What Causes Second Trimester Miscarriages?

While rare, there are a number of potential culprits

pregnancy ultrasound
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A second-trimester miscarriage is a pregnancy loss that happens specifically between 12 and 20 weeks. A pregnancy loss after 20 weeks, when you're halfway along, would not be classified as a miscarriage. It would be labeled a stillbirth or a neonatal death.

A number of factors can lead to a second-trimester miscarriage. If your miscarriage was diagnosed early in the second trimester, it might have actually occurred in the first trimester.

A missed miscarriage is any miscarriage that was not detected immediately.

Causes of Second Trimester Miscarriage

Miscarriages that occur after 12 weeks of gestation can be due to chromosomal abnormalities, which are a major culprit in almost all forms of pregnancy loss. Second trimester miscarriages are more likely to be caused by other factors. These may include:

Cervical insufficiency: This 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 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, as compared to the U.S. 

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're a victim of domestic violence, immediately call 1-800-799-SAFE, the National Domestic Violence Hotline.

Thrombophilia: This 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. 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. 

Fetal Issues That Can Cause a Miscarriage

Congenital birth defectsBirth 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.  

Unknown causes: Sometimes none of the above is at play, and the true cause remains a mystery. 

If you have had second-trimester pregnancy loss and are pregnant again, consult your doctor early in your pregnancy in case extra monitoring is needed.

Sources:

March of Dimes. (July 2012). Miscarriage. 

Miscarriage Association. (2016). Late Miscarriage.

UC Davis Health System: Department of Obstetrics and Gynecology. (2015). Understanding Second Trimester Pregnancy Loss. 

National Health System (November 2016). What is the placenta? National Health System (2015).

Michels TC, Tiu AY. Second-trimester pregnancy loss. Am Fam Physician. 2007 Nov 1;76(9):1341-46.

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