Most human embryos die naturally after conception – restrictive abortion laws do not account for this loss of embryos

(The Conversation) – Many state legislatures are seriously considering human embryos in the early stages of legal personality development. Comprehensive abortion bans that deem human beings to have every right from the moment of conception have created a confusing legal realm that affects a wide range of areas including assisted reproductive technologies, contraception, medical care essentials and parental rights, among others.

However, an important biological feature of human embryos has been left out of many ethical and even scientific discussions of reproductive politics – most human embryos die before anyone, including doctors, even knows they exist. . This loss of embryos usually occurs within the first two months after fertilization, before the cell cluster develops into a fetus with immature forms of major body organs. Total bans on abortion that define personality at conception mean that all legal rights exist for a 5-day-old blastocyst, a hollow ball of cells about 0.008 inches (0.2 millimeters) in diameter with a high probability of disintegrate in a few days.

As an evolutionary biologist whose career has focused on how embryos develop in a wide variety of species during evolution, I was struck by the extraordinarily high probability that most human embryos die due to random genetic errors. About 60% of embryos disintegrate before people even know they are pregnant. Another 10% of pregnancies end in miscarriage after the person knows they are pregnant. These losses clearly show that the vast majority of human embryos do not survive until birth.

The emerging scientific consensus is that a high rate of early embryo loss is a common and normal occurrence 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, such as cows and sheep. This persistent “reproductive waste” has frustrated breeders who attempt to increase animal production but are unable to eliminate high embryo mortality.

In contrast, most embryo loss in animals that lay eggs like fish and frogs is due to external factors, such as predators, disease, or other environmental threats. These lost embryos are effectively “recycled” in the ecosystem as food. These laying animals have little or no intrinsic embryo loss. Each square shows the first 24 hours of embryo development in a different animal species. From left to right: 1. zebrafish (Danio rerio), 2. sea urchin (Lytechinus variegatus), 3. black widow spider (Latrodectus), 4. tardigrade (Hypsibius dujardini), 5. sea squirt (Intestinal Ciona), 6. comb jelly (Ctenophore, Mnemiopsis leidyi), 7. parchment tube worm (Chaetopterus variopedatus), 8. roundworm (Caenorhabditis elegans), and 9. sea snail (Crepidula fornicata).

In humans, by far the most common outcome of reproduction is the loss of embryos due to random genetic errors. It is estimated that 70% to 75% of human conceptions do not survive birth. This 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 engine for embryo loss

In humans, an evolutionary force called the meiotic drive plays a role in the early loss of embryos. Meiotic drive is a type of competition within the genome of unfertilized eggs, where variations in different genes can manipulate the process of cell division to favor their own transmission to offspring over other variations.

Statistical models attempting to explain why most human embryos fail to develop typically begin by observing that a massive 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 and result in a condition called aneuploidy, which basically means “the wrong number of chromosomes.” With developmental instructions 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 – the researchers hypothesized that these early losses have little effect on success. parent’s breeder. This may allow humans and other mammals to tolerate meiotic entrainment over evolutionary time.

Unexpectedly, there may even be benefits to the high rates of genetic errors that lead to embryo loss. Early loss of aneuploid embryos may direct maternal resources toward healthier single newborns rather than twins or multiples. Additionally, in the deeper evolutionary history of a species, having a huge pool of genetic variants could sometimes provide a beneficial new adaptation that could aid human survival in changing environments.

Spontaneous abortion is natural

Biological data on human embryos raise 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 happens before the person knows they are pregnant and often coincides with the next menstrual period. Until relatively recently, researchers were unaware of the extremely high rate of early embryo loss in humans, and “conception” was an imaginary time estimated from the last menstrual period.

How does massive, early, naturally embedded embryo loss affect legal protections for human embryos?

Errors that occur during chromosomal replication are essentially random, which means that development can be disrupted in different ways in different embryos. However, while early embryos and late fetuses can become inviable due to genetic errors, early and late abortions are regulated very differently. Some states still require doctors to wait until the pregnant person’s health is at risk before allowing induced abortion of non-viable fetuses. In the wake of anti-abortion laws, doctors have refused to treat patients who have miscarried because it uses the same procedures as abortions.

Since so many pregnancies end naturally in their very first days, early embryo loss is extremely common, although most people are unaware they have experienced it. I believe that new laws ignoring this natural occurrence lead to a slippery slope that can put lives and livelihoods at risk.

Between 1973 and 2005, more than 400 women were arrested for miscarriage in the United States. With the current move towards restrictive abortion policies, the continued criminalization of pregnancies that do not result in delivery, despite their frequency, is a growing concern.

I believe that recognizing mass loss of early embryos as part of human life is a step forward in helping society make rational decisions about reproductive health policy.


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