A miscarriage refers to the loss of a pregnancy before it reaches viability, typically defined as before 20 weeks of gestation. While most miscarriages occur during the first trimester, some pregnancies are lost in the second trimester. Second-trimester losses, though less common, concern expectant parents. This article explores the various causes behind second-trimester miscarriages and their underlying factors.
Understanding Second Trimester Miscarriage
A second-trimester miscarriage typically occurs between the 13th and 24th weeks of pregnancy. Reasons for loss often differ from those in the first trimester. While first-trimester miscarriages are frequently linked to chromosomal abnormalities, second-trimester losses can stem from a broader range of maternal, placental, or fetal issues.
Approximately 1% to 5% of pregnancies result in a loss between weeks 13 and 19. After 20 weeks of gestation, the rate of fetal demise is less than 0.5%. Miscarriage is often defined up to 20 weeks, with later losses sometimes categorized as stillbirths, though definitions can overlap.
Maternal Health Conditions
Various health conditions affecting the pregnant individual can contribute to a second-trimester miscarriage, especially chronic conditions if not well-managed. Poorly controlled diabetes, severe high blood pressure, and thyroid disorders can increase the likelihood of pregnancy loss. These conditions disrupt the balance needed for healthy fetal development and placental function.
Autoimmune diseases also play a role in some second-trimester miscarriages. Conditions such as lupus (systemic lupus erythematosus) and antiphospholipid syndrome (APS) can lead to complications. Antiphospholipid syndrome, in particular, involves the immune system producing antibodies that can cause blood clots, potentially blocking blood flow to the placenta and affecting nutrient supply to the fetus.
Acute infections can also trigger second-trimester miscarriages. Bacterial vaginosis, toxoplasmosis, listeria, rubella, and cytomegalovirus are examples of infections that can impact pregnancy. These infections can lead to inflammation around the fetus or within the amniotic fluid, potentially causing premature rupture of membranes or early labor.
Certain blood clotting disorders, known as thrombophilias, can increase the risk of second-trimester loss. These conditions, which include inherited factors like factor V Leiden, prothrombin gene mutation, and protein S deficiency, can lead to the formation of blood clots. Such clots can interfere with blood flow in the placenta, thereby impairing the delivery of oxygen and nutrients to the fetus.
Uterine and Cervical Issues
Structural or functional problems within the uterus or cervix can also be direct causes of second-trimester miscarriage. Cervical insufficiency, also known as an “incompetent cervix,” is a condition where the cervix begins to dilate and thin too early in the pregnancy without contractions or pain. This premature opening of the cervix can lead to the expulsion of the fetus. Prior cervical surgery or trauma, such as a cone biopsy or LEEP procedure, can increase the risk of cervical insufficiency.
Abnormalities in the shape or structure of the uterus can hinder a pregnancy’s progression. Conditions like a septate uterus, where a wall of tissue divides the uterine cavity, or a bicornuate uterus, which is heart-shaped, can reduce the space available for fetal growth or interfere with implantation. Uterine fibroids, which are non-cancerous growths, can also cause issues if they are large or positioned in a way that disrupts the uterine cavity or blood supply.
Asherman’s syndrome, characterized by the formation of scar tissue or adhesions inside the uterus, can also lead to pregnancy loss. These adhesions can develop after uterine surgery, such as a dilation and curettage (D&C), or due to infection. The presence of scar tissue can prevent proper implantation or compromise the uterine environment necessary for continued fetal development.
Placental Complications
The placenta is the organ responsible for providing oxygen and nutrients to the developing fetus and removing waste products. Problems with the placenta’s development or function can therefore lead to second-trimester miscarriage.
Placental abruption occurs when the placenta prematurely detaches from the inner wall of the uterus. This separation can interrupt the supply of oxygen and nutrients to the fetus and cause significant bleeding, endangering both the fetus and the pregnant individual.
Placenta previa is another placental complication where the placenta implants low in the uterus, sometimes covering the cervix. While more commonly associated with bleeding in the third trimester, if the placenta covers the cervix, it can lead to bleeding and complications that might necessitate early delivery or result in miscarriage.
Placental insufficiency, also known as uteroplacental or fetoplacental insufficiency, describes a condition where the placenta does not function adequately to meet the growing demands of the fetus. This can result from issues with blood flow within the placenta or its overall development, leading to insufficient delivery of nutrients and oxygen. When the placenta cannot support fetal growth effectively, it can result in restricted fetal growth or, in severe cases, pregnancy loss.
Fetal Chromosomal or Structural Abnormalities
While chromosomal abnormalities are the most frequent cause of first-trimester miscarriages, they can also contribute to losses in the second trimester, though less often. These abnormalities involve an incorrect number of chromosomes or structural changes within them. Examples include trisomies, such as Trisomy 13, 18, or 21, where an extra copy of a chromosome is present, or monosomy X (Turner syndrome).
Severe structural malformations in the fetus can also be a cause of second-trimester miscarriage. These are physical defects in the baby’s body that are incompatible with life. Such malformations can include severe heart defects or neural tube defects that significantly impair vital functions. When these abnormalities are severe, the pregnancy may not be able to continue to term.