Miscarriage is more common than a successful pregnancy. Finding out why may change the way we think about fertility.
Infertility is an eternal topic for human beings. In history, how many emperors want to pass the throne to their descendants, and the kingdom will last forever, but they happen to encounter such an embarrassing thing as infertility.
Of course, the responsibility for infertility may be both in the man and in the woman. Women are generally blamed, usually for miscarriages. When encountering this kind of thing, the mother-to-be could not help but blame herself: "What did I do wrong?" Did you eat the wrong thing? Sleeping too little? Too much pressure to work? Or is it too much of a workout? "In some countries, miscarriages even send women to jail.
So, is miscarriage really a woman's fault? Although the phenomenon of miscarriage has been with us since the birth of mankind, it is only today that scientists are beginning to unveil its mysteries.
Abortion rates in humans are staggeringly high
Until a few decades ago, even medical professionals had little idea what the true abortion rate was for humans. In 1975, the Lancet published an article titled "Where Have All These People Gone?" For the paper, the authors calculated how many children married women in their 20s in England and Wales have each year. They found that the real number was 78 percent lower than they expected. It's amazing. Where have all these people gone? At the end of the article, the authors provide a reference that most pregnancies may terminate naturally before a woman knows she is pregnant. Abortion is a means by which nature helps us control the quality of our population. It also implies that most miscarriages may not be the fault of humans themselves, but the result of natural selection. However, they cannot give further proof of this view.
The developing embryo sends signals, and once received by the uterus, it can decide whether to accept or reject.
Since then, their views have been confirmed as the level of techniques used to detect early pregnancy, such as urine tests, has increased. Now we know that humans do have very high miscarriage rates. The latest estimates suggest that for women in their early 30s, 60% of pregnancies end in miscarriage, with most occurring in the first trimester. This miscarriage rate is 6 times that of rats and rabbits, and 2 times that of pigs and sheep. Among women in their early 20s, the miscarriage rate is 50 percent; among women in their early 40s, the rate is as high as 80 percent.
Many may question this figure, as if in their own experience, the miscarriage rate would not be so outrageous. This is not surprising. First, most women see miscarriage as a stigma and don't tell others easily. Secondly, most miscarriages occur in the first trimester of pregnancy, before they can announce that there is joy to the outside world, or even before they are aware of it, so outsiders rarely know about it.
Who is responsible for miscarriage?
Once a miscarriage occurs, the responsibility is generally passed on to the woman. There is evidence that some poor lifestyles in women, such as stressfulness, smoking, and heavy alcohol consumption, do slightly increase the risk of miscarriage; so do infectious diseases such as endometriosis, thyroid disease, and influenza.
However, most miscarriages are beyond human control, and scientists have found that miscarriages are caused by chromosomal errors in embryos and are unavoidable. The older the woman, the higher the risk of miscarriage. The reason is that as women age, the probability of chromosomal abnormalities in embryos also increases. As the father ages, the probability of chromosomal abnormalities in the embryo increases slightly.
Our chromosomes contain all the genes needed to build a normal person, so embryos with abnormal chromosomes often do not survive. The probability of chromosomal errors in human embryos is much higher than in most other mammals. Up to 80% of embryos obtained through in vitro fertilization contain at least one cell with abnormal chromosomal abnormalities. This number may be similar to an embryo conceived naturally. By comparison, less than 10 percent of mouse embryos have such abnormalities.
These are just the proof of the idea of an article in The Lancet more than 40 years ago that abortion is a means by which nature helps us control the quality of our population. Therefore, miscarriage is a survival of the fittest selection process, and from an evolutionary point of view, the reason why humans have a high elimination rate of embryos is because we can usually only conceive one child at a time. For animals that can give birth to a litter at a time, maybe pregnant with 10 cubs at a time, even if only 8 survive after birth, is also a high reproductive rate; and we usually can only conceive one child at a time, so if the embryo is not good, instead of letting him die after birth, it is better to let it abort as soon as possible, and it is more cost-effective to wait until the next time to create a healthy one. This is a clever strategy that nature has given us.
Endometrium – the gatekeeper of the embryo
How does this human screening system work? This has long been an ethically off-limit, but research has made progress over the past decade.
The researchers first looked at what happens when an embryo implants into the uterine lining (a process known as "implantation"), which usually occurs about 5 days after conception. Fearing that it would affect a pregnant woman's normal pregnancy, the researchers did not study the real embryo implantation process. They studied situations outside the body.
The researchers placed embryos donated by in vitro fertilizers on a layer of cells collected from the woman's uterine lining. Soon after the embryo is placed on, endometrial cells usually gather around it and release chemicals to facilitate implantation and growth of the embryo. But when an embryo has chromosomal abnormalities, the endometrial cells avoid it and stop secreting chemicals that can help it implant, depriving it of the possibility of getting nutrients from the endometrium so that it can't survive.
To learn more about this process, the researchers collected two more types of fluids grown in human embryos during IVF and injected them into the uterus of mice. The fluid of a "good embryo" activates 90 genes associated with implantation and growth in embryos. In contrast, the fluid of the "bad embryo" triggered an unusually strong response in the mice's uterus, activating 544 genes. These genes work together to create a rejection reaction to the "bad embryo".
Taken together, these findings suggest that during the implantation phase, the endometrium is a gatekeeper, checking the embryos most likely to survive, doing everything possible to keep other embryos out. This is also why the majority of miscarriages (about 75% of miscarriages) occur during the implantation phase. In the past, these early miscarriages were not easy to notice because they looked like they were just a few days late for menstruation.
Understanding how the endometrium plays the role of a gatekeeper can also explain why some couples cannot get pregnant or have repeated miscarriages. Both of these conditions may be due to tight or wide endometrial controls. For example, if the endometrium is too tightly guarded, it will prevent all embryos from implanting, which will lead to infertility. Conversely, if it is too wide, it will implant an embryo with abnormal chromosomal abnormalities, leading to a later miscarriage or stillbirth.
As for why some endometrium is too tight or too wide, this is the future of the problem, if we figure it out, we may be able to treat some women's infertility.