by Many state legislatures are seriously considering human embryos in the earliest stages of their development for legal personhood. Complete bans on abortion that deem people to have full rights from the point of conception have created a confusing legal field that affects a wide range of areas, including assisted reproductive technologies, contraception, essential medical care and parental rights, among others. another.
However, an important biological aspect of human embryos has been left out of many ethical and even scientific discussions that inform reproductive policy – most human embryos die before anyone, including doctors, even knows they exist. This embryo loss usually occurs in the first two months after fertilization, before the lump of cells becomes a fetus with immature forms of the body’s major organs. Complete bans on abortion that define personhood at conception mean full legal rights for a 5-day-old blastocyst, a hollow ball of cells about 0.008 inches (0.2 millimeters) across and highly likely to disintegrate within a few days.
As an evolutionary biologist whose careers focus on how embryos develop in a wide variety of species during evolution, I was struck by the extremely high probability that most human embryos die from random genetic errors. About 60% of embryos disperse before people may be aware that they are pregnant. Another 10% of pregnancies end in miscarriage, after the person knows they are pregnant. These losses indicate that the vast majority of human embryos do not survive to birth.
The emerging scientific consensus is that a high rate of early embryo loss is a normal occurrence in humans. Research into the causes and evolutionary causes of early embryo loss sheds light on this fundamental aspect of human biology and its implications for reproductive health decisions.
Intrinsic embryo loss is common in mammals
Intrinsic embryo loss, or the death of an embryo due to internal factors such as genetics, is common in many mammals, such as cows and sheep. This ongoing “reproductive waste” has frustrated breeders who want to increase livestock production but are unable to eliminate high embryonic mortality.
In contrast, most embryo loss in egg-laying animals such as fish and frogs is caused by external factors, such as predators, disease or other environmental threats. These lost embryos are effectively “recycled” into the ecosystem as food. These calving animals have very little intrinsic embryo loss.
In humans, the most common result of extreme reproduction is embryo loss due to random genetic errors. It is estimated that 70% to 75% of human embryos fail to survive to birth. That number includes both embryos reabsorbed into the parent’s body before anyone knows an egg has been fertilized and miscarriages that occur later in the pregnancy.
An evolutionary drive for embryo loss
In humans, an evolutionary force called meiotic drive plays a role in early embryo loss. Meiotic drive is a form of competition within the genome of unfertilized eggs, where different gene variations can manipulate the cell division process to transmit themselves to the offspring over other variations.
Statistical models that attempt to explain why most human embryos fail to develop usually begin by noting that a huge number of random genetic errors occur in the mother’s eggs even before fertilization.
When sperm fertilizes eggs, the resulting embryo’s DNA is packaged into 46 chromosomes – 23 from each parent. This genetic information guides the embryo through the process of development as the cells divide and grow. When random mistakes occur during chromosome replication, fertilized eggs can inherit cells with these errors and lead to a condition called aneuploidy, which basically means “the wrong number of chromosomes.” Because the developmental directions are now disorganized due to mixed chromosomes, embryos with aneuploidy are usually doomed.
Because human and other mammal embryos are highly protected from environmental threats—unlike animals that lay eggs outside their bodies—researchers theorize that these early losses have little effect on the parent’s reproductive success. This may allow humans and other mammals to experience meiotic drive over evolutionary time.
Counterintuitively, the high rates of genetic errors leading to embryo loss may even have advantages. Early loss of aneuploid embryos can direct mothers’ resources to healthier singletons rather than twins or multiples. Also, in the deeper evolutionary history of a species, a large number of genetic variants may occasionally provide a new beneficial adaptation that may help humans survive in changing environments.
Spontaneous abortion is natural
Biological data on human embryos brings new questions to consider for abortion policies.
Although required in some states, early embryo loss is usually not documented in the medical record. This is because it occurs before the person knows they are pregnant and often coincides with the next menstrual period. Until recently, researchers were unaware of the extremely high rate of embryo loss in humans, and the “conception” considered from the last menstrual period was an imaginary moment.
How does massive early loss of life naturally affect legal protections for human embryos?
Errors that occur during chromosomal replication are essentially random, meaning that development can be affected in different ways in different embryos. However, while early embryos and late fetuses can be non-viable due to genetic errors, early and late abortion are regulated very differently. Some states still require doctors to wait until the health of the pregnant person is at risk before allowing an induced abortion on a non-viable fetus.
Since so many pregnancies end naturally in their early days, early embryo loss is extremely common, although most people will not know they have experienced it. I believe new laws that ignore this natural occurrence that could endanger lives and livelihoods lead to a slippery slope.
Between 1973 and 2005, over 400 women were arrested for miscarriage in the U.S. With the current shift toward restrictive abortion policies, there is a growing concern that the criminalization of pregnancies that do not result in birth, despite their prevalence, will continue they.
I believe that acknowledging the massive loss of embryos as a normal part of human life is one step forward in helping society make rational decisions about reproductive health policy.
This article is republished from The Conversation, a non-profit, independent news organization that brings you reliable facts and analysis to help you make sense of our complex world. Written by: Kathryn Kavanagh, UMass Dartmouth
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Kathryn Kavanagh does not work for, consult with, own shares in, or receive funding from any company or organization that would benefit from this article, and She has disclosed no relevant affiliations beyond her academic appointment.
