More than two-thirds of early miscarriages seem to be due to abnormal chromosomes, the packages of DNA within all our cells, in the embryo or fetus. The remaining third are probably also largely due to genetic factors, which we may not be able to detect yet.
Previously, it had been thought that about half of miscarriages were due to these chromosomal abnormalities. Now, a new technique has found that, in fact, the majority of early miscarriages are the result of such problems.
The finding may give some comfort to those who have had a miscarriage, as it shows the event probably didn鈥檛 happen because of anything they did, says at Maastricht University Medical Center in the Netherlands.
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Miscarriages 鈥� generally defined as 鈥� often have no clear cause, although if someone has more than one, the expelled tissue may be analysed to try to understand why.
This is usually done by multiplying the cells in a dish and then analysing their chromosomes, a process called karyotyping. This may find, for instance, that among the 23 pairs of human chromosomes, one chromosome is missing or duplicated.
The drawback with this method is that some cells with abnormal chromosomes can鈥檛 grow and divide successfully, so certain miscarriages caused by abnormal chromosomes are missed.
To overcome that issue, Zamani Esteki and his colleagues used a new genetic analysis method. Looking at the tissue from 1745 miscarriages, with an average gestation of seven weeks, they first used standard karyotyping. The researchers found that 50 per cent of these cases had chromosomal abnormalities, as expected.
Next, the team took a random subset of 94 of the tissue samples that had been found to have normal chromosomes and put them through a technique called haplarithmisis.
This doesn鈥檛 require growing the cells in a dish, but instead involves analysing the DNA of many of the cells together before comparing them with the DNA of the parents, using donated blood samples.
This found that about a third of the cells that were initially classed as having normal chromosomes actually had genetic errors. Put together, this suggests that, in the whole group of miscarriages, 68 per cent would have had such errors. 鈥淐hromosomal abnormalities have a much bigger contribution to pregnancy loss [than found in] previous studies that used conventional methods,鈥� says Zamani Esteki.
at the University of Warwick, UK, says people often wrongly blame themselves for a miscarriage, believing it happened, for instance, because they felt stressed at work. 鈥淏eing told that the pregnancy was abnormal takes away the blame,鈥� she says.
The cause of the remaining third of miscarriages is probably largely also genetic errors that mean the embryo or fetus couldn鈥檛 have survived, albeit ones that are more minor than chromosomal abnormalities.
Nature Medicine
