What Causes Spontaneous Abortion (Miscarriage)?
Spontaneous abortion is most commonly caused by chromosomal abnormalities in the embryo, representing natural selection, though other established causes include antiphospholipid syndrome, parental chromosomal translocations, polycystic ovarian disease (PCOD), advanced maternal age (>40 years), anatomical uterine malformations, thyroid dysfunction, and reproductive tract infections.
Primary Etiologic Categories
Genetic Causes
Chromosomal abnormalities are the predominant cause of first-trimester pregnancy loss, representing the body's natural selection mechanism 1. When evaluating recurrent losses, pre-conception chromosomal analysis of both partners (or aborted tissue) is recommended to identify parental translocations 2.
Endocrine Factors
- Thyroid dysfunction should be excluded in all cases of recurrent loss 2
- Polycystic ovarian disease (PCOD) is a well-substantiated cause 1
- Corpus luteum defect has largely been refuted as a common cause by current evidence 1
Anatomical Abnormalities
Müllerian duplication defects likely cause pregnancy loss in some women and should be investigated 1. Guidelines consistently recommend excluding anatomical malformations 2.
Immunologic Factors
Antiphospholipid syndrome is an established, treatable cause that must be excluded 2, 1. Other immunologic factors include:
- Autoimmune antibodies (APA, ANA, ACA, ATA, AECA)
- Alloimmune antibodies (APCA, Ab2, MLR-Bf)
- Altered HLA-G expression
- Th-1 cytokine patterns
- Natural killer cell activity 3
Infectious Causes
Recent evidence identifies reproductive tract infections as key factors 4:
- Bacterial vaginosis shows significant association
- Chlamydia trachomatis
- Ureaplasma urealyticum and parvum
- Vaginal dysbiosis promotes abortion through inflammatory responses, disrupted maternal-fetal immune tolerance, and direct placental damage 4
The association with viruses (HPV, HSV, CMV), Mycoplasma hominis, and Trichomonas vaginalis remains controversial 4.
Environmental and Lifestyle Factors
- Maternal age >40 years is a well-documented risk factor 1
- Cigarette smoking and alcohol consumption likely contribute to pregnancy wastage 1
- Exposure to environmental toxins
- Psychological trauma and stressful life events 3
Clinical Context and Frequency
Approximately 12-24% of recognized pregnancies result in miscarriage, with 85% occurring in the first trimester 5. The causes are heterogeneous, including genetic, environmental, and immunological factors 6. Recurrent spontaneous abortion (defined as 2 or more clinical pregnancies ending in loss) affects 1-3% of couples 2.
Critical Diagnostic Approach
Investigation is indicated after 2 clinical pregnancy losses 2. The diagnostic workup should systematically exclude:
- Parental chromosomal abnormalities (karyotyping or tissue analysis)
- Antiphospholipid syndrome
- Thyroid dysfunction
- Anatomical uterine malformations
- Reproductive tract infections (particularly bacterial vaginosis and Chlamydia)
Important Caveats
Most spontaneous abortions are random events 1. Even with recurrent losses, the majority of cases remain unexplained despite thorough evaluation. Couples with unexplained recurrent abortion demonstrate excellent outcomes in subsequent pregnancies without treatment, making it difficult to justify invasive or expensive unproven therapies 1.
The pathologist's role extends beyond excluding molar pregnancy; histological examination can identify lesions with high recurrence risk that may lead to habitual abortion 6.