Why do membranes rupture before the onset of labor?

by Carolyn Salafia, MD, Ph.D.

The extraplacental membranes form the walls of the fetal sac. They are composed of three layers (decidua, chorion and amnion) and their function is to keep the amniotic fluid in around the pregnancy. The membranes expand as the pregnancy grows. However, the number of cells in the membranes increase only until the middle of pregnancy. After that time, cell number remains the same even though the membranes continue to expand. As the membranes get bigger, they get progressively thinner. Since the membranes have to rupture to let the baby out, this is part of the way the body prepares for delivery. 

Rupture of the membranes is a normal process that is brought on by the stress of uterine contractions. What's abnormal is if they rupture in any of these three ways: 

* Premature, prior to the onset of labor.
* Preterm, before the baby is ready to be born. 
* Prolonged. This means you have a prolonged interval--generally greater than 24 hours--between ruptured membranes and delivery. 

The term PROM is vague and is, at times, used to apply to any of the above scenarios. You can have a PPPROM scenario if your membrane rupture is preterm, premature and prolonged. 

When I examine tissues under a microscope, I see that one of the most common causes of membranes rupturing too soon (prematurely) is a bacterial infection that is damaging the strength of the membranes. Whether this is the most common reason depends on what hospital I am at. The risk of infection varies with different patient populations who may have different risks and behaviors that may infection more or less likely to complicate a pregnancy. If there's no infection, the next most common cause is a blood flow problem. If you don't get nutrients and oxygen to the membranes via proper blood flow, then the membranes will become weak. Far less frequently, I see chronic inflammation and immune problems associated with spontaneous membrane rupture. Hormonal imbalances could in theory affect membrane integrity as well. If preterm labor followed premature rupture of the membranes, it's often thought that the membrane rupture brought on labor. If that's the case, then the above causes apply here as well. If the membranes did not rupture before labor began, then a reproductive pathologist will look for the reason labor started. Often, many of the same things that can cause the membranes to rupture spontaneously can cause the uterus to contract. It's not known why the same tissue injury process can present clinically in different ways in different people. 

Prolonged membrane rupture is considered a risk for the development of intramniotic infection (infection in the amniotic fluid around the baby, that the baby floats in, breathes into its lungs, and swallows). However, as a pathologist, I often see signs that the membrane rupture was caused by the infection. Often it takes 24 hours for the infection to progress up the cervical canal to the point where it's clinically detectable. 

Membrane rupture has a risk of happening again and the cause tends to remain the same in the same woman. For example, if your membranes ruptured prematurely in one pregnancy due to bacterial infection it is reasonable that a good component of your risk for future membrane rupture ought to be related to bacterial infection. Sending your tissue or slides from a pregnancy loss to a perinatal pathologist will enable you to identify the appropriate mechanism underlying the chain of events that led to your membrane rupture. This will allow your doctor to monitor and treat you properly. 

Carolyn Salafia, M.D., Ph.D., is board certified in Anatomic and Clinical Pathology and in Pediatric Pathology.

Note: This communication is for educational purposes only and should not be used as a substitute for a consultation with your physician.