Causes of second-trimester miscarriage vary, but, in most cases, the pregnancy loss isn’t due to anyone’s actions. Oftentimes, even your doctor won’t be able to explain why it happened.
Chromosomal Abnormalities
Chromosomal abnormalities, such as trisomies, are a major cause of almost all forms of pregnancy loss. Second-trimester miscarriages are caused by these abnormalities about one-quarter of the time. Prenatal testing may have made you aware of the potential for this, but that is not always the case.
Congenital Defects
Congenital birth defects are those that are often inherited but may also be caused by an infection during pregnancy or exposure to certain teratogenic (defect-causing) medications, chemicals, or toxins. Birth defects such as heart or brain malformations in your developing baby can make your pregnancy a nonviable one. Depending on the type and severity of the birth defect, your doctor may be able to diagnose any potential problems via ultrasound. Others may be detected during routine prenatal screenings.
Placental Problems
One such problem is called a placental abruption. If your placenta—the structure that’s attached to the wall of the uterus and gives nutrients to your baby via the umbilical cord—suddenly peels off the wall of the uterus before you’re ready to give birth, this can prevent your developing baby from getting necessary nutrients and oxygen.
Cervical Insufficiency
Cervical insufficiency, also known as an incompetent cervix, means your cervix is weak and begins dilating and opening too soon. Some women experience cervical insufficiency after having a challenging birth or after having a cervical procedure such as loop electrosurgical excision procedure (LEEP), laser ablation, or cold knife conization. Others develop the condition due to congenital uterine malformations.
Infections That Cause Miscarriages
Uterine infection is another possible cause of miscarriage, although it is a more common cause in developing countries than in the United States. Still, any untreated or severe uterine infection may trigger the spontaneous loss of a pregnancy, including:
Bacterial vaginosisChlamydiaGonorrheaTrichomoniasisViral vaginitis
Poorly Controlled Chronic Conditions
Chronic diseases in the mother can also increase the risk for pregnancy loss. In many cases, the risk increases if the disease is poorly controlled with medications or lifestyle. Other chronic conditions carry an inherent risk of pregnancy loss depending on their severity. Examples include:
Autoimmune diseases like scleroderma and antiphospholipid syndrome Diabetes Hypertension (high blood pressure) Polycystic ovary syndrome (PCOS) Thyroid disease
The risk can be further increased in women who smoke, are obese, or have hormone problems.
Thrombophilia During Pregnancy
Thrombophilia is an increased risk of forming blood clots in blood vessels like veins and arteries that may be due to a genetic abnormality or a problem with your immune system, such as in lupus. This can cause problems involving your placenta and your umbilical cord. Thrombophilia is sometimes treated during pregnancy with blood-thinning drugs or with low-dose aspirin.
Abdominal Trauma
Any trauma such as an automobile accident, fall, or being hit in the abdomen can hurt both you and your developing baby and potentially cause a miscarriage. In a car, always wear a seat belt, placing the lap belt under the uterus and putting the shoulder strap between your breasts. Avoid high-impact physical activities that might cause you to lose your balance. Try not to let your kids roughhouse with you, if you can avoid it.
Drug and Alcohol Use
Cocaine and methamphetamine are especially linked to miscarriage. Studies have shown that the use of illegal drugs has been associated with low birth weight, premature labor, placental abruption, and fetal death. Alcohol has also been cited as a cause of pregnancy loss during not only the first trimester but the second trimester as well. According to research from the University of Copenhagen, light drinking (defined as one to two drinks per week) increases the odds of an early second-trimester miscarriage by 70%.
Unknown Causes of Miscarriage
Sometimes a pregnancy loss occurs for no apparent reason and, no matter how much a doctor investigates, no cause can be found. As distressing as this may be, it does happen and more frequently that one might think. Moreover, having had a prior second-trimester miscarriage in and of itself is a risk factor for subsequent second-trimester miscarriages. While the cause may be identified in some women (like cervical insufficient or uterine defects), others will remain a mystery.
A Word From Verywell
If you’ve had second-trimester pregnancy loss and are pregnant again, consult your doctor early in your pregnancy in case extra monitoring may be needed. A miscarriage of a prior pregnancy doesn’t necessarily mean that a woman is at high risk, as most women go on to have healthy pregnancies.
