What Causes Miscarriage in the Second Trimester?

A miscarriage in the second trimester is a pregnancy loss that happens specifically between 12 and 20 weeks of gestation. (After this time, the loss is more accurately classified as a stillbirth or neonatal death.) No matter the timing, it's only natural to want answers when such an event occurs.

Causes of second trimester miscarriage vary—maternal conditions, chromosomal abnormalities, congenital birth defects, and trauma, for example. But almost always, the pregnancy loss isn't due to anyone's actions and even doctors can't always explain why it happened.

These are the most common causes of second trimester pregnancy loss. Know, too, that if your miscarriage was diagnosed early in the second trimester, it might have actually occurred in the first trimester.

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 Birth Defects

Congenital birth defects are structural problems not due to chromosomal abnormalities. Birth defects such as heart 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 issues via ultrasound.

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 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 performed like loop electrosurgical excision procedure (LEEP), laser ablation, or a cold knife conization. Others develop the condition due to congenital uterine malformations.

Infections

A uterine infection is another possible cause of miscarriage, though this is more common in developing countries than the United States.

Poorly Controlled Maternal Conditions

Studies have shown that a pregnant woman's poorly controlled diabetes, hypertension, or thyroid disease can cause her to be at increased risk for miscarriage.

Thrombophilia

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.

If you are experiencing domestic violence, immediately call 1-800-799-SAFE, the National Domestic Violence Hotline.

Drug 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
  • Fetal death

Unknown Causes

Sometimes none of the above is at play, and the true cause remains a mystery. Not knowing the cause may be unsettling, but know that this is quite common.

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—it's usually a one-time thing. But one percent of women do have two or more miscarriages, as the risk for future pregnancy loss is at about 14 percent. The risk of having a consecutive second trimester miscarriage is generally quite low, so be honest with your doctor about your concerns and your risks.

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Article Sources
  • Miscarriage Association. Late Miscarriage. https://www.miscarriageassociation.org.uk/information/miscarriage/late-miscarriage/.

  • March of Dimes. Miscarriage. https://www.marchofdimes.org/complications/miscarriage.aspx.

  • UC Davis Health System: Department of Obstetrics and Gynecology. Understanding Second Trimester Pregnancy Loss. https://www.ucdmc.ucdavis.edu/obgyn/services/FP/trimester_loss.html.

  • What Is the Placenta? National Health System. http://www.nhs.uk/chq/Pages/2309.aspx?CategoryID=54.
  • Using Illegal Drugs During Pregnancy American Pregnancy Association.

    http://americanpregnancy.org/pregnancy-health/illegal-drugs-during-pregnancy/