Many state legislators are seriously considering human embryos in the earliest stages of development for legal personality. Total abortion bans where people have full rights from the moment of conception have created a confusing legal domain that affects many areas, including assisted reproductive technologies, birth control, essential medical care and parental rights.
However, an important biological feature of human embryos has been neglected in many ethical and even scientific discussions of 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 conception, before the clump of cells has developed into a fetus with immature forms of the body’s major organs. Total abortion bans defining personality at conception mean full legal rights exist for a 5-day-old blastocyst, a hollow ball of cells about 0.008 inches (0.2 millimeters) wide with a high probability of dying within a few days. to disintegrate.
As an evolutionary biologist whose career has focused on developing embryos in a wide variety of species over the course of evolution, I was struck by the extraordinary probability that most human embryos die as a result of random genetic errors. About 60% of embryos disintegrate before people are even aware that they are pregnant. Another 10% of pregnancies end in miscarriage after the person knows they are pregnant. These losses make it clear that the vast majority of human embryos do not survive birth.
The emerging scientific consensus is that a high rate of early embryo loss is a common and normal phenomenon in humans. Exploring the causes and evolutionary reasons for early embryo loss provides insight into this fundamental feature of human biology and its implications for reproductive health decisions.
Intrinsic embryo loss is common in mammals
Intrinsic embryo loss, or embryo death 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 are trying to increase livestock but can’t eliminate the high embryonic mortality rate.
In contrast, most embryo loss in egg-laying animals, such as fish and frogs, is due to external factors, such as predators, disease, or other environmental threats. These lost embryos are effectively “recycled” into the ecosystem as food. These egg-laying animals have little to no intrinsic embryo loss.
In humans, by far the most common outcome of reproduction is the loss of embryos due to random genetic errors. An estimated 70 to 75% of human conceptions do not survive birth. That number includes both embryos that are reabsorbed into the parent’s body before anyone knows an egg has been fertilized, and miscarriages that happen 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 type of competition within the genome of unfertilized eggs, where variations of different genes can manipulate the cell division process to favor their own transmission to the offspring over other variations.
Statistical models that attempt to explain why most human embryos fail to develop usually begin with the observation that a huge number of random genetic errors occur in the mother’s eggs, even before fertilization.
When sperm fertilizes eggs, the DNA of the resulting embryo is packaged into 46 chromosomes — 23 from each parent. This genetic information guides the embryo through the development process as the cells divide and grow. When random errors occur during chromosome replication, fertilized eggs can inherit cells with these errors and result in a condition called aneuploidy, which essentially means “the wrong number of chromosomes.” With development instructions disorganized due to chromosome swapping, embryos with aneuploidy are usually doomed to failure.
Because human and other mammalian embryos are highly protected from environmental threats — unlike animals that lay eggs outside their bodies — researchers have theorized that these early losses have little effect on the parent’s reproductive success. This allows humans and other mammals to tolerate the meiotic drift over evolutionary time.
Counterintuitively, there may even be benefits to the high rates of genetic errors that result in embryo loss. Early loss of aneuploid embryos can focus the mother’s resources on healthier single newborns rather than twins or multiples. Also in the deeper evolutionary history of a species, having a huge amount of genetic variants can occasionally yield a beneficial new adaptation that can aid in the survival of humans in changing environments.
Spontaneous abortion is natural
Biological data on human embryos raise new questions for abortion policy.
Although required in some states, early embryo loss is usually not documented in the medical record. This is because it happens 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 early embryo loss in humans, and “conception” was an imagined moment estimated from the last menstrual period.
How does naturally built-in, massive early embryo loss affect the legal protection of human embryos?
Errors that occur during chromosomal replication are essentially random, meaning that development in different embryos can be disrupted in different ways. While both early embryos and late fetuses can become invincible due to genetic errors, early and late abortions are regulated very differently. Some states still require doctors to wait until the health of the pregnant person is at risk before allowing an abortion of non-viable fetuses.
Because so many pregnancies end naturally in their earliest days, early embryo loss is extremely common, even though most people don’t know they’ve experienced it. I believe that new laws that ignore this natural occurrence lead to a slippery slope that could endanger lives and livelihoods.
More than 400 women were arrested for miscarriage in the US between 1973 and 2005. With the current shift toward restrictive abortion policies, the continued criminalization of pregnancies that do not result in birth, despite how common they are, is a growing concern.
I believe that recognizing massive early embryo loss as a normal part of human life is a step forward in helping society make rational decisions about reproductive health policy.
Kathryn Kavanagh is an associate professor of biology at the University of Massachusetts, Dartmouth.
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