Causes of Second Trimester Spontaneous Abortion
Second trimester spontaneous abortion (pregnancy loss between 13-27 weeks) is primarily caused by placental complications, cervical insufficiency, uterine anatomic abnormalities, maternal medical conditions, and infections—with chromosomal abnormalities playing a lesser role than in first trimester losses.
Major Causes by Category
Placental Complications
- Placental abruption affects approximately 1% of pregnancies and represents a significant cause of second trimester loss, with central abruption associated with worse perinatal outcomes than marginal placental separation 1.
- Placenta previa occurs in approximately 1 in 200 pregnancies at delivery and can lead to pregnancy loss through hemorrhage 1.
- Vasa previa is less frequent (1 in 2,500 to 1 in 5,000 deliveries) but carries high risk of fetal mortality if undiagnosed 1.
Cervical Insufficiency
- Cervical incompetence is a major cause of second trimester loss, particularly in women with müllerian anomalies who may benefit from prophylactic cervical cerclage 2.
- Women with anatomic uterine defects and history of second-trimester losses should be considered for cervical cerclage 2.
Uterine Anatomic Abnormalities
- Müllerian anomalies are found in 8-10% of women with three or more consecutive spontaneous abortions, with the uterine septum being the most common 2.
- These anomalies predispose to pregnancy loss through inadequate vascularity to the developing embryo and placenta, reduced intraluminal volume, or cervical incompetence 2.
- Intrauterine adhesions and leiomyomata (fibroids) can cause recurrent second trimester losses and are amenable to surgical correction 2.
- Bicornuate uterus is associated with poor reproductive outcomes and second trimester losses 2.
Maternal Medical Conditions
- Gestational hypertension and pre-eclampsia are associated with second trimester complications and pregnancy loss 1.
- Gestational diabetes increases risk of pregnancy complications including loss 1.
- Antepartum hemorrhage significantly increases risk of adverse outcomes 1.
- Maternal schizophrenia is associated with increased risk of stillbirth and pregnancy complications, with JAK2V617F mutation in myeloproliferative neoplasms showing higher rates of late fetal loss around 10% 1.
Infections
- Septic abortion represents a life-threatening complication requiring immediate intervention 3, 4.
- Maternal infections should be treated appropriately to prevent pregnancy loss 5.
Premature Rupture of Membranes and Preterm Labor
- Threatened preterm labor is a recognized cause of second trimester loss 1.
- Premature rupture of membranes can lead to pregnancy loss, particularly when complicated by infection 1.
Iatrogenic and Procedural Factors
- Prior cesarean delivery, particularly multiple cesarean births, is the most significant risk factor for complications during second trimester abortion due to increased risks of hemorrhage, placenta accreta spectrum, and uterine perforation 6.
- Surgery during pregnancy carries slightly higher risk of miscarriage during the first trimester, with risks extending into early second trimester 7.
Chromosomal and Genetic Factors
- While chromosomal abnormalities cause approximately 50-60% of first trimester losses, they play a lesser role in second trimester losses 5, 3.
- The earlier in gestation the diagnosis of maternal conditions like leukemia, the higher the incidence of spontaneous abortion 1.
Important Clinical Considerations
Risk Assessment
- Advanced maternal age (≥35 years) significantly increases miscarriage risk (OR 1.85; 95% CI, 1.35-2.52) 7.
- Low serum AMH levels are associated with higher miscarriage risk (OR 1.91; 95% CI, 1.40-2.60), with even higher risk in women over 35 with diminished ovarian reserve (OR 2.05; 95% CI, 1.42-2.95) 7.
Diagnostic Approach
- Digital pelvic examination should be avoided until placenta previa, low-lying placenta, and vasa previa have been excluded by ultrasound 1.
- Transabdominal and transvaginal ultrasound are the mainstays for diagnosis, with transvaginal ultrasound being both accurate and safe for evaluating cervical and placental causes 1.