressing because they made the embryos and fetuses incompatible with life, says Sahoo.
About 65 per cent of the samples were from so-called sporadic miscarriages, “random, out-of-the-blue pregnancy losses”, says Sahoo. Another 22 per cent were from people with what is known as recurrent pregnancy loss and 10 per cent were from stillbirths, pregnancy losses after 20 weeks’ gestation. The remaining samples were from rarer forms of pregnancy loss.
All the miscarriages occurred at eight weeks’ gestation or later.
Chromosomal abnormalities are common in human embryos because of random mistakes that can happen when egg and sperm cells divide and multiply in the ovaries or testes, before conception occurs. If chromosomes are imperfectly copied during this cell division and “if the resulting egg or sperm gets fertilised, the result is a genetically unbalanced embryo”, says Sahoo.
Similar chromosomal errors can also sometimes occur when cells in the early embryo are dividing and multiplying, says at Oregon Health & Science University.
In the 56 per cent of cases where a chromosomal abnormality was found, it may have been reassuring for people to know a pregnancy loss wasn’t because of anything they did or anything that was wrong with them, says Sahoo. “But we’re still trying to find answers for the other 44 per cent,” he says.
A proportion of these other miscarriages may have been caused by mutations in single genes that couldn’t be detected by current methods, whose resolutions are not sufficiently high to pick up smaller mutations, says Sahoo. His team is working on ways to identify these.
Some of the other 44 per cent of miscarriages may have been caused by certain medical conditions, like hypothyroidism and endometriosis, which are known to increase the risk of pregnancy loss, says Chavez.
A number of environmental factors, such as stress, smoking and exposure to air pollution, have also been associated with a slightly higher miscarriage risk, but they probably only cause a small fraction of pregnancy losses, she says. “They play a much more minor role than chromosomal abnormalities for sure.”
Reproductive BioMedicine Online
