Women (and society) often believe it is something they did, or did not do, that caused the miscarriage. Early pregnancy tests mean that women often know they have miscarried when in earlier times it would have been put down to a late period. Much scientific research has gone into how common miscarriage is, and why it happens. An article in New Scientist brings together the latest findings.
- In 1975, researchers calculated that the number of babies born to married women in their 20's was 78% lower than expected. They proposed that miscarriages typically happen early and are a method of quality control. New research supports this theory.
- Today it is thought that for women in their early 30s, 60% of pregnancies end in miscarriage.
- The general population think miscarriages happen infrequently because they usually happen before the 12 week mark when it is socially acceptable to announce a pregnancy. Couples therefore often feel very alone and isolated when it happens to them.
- There is some evidence that stress, smoking and heavy alcohol consumption slightly increase the risk of miscarriage, as do conditions such as endometriosis and thyroid disorders, and infectious diseases like flu.
- But most miscarriages are unavoidable because they are caused by chromosomal errors in the embryo. The risk of miscarriages rise with maternal age as chromosomal abnormalities in embryos rises as women get older.
- Human embryos contain far more chromosomal errors than those of most mammals. Up to 80% of in-vitro fertilized embryos contain at least one chromosomally abnormal cell, and it is thought that the figure is probably similar for naturally conceived embryos. [The figure for mouse embryos is few than 10%.]
- Chromosomes contain all the genes needed to build a functioning human, so embryos with significant abnormalities usually don't survive.
- Humans only have one child at a time so it is extra important that there are no abnormalities. [Animals with litters of 10 can afford to lose a couple and still have 8 live births.]
- An embryo starts burrowing into the endometrium (lining of the uterus) around 5 days after conception. It seems that endometrial cells usually flock to them and release chemicals that promote implantation and growth. But endometrial cells avoid embryos with chromosomal abnormalities and stop secreting the implantation chemicals. It is now estimated that about three-quarters of miscarriages occur at the implantation stage.
Whenever the miscarriage occurs, it is distressing, as many people start planning for the future as soon as they know they are pregnant. It can be a huge source of grief.
Understanding how the endometrium works as a gatekeeper may also explain the small proportion of couples who struggle to have children, either because they can't get pregnant or suffer repeated miscarriages. This might be that the endometrium sensor is programmed on a slightly wrong setting.
- If the sensor is overly selective, it will block the implantation of all embryos, leading to infertility.
- If it is overly receptive, it will let through embryos with chromosomal abnormalities, potentially resulting in later miscarriage or stillbirth.
Other factors may also govern the outcome. It has been noted that in the three to five months after traumatic events (such as New York September 11 attacks, or the Fukushima Daiichi nuclear disaster in Japan) birth records show there were fewer male births than usual in affected areas.
- Normally more males babies are born than female, as fewer survive infancy.
- But exposure to stressful events during pregnancy may promote the loss of male fetuses because males are more vulnerable in infancy than females. In very troubled times, stress may trigger a long-evolved process not to invest in a male that may not survive.
Source: Article Rethinking Miscarriage by Alice Klein in New Scientist, 8 Aug. 2020.
