Many state legislatures are seriously considering human embryos in the earliest stages of development for legal personhood. Total abortion bans that grant people full rights from the moment of conception have created a confusing legal landscape that affects a wide range of areas, including assisted reproductive technologies, contraception, essential medical care, and parental rights, among others.
However, a key biological feature of human embryos has been ignored in many ethical and even scientific discussions that influence reproductive policy: Most human embryos die before anyone, including doctors, even knows they exist. This embryo loss typically occurs in the first two months after fertilization, before the clump of cells has developed into a fetus with immature forms of the body’s major organs. Total abortion bans that define personhood at conception mean that full legal rights exist for a 5-day-old blastocyst, a hollow ball of cells about 0.008 inches (0.2 millimeters) across that is highly likely to disintegrate within a few days.
As an evolutionary biologist whose career has focused on how embryos develop in a wide variety of species over the course of evolution, I was struck by the extraordinarily high probability that most human embryos die from random genetic errors. About 60% of embryos disintegrate before people are even aware they are pregnant. Another 10% of pregnancies end in miscarriage, after the person knows she is pregnant. These losses make it clear that the vast majority of human embryos do not survive birth.
The emerging scientific consensus is that high rates of early embryo loss are common and normal in humans. Research into 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, including cattle and sheep. This persistent “reproductive waste” has frustrated breeders who are trying to increase livestock production but are unable to eliminate the high embryonic mortality.
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” back into the ecosystem as food. These egg-laying animals have little to no intrinsic embryo loss.
In humans, the most common reproductive outcome by far is embryo loss due to random genetic errors. It is estimated that 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 occur later in pregnancy.
An evolutionary driver 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 offspring over other variations.
Statistical models that attempt to explain why most human embryos fail to develop usually start with the observation that a large number of random genetic errors occur in the mother’s eggs, even before fertilization.
When sperm fertilize eggs, the resulting embryo’s DNA is packaged into 46 chromosomes—23 from each parent. This genetic information guides the embryo through the development process as its cells divide and grow. When random errors occur during chromosome replication, fertilized eggs can inherit cells with these errors, resulting in a condition called aneuploidy, which essentially means “the wrong number of chromosomes.” With the instructions for development disorganized due to mixed-up chromosomes, embryos with aneuploidy are usually doomed.
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 may allow humans and other mammals to tolerate meiotic drift over the course of evolution.
Counterintuitively, there may even be advantages to the high rates of genetic error that lead to embryo loss. Early loss of aneuploid embryos could redirect maternal resources toward healthier singletons rather than twins or multiples. Also, in the deeper evolutionary history of a species, having a vast array of genetic variants might occasionally provide a beneficial new adaptation that could help humans survive in changing environments.
Spontaneous abortion is natural
Biological data on human embryos raises new questions about abortion policy.
Although required in some states, early embryo loss is not typically recorded in the medical record. This is because it occurs before the person knows she is 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 the 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 can be disrupted in different ways in different embryos. Although both early embryos and late fetuses can be rendered unviable by genetic errors, early and late abortions are regulated very differently. Some states still require that doctors wait until the health of the pregnant woman is at risk before allowing induced abortions of nonviable fetuses.
Because so many pregnancies end naturally in their very first days, early embryo loss is extremely common, even though most people are unaware that they have experienced it. I believe that new laws that ignore this natural phenomenon are leading to a slippery slope that could endanger lives and livelihoods.
Between 1973 and 2005, more than 400 women were arrested in the United States for miscarriage. With the current shift toward restrictive abortion policies, the continued criminalization of pregnancies that do not result in birth, despite their frequency, is a growing concern.
I believe that recognizing mass early embryo loss as a normal part of human life is a step forward in helping society make rational decisions about reproductive health policies.
This article is republished from The Conversation, a non-profit, independent news organization that brings you facts and reliable analysis to help you understand our complex world. It was written by: Kathryn Kavanagh, UMass Dartmouth
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Kathryn Kavanagh is not an employee of, an advisor to, an owner of stock in, or a recipient of funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond her academic appointment.