What is the most common chromosomal abnormality in a 12‑week abortus—aneuploidy or autosomal trisomy?

Most Common Chromosomal Abnormality in 12-Week Abortus

Autosomal trisomy is the most common chromosomal abnormality in 12-week abortuses, representing the predominant subtype within the broader category of aneuploidy.

Understanding the Terminology

The question presents a false dichotomy—autosomal trisomy is actually a specific type of aneuploidy, not a separate category. Aneuploidy refers to any abnormal number of chromosomes, which includes:

• Autosomal trisomies (extra autosomal chromosome)
• Monosomy X (missing sex chromosome)
• Polyploidy (complete extra chromosome sets)
• Sex chromosome aneuploidies

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Frequency of Chromosomal Abnormalities

Overall chromosomal abnormality rates in first-trimester miscarriages range from 47-70%, with the highest quality studies reporting:

• 70.3% in a series of 1,011 consecutive chorionic villi samples 1
• 67.25% in a large study of 7,118 miscarriages 4
• 61% in 259 consecutive first-trimester miscarriages 3
• 58.59% in 990 early missed abortion cases 5

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Autosomal Trisomy as the Predominant Abnormality

Among all chromosomal abnormalities detected in first-trimester losses, autosomal trisomies consistently represent 56-67% of abnormal cases:

• Single autosomal trisomy accounted for 64.6% of abnormal karyotypes in the largest systematic study 1
• Trisomies represented 67.24% of chromosomal abnormalities in recent CNV-seq analysis 5
• 88.62% of abnormalities were numerical chromosomal abnormalities, with autosomal trisomies being the dominant subtype 5
• Autosomal trisomies were found in 37% of all karyotyped samples (both normal and abnormal) 3

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Specific Trisomy Distribution

The most frequently encountered autosomal trisomies in 12-week abortuses are:

• Trisomy 16: Most common overall, representing 14-19% of chromosomal abnormalities 5, 6, 3
• Trisomy 22: Second most common at approximately 19% 5
• Trisomy 21,18,13, and 15: Also frequently observed but less common than trisomy 16 1, 4

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Other Chromosomal Abnormalities (Less Common)

Following autosomal trisomies in frequency:

• Triploidy: 8-13% of abnormal cases 5, 1, 2
• Monosomy X: 6-10% of abnormal cases 1, 2, 3
• Structural rearrangements: 5-11% of abnormal cases 1, 2
• Tetraploidy and other polyploidies: 1-2% 5, 6

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Clinical Implications

The high rate of autosomal trisomies in early pregnancy losses reflects natural selection against chromosomally abnormal embryos:

• Approximately 70% of trisomy 18 fetuses alive in the second trimester will be spontaneously lost by term 7
• Most autosomal trisomies (except 13,18, and 21) are incompatible with survival beyond the first trimester 7
• Trisomy 16, the most common abnormality in abortuses, is never seen in live births 3

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Important Caveats

When counseling patients about chromosomal abnormalities in miscarriage:

• Maternal age significantly affects the rate of chromosomal abnormalities, increasing from 54.52% in women <35 years to 73.97% in women ≥40 years 5, 4
• The proportion of autosomal trisomies increases with maternal age, while monosomy X and polyploidy decrease 5, 4
• Cell-free DNA screening detects only trisomies 21,18, and 13, missing 16.9% of chromosomal abnormalities that would be detected by diagnostic testing 7
• Cytogenetic analysis of miscarriage tissue provides definitive information for genetic counseling and future pregnancy planning 8, 1, 2

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