A late miscarriage is defined as the loss of a pregnancy occurring between the 13th and 20th week of gestation. This second-trimester loss differs significantly from an early miscarriage, which happens before the 12th week and is usually caused by random chromosomal abnormalities in the developing fetus. A late miscarriage is more often linked to identifiable underlying issues within the mother’s body or the environment sustaining the pregnancy. These causes typically relate to the structural integrity of the reproductive organs, the health of the placenta, or systemic medical conditions of the mother.
Structural Factors of the Uterus and Cervix
A significant proportion of late miscarriages can be traced to physical abnormalities or weaknesses in the organs designed to hold the pregnancy. The cervix normally remains firm and closed until labor begins. A condition known as cervical insufficiency occurs when the cervical tissues are too weak to handle the increasing pressure of the growing fetus, causing it to dilate prematurely without painful contractions. This painless opening allows the fetal membranes to bulge into the vagina, often leading to the premature rupture of the amniotic sac and the subsequent loss of the pregnancy.
Previous procedures on the cervix, such as a Loop Electrosurgical Excision Procedure (LEEP) or cone biopsy, can weaken the cervical structure and contribute to insufficiency. Trauma from a previous difficult childbirth or a dilation and curettage (D&C) procedure may also compromise the cervix’s ability to remain closed during the second trimester. The structure of the uterus itself can also play a role in late loss.
Congenital uterine anomalies, which are structural issues present from birth, can interfere with the proper growth of the pregnancy. Examples include a uterine septum, where a wall of tissue divides the uterine cavity, or a bicornuate uterus. Acquired issues, such as large fibroids or significant intrauterine scar tissue, can also reduce the space available for the fetus or disrupt blood flow to the placenta. These physical obstructions can prevent successful implantation or cause mechanical irritation, leading to a breakdown of the pregnancy environment.
Placental Issues and Infection-Related Causes
The placenta is the temporary organ that acts as the lifeline between the mother and the fetus, supplying oxygen and nutrients. Failure of this organ, known as placental insufficiency, can lead to late miscarriage by restricting the flow of resources to the fetus. This failure often stems from the mother’s arteries not transforming correctly to accommodate the high blood flow needed for the placenta.
Another serious placental issue is placental abruption, where the placenta prematurely detaches from the wall of the uterus, cutting off the fetus’s oxygen and nutrient supply. Both poor blood flow and premature detachment are failures that create an environment hostile to the development of the fetus. The health of the placenta is dependent on the mother’s overall health and the absence of infection.
Infection is a common cause of late miscarriage, often occurring when bacteria ascend from the vagina into the uterus, triggering inflammation. This inflammation can lead to chorioamnionitis, an infection of the membranes surrounding the fetus and the amniotic fluid. The infection weakens the amniotic sac, causing the membranes to rupture prematurely, which initiates labor and loss of the pregnancy. Systemic maternal infections, such as those caused by Listeria, cytomegalovirus (CMV), or bacterial vaginosis, can also directly affect fetal health or cause inflammation leading to sudden loss.
Systemic Maternal Health Conditions
Chronic or acquired health conditions in the mother can create a systemic environment that interferes with a successful second-trimester pregnancy. Thrombophilias, or blood clotting disorders, increase the risk of micro-clots forming in the small blood vessels of the placenta. These clots impede the exchange of nutrients and oxygen, causing placental damage that leads to fetal loss.
Antiphospholipid Syndrome (APS) is an autoimmune disorder where the body produces antibodies that target blood components, leading to excessive clotting. Inherited thrombophilias, such as Factor V Leiden mutation, also increase the risk of blood clots and are associated with a higher likelihood of late pregnancy loss. The disruption of blood flow caused by these clotting issues creates chronic oxygen and nutrient deprivation for the fetus.
Other systemic autoimmune disorders, including Lupus or Sjogren’s syndrome, can involve a similar immune-mediated attack on the placental tissue or the blood vessels supplying the uterus. Poorly managed chronic diseases also heighten the risk of late miscarriage. Uncontrolled diabetes or chronic hypertension can damage the maternal blood vessels that supply the placenta. This vascular damage contributes to placental insufficiency, resulting in a failure to sustain the pregnancy.
Diagnostic Steps After a Miscarriage
Following a late miscarriage, a thorough medical investigation is initiated to identify a cause, which is crucial for future pregnancy planning. Testing of the fetal and placental tissue is a primary step, often involving a karyotype analysis to check for chromosomal abnormalities. The placenta is also sent for pathology examination to look for signs of infection, inflammation, or clotting issues.
Maternal blood tests are ordered to check for systemic causes, focusing on autoimmune and clotting disorders. This includes a full thrombophilia profile that screens for acquired conditions like Antiphospholipid Syndrome and checks for inherited mutations such as Factor V Leiden, Protein C, and Protein S deficiencies. Blood work may also assess for uncontrolled chronic conditions, including thyroid dysfunction or diabetes.
To evaluate the structural integrity of the uterus and cervix, specialized imaging studies are performed. A three-dimensional (3D) ultrasound provides a detailed view of the uterine shape to detect congenital anomalies like a septum or bicornuate uterus. A hysteroscopy, where a thin, lighted telescope is inserted through the cervix, allows a doctor to visually inspect the uterine cavity for scar tissue, polyps, or other structural defects.