The cause is not known. It is likely due to prior injury and scarring of the lining of the womb. This can happen if you have had many pregnancies, a cesarean section (C-section), or prior placenta previa.
This health problem is more common in mothers who are older.
Factors that may raise your chance of this are:
Pregnancy with more than one fetus
Prior placenta previa
Symptoms differ and depend on how much of the cervix is covered. The main sign is painless bleeding from the birth canal. It can range from light to very heavy. It most often happens during late pregnancy. It often happens after sex.
The doctor will ask about your symptoms and health history. A physical exam will be done.
Podrasky AE, Javitt MC, Glanc P, et al. American College of Radiology (ACR) Appropriateness Criteria for second and third trimester bleeding. Available at: http://www.acr.org/~/media/ACR/Documents/AppCriteria/Diagnostic/SecondAndThirdTrimesterBleeding.pdf. Updated 2013. Accessed June 6, 2016.
Pregnancy complications. Office on Women's Health website. Available at: http://www.womenshealth.gov/pregnancy/you-are-pregnant/pregnancy-complications.html. Updated June 6, 2018. Accessed August 8, 2018.
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