Causes of First Trimester Threatened Abortion
Threatened abortion in the first trimester has multiple etiologies, with chromosomal abnormalities accounting for 50-60% of cases, followed by maternal factors including hormonal deficiencies (particularly luteal phase deficiency), infections, uterine abnormalities, immunologic disorders, and environmental exposures 1.
Fetal/Genetic Causes
Chromosomal abnormalities are the predominant cause, representing the majority of first-trimester pregnancy losses 1. This represents a natural "quality control" mechanism in human reproduction, where genetically abnormal embryos fail to progress.
Maternal Endocrine Factors
- Luteal phase deficiency/progesterone insufficiency is the most common treatable maternal cause 1
- Hormonal imbalances: Recent evidence demonstrates that women with threatened abortion have significantly lower levels of:
- Thyroid dysfunction (hypothyroidism) 1
- Diabetes mellitus (poorly controlled) 1
Maternal Anatomic/Structural Causes
- Uterine abnormalities: septate uterus, submucous leiomyomata 1
- Cervical incompetence (though typically manifests later in pregnancy) 1
Infectious Causes
Abnormal vaginal flora has been implicated, with 65% of women with threatened abortion showing bacterial vaginosis (Nugent score >4) in one study 3. Specific infections can trigger pregnancy loss through inflammatory mechanisms 1.
Immunologic Causes
- Antiphospholipid antibodies are associated with reproductive failure 1
- Maternal-fetal histocompatibility issues 1
- Downregulation of ESRRG (estrogen-related receptor gamma) in trophoblasts leads to mitochondrial dysfunction and impaired placental function 4
Environmental and Lifestyle Factors
- Excessive smoking 1
- Environmental toxicants and chemical exposures 1
- Gut microbiota dysbiosis: Recent Mendelian randomization studies suggest causal relationships between specific bacterial genera (Paraprevotella, Clostridium) and spontaneous abortion 5
Clinical Context
Approximately 25% of pregnant women experience first-trimester vaginal bleeding 6, and about 15% of clinically recognized pregnancies end in spontaneous abortion 1. The key clinical distinction is that threatened abortion presents with bleeding and/or cramping while the cervix remains closed 1, differentiating it from inevitable or incomplete abortion.
Important Caveat
Treatment to maintain pregnancy is only reasonable when fetal factors are not the primary cause 1. Interventions that allow chromosomally or anatomically abnormal embryos to survive to term provide no benefit and may cause harm. Therefore, identifying the underlying etiology is crucial before initiating any therapeutic intervention beyond expectant management.