What Causes Second Trimester Miscarriages?

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

These are causes of second-trimester fetal demise:

  • Chromosomal abnormalities: These are a major cause in almost all forms of pregnancy loss. Second-trimester miscarriages are caused by these abnormalities about one-quarter of the time.
  • Congenital birth defects: These 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: 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.
  • Use of drugs: Cocaine is especially linked to miscarriage.
  • Infections: A uterine infection is another possible cause of miscarriage, though this is more common in developing countries, as compared to the U.S.
  • Poorly controlled maternal conditions: The pregnancy may be affected by diabetes, hypertension, or thyroid disease.
  • 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.
  • 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, 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. 
  • 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.

View Article Sources
  • Late Miscarriage. Miscarriage Association. https://www.miscarriageassociation.org.uk/information/miscarriage/late-miscarriage/.
  • Miscarriage. March of Dimes. https://www.marchofdimes.org/complications/miscarriage.aspx.
  • Understanding Second Trimester Pregnancy Loss. UC Davis Health System: Department of Obstetrics and Gynecology. 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.