Miscarriage is the spontaneous loss of a pregnancy, typically defined as occurring before 12 weeks of gestation, though some definitions extend this to 20 weeks. These losses are common, with studies suggesting that between 10 and 25 percent of clinically recognized pregnancies end in miscarriage. The actual rate is likely higher when including very early losses that occur before confirmation. Understanding the causes helps normalize this experience and alleviate the common feelings of guilt that often accompany it.
Genetic Errors: The Most Common Cause
The most frequent cause of early pregnancy loss is a genetic error in the embryo, occurring randomly at the earliest stages of development. Chromosomal abnormalities account for the majority of these losses, estimated to be between 50 and 70 percent of all early miscarriages. This means the developing embryo received the wrong number of chromosomes, the structures holding the genetic instructions for development.
These errors are typically non-hereditary, arising spontaneously during the formation of the egg, sperm, or shortly after fertilization. Examples include Trisomy (three copies of a chromosome instead of two) or Monosomy (one copy is missing). These severe abnormalities prevent the pregnancy from developing normally, leading to miscarriage as a form of natural quality control.
This process is generally a random occurrence and is not preventable by parental actions or lifestyle choices. Advancing maternal age does increase the risk because older eggs are more prone to these spontaneous chromosomal errors. The risk increases from around 15 to 20 percent for women under 35 to over 50 percent for women over 40.
Hormonal Imbalances and Maternal Health Conditions
Issues with the mother’s hormonal environment or pre-existing health conditions can affect the pregnancy’s viability. One common hormonal factor is insufficient progesterone, which stabilizes the uterine lining and sustains the early embryo. If progesterone levels are too low, the uterine environment may not support a healthy pregnancy, leading to loss.
It is not always clear whether low progesterone causes the miscarriage or if the level drops because the pregnancy was already failing. Endocrine disorders like thyroid dysfunction also pose a risk. Both an underactive thyroid (hypothyroidism) and an overactive thyroid (hyperthyroidism) are correlated with an increased chance of miscarriage, especially when left untreated.
Systemic Maternal Conditions
Poorly managed diabetes, characterized by high blood sugar levels, can significantly increase the risk of early pregnancy loss. Certain autoimmune disorders, such as Antiphospholipid Syndrome (APS), can cause the body to produce antibodies that interfere with blood flow or attack the pregnancy itself. These systemic maternal conditions require careful management to support a successful pregnancy.
Lifestyle and Environmental Factors
While genetic errors are the primary cause of early loss, certain lifestyle and environmental exposures contribute to an increased risk. Heavy smoking is an established risk factor, as toxins interfere with the developing fetus and placental function. Similarly, excessive alcohol consumption and the use of recreational drugs are linked to a higher risk of miscarriage.
The consumption of high levels of caffeine, typically above 300 milligrams per day, is associated with a modestly increased risk. Exposure to high concentrations of environmental toxins, such as heavy metals or workplace chemicals, can also be a factor. These elements increase risk but remain secondary to the more common random chromosomal issues.
Addressing Common Misconceptions About Miscarriage
It is common to search for an explanation and assign blame after a miscarriage, but many popular beliefs about the causes are incorrect. One persistent myth is that emotional stress, anxiety, or crying can cause a pregnancy loss. Studies have consistently shown that daily stress is not a direct cause of miscarriage.
The fear that moderate exercise, heavy lifting, or sexual activity caused the loss is also unfounded. These activities are not linked to an increased risk of miscarriage in an otherwise healthy pregnancy. Likewise, a sudden shock or fright does not trigger a miscarriage. In the vast majority of early losses, the cause is random, non-preventable genetic errors.