Pregnancy Is a High-Stakes Biological Trial, and Loss Is Its Most Common Outcome
Pregnancy is often described as a predictable sequence that begins with fertilization and ends with birth. But the biology of reproduction tells a far more uncertain story, according to Kate Clancy, an anthropology professor at the University of Illinois Urbana-Champaign. In her new book, Pregnancy Interrupted: The Science and Stories of How Pregnancies Really End, Clancy argues that pregnancy is not a single uninterrupted process but a succession of biological tests. At every stage, eggs, embryos, placental tissues and the pregnant body must meet demanding developmental conditions. Many pregnancies end before a person ever knows they began, while others are interrupted later by genetic, environmental, medical or social factors. Together, these outcomes reveal that pregnancy loss is not an unusual deviation from reproduction, but a central feature of it.
The process begins long before a pregnancy can be detected. Menstruation represents the breakdown and removal of tissue prepared during the previous reproductive cycle, while ovulation releases an egg that has only a limited window in which fertilization can occur. After sperm and egg fuse, the resulting embryo must divide repeatedly, regulate its genetic activity and travel through the fallopian tube toward the uterus. It must then attach to the uterine lining at precisely the right time. Implantation is not simply a matter of the embryo making contact with the uterus: it requires coordinated molecular signals between embryonic cells and maternal tissue. If those signals fail, or if the embryo cannot continue developing, the pregnancy may end before symptoms appear or a test can detect it.
Even when implantation occurs, another demanding phase begins. The developing placenta must establish a functional connection with the pregnant body, allowing oxygen, nutrients and waste products to pass between maternal and embryonic circulatory systems without directly mixing the two blood supplies. Specialized placental cells invade the uterine lining and remodel maternal blood vessels so that they can accommodate the increased flow needed for fetal growth. This process is tightly regulated. Too little invasion can restrict blood flow, while abnormal placental development can contribute to complications later in pregnancy. Clancy describes these transitions as biological “load tests,” in which the embryo and placenta must pass successive developmental milestones before the pregnancy can progress.
The scale of pregnancy loss becomes clearer when both recognized and unrecognized pregnancies are considered. In the United States, Clancy cites estimates of approximately 3.6 million births, 1 million miscarriages, 620,000 abortions and 20,000 stillbirths each year. These figures do not include the millions of very early losses that occur between fertilization and the first signs of pregnancy. Estimates suggest that such early losses may account for 30% to 50% of all pregnancies, although the precise proportion is difficult to measure because many are never clinically identified. Chromosomal abnormalities are believed to contribute substantially to early miscarriage, often because an embryo has gained or lost genetic material during the formation of eggs, sperm or the earliest cell divisions. In many cases, these changes prevent normal development before a fetus could become viable.
This high level of biological attrition is not evidence that human reproduction is poorly designed in a simple sense. Rather, it reflects the evolutionary compromises of mammalian reproduction. Humans produce relatively few eggs compared with many other species, yet each reproductive event requires coordination among gametes, the uterine lining, the placenta, the immune system and multiple hormonal pathways. The body must allow an embryo carrying unfamiliar genetic material to implant, while also defending itself against infection and maintaining the integrity of its tissues. The placenta, an organ that exists only during pregnancy, must simultaneously support fetal development and manage the physiological demands placed on the pregnant person. The result is a system capable of producing healthy births but also vulnerable to failure at nearly every transition.
Biology is only one part of the risk landscape. During pregnancy, substances encountered in the environment may cross into the maternal bloodstream and affect fetal development. Chemicals associated with food packaging, industrial pollution and other exposures have been investigated for their potential effects on growth, hormone signaling and organ formation. Medications can present difficult decisions because a drug that is safe under ordinary circumstances may have different effects during embryonic or fetal development. At the same time, withholding treatment can also create serious dangers. The risk-benefit balance is especially complicated when evidence from pregnant patients is limited, since pregnant people have historically been excluded from many clinical trials. This lack of data can leave clinicians and patients navigating uncertainty precisely when timely decisions are most important.
Some medical treatments associated in public debate with abortion are also used to manage miscarriage, stillbirth and labor. Misoprostol, for example, can help end a pregnancy, treat an incomplete miscarriage, induce labor and manage certain obstetric emergencies. Restrictions on medications or procedures because of their association with abortion may therefore delay care for patients whose pregnancies have already ended or whose health is in immediate danger. In cases of miscarriage or stillbirth, prolonged waiting can increase the risk of hemorrhage, infection and severe psychological distress. Clancy warns that laws and institutional policies written without distinguishing among different pregnancy outcomes can interfere with medically necessary treatment, particularly when clinicians fear legal consequences.
The social meaning assigned to pregnancy loss can add another layer of harm. Miscarriage has often been interpreted as a personal failure, with patients blamed for stress, behavior, age or imagined mistakes even when the underlying cause was outside their control. In countries and regions where abortion is criminalized, a miscarriage may also attract suspicion from authorities, exposing patients to investigation, detention or prosecution. Access to pain relief can be restricted or delayed, leaving people experiencing miscarriage or stillbirth to endure avoidable physical suffering. The burden is not distributed equally: racism, economic inequality, discrimination against people with obesity or transgender patients, inadequate prenatal care and exposure to violence all influence pregnancy-related morbidity and mortality. These social determinants can transform biological uncertainty into preventable danger.
Clancy’s central argument is that acknowledging uncertainty should not produce fatalism; it should lead to better care and more honest public discussion. People deserve clear information about what can happen during pregnancy, including the possibility of loss before a heartbeat is detected, after a diagnosis of fetal abnormality or near the end of gestation. They also deserve choices about how a miscarriage or stillbirth is managed when medical circumstances allow. Some may prefer expectant management, waiting for the body to complete the process naturally; others may choose medication or a surgical procedure. Each option carries different considerations involving timing, pain, bleeding, infection risk and emotional needs. Respectful care requires clinicians to explain these choices without judgment and to provide adequate pain control, follow-up and psychological support.
By placing pregnancy loss at the center of reproductive biology rather than treating it as an exceptional failure, Pregnancy Interrupted challenges the cultural expectation that pregnancy is inevitable once conception occurs. The book also distinguishes between losses that arise from the intrinsic vulnerability of reproduction and those shaped by social conditions, environmental exposures, unequal medical access or restrictive policies. That distinction matters because not every pregnancy ending can be prevented, but many forms of suffering can be reduced. A more scientifically accurate understanding of pregnancy would recognize both realities: embryos and fetuses must navigate a series of demanding biological checkpoints, and pregnant people require care that responds to uncertainty rather than concealing it. Clancy’s message is ultimately that better knowledge can support more humane medicine, stronger self-advocacy and public policies grounded in reproductive biology instead of stigma.
Subject of Research: The biological, medical and social causes and consequences of pregnancy loss.
Article Title: Pregnancy Is a High-Stakes Biological Trial, and Loss Is Its Most Common Outcome
Web References: https://press.princeton.edu/books/hardcover/9780691254739/pregnancy-interrupted; https://anthro.illinois.edu/directory/profile/kclancy
References: Kate Clancy, Pregnancy Interrupted: The Science and Stories of How Pregnancies Really End, Princeton University Press.
Image Credits: Photo by Joanna Strauss.
Keywords: pregnancy loss, miscarriage, stillbirth, reproductive biology, implantation, placenta, embryonic development, pregnancy research, maternal health, abortion care, misoprostol, reproductive medicine
