What is the most common chromosomal abnormality in a spontaneously aborted fetus at approximately 12 weeks gestation?

Most Common Chromosomal Abnormality in 12-Week Abortus

Autosomal trisomy (Option B) is the most common chromosomal abnormality in spontaneous abortions at 12 weeks gestation, accounting for approximately 37-49% of all chromosomal abnormalities detected in first-trimester losses. 1, 2

Evidence Hierarchy and Frequency Distribution

The research evidence consistently demonstrates that autosomal trisomies dominate the chromosomal abnormality spectrum in early pregnancy loss:

• Autosomal trisomies represent 48.8% of all chromosomal abnormalities detected in spontaneous abortion materials, making them the single most common category 1
• In a large Scandinavian study of 259 first-trimester miscarriages, autosomal trisomies were found in 37% of all karyotyped samples, compared to polyploidies (9%) and monosomy X (6%) 2
• Among 1,210 products of conception analyzed by next-generation sequencing, autosomal trisomies collectively outnumbered all other abnormalities in first-trimester losses 3

Specific Trisomy Patterns at 12 Weeks

The most frequently encountered autosomal trisomies in first-trimester abortions include:

• Trisomy 16 is the single most common specific abnormality, representing 14% of chromosomally abnormal abortions and 14.38% in NGS-based studies 2, 3
• Trisomy 22 follows as the second most common, accounting for 7.90% of first-trimester chromosomal abnormalities 3
• Trisomy 15 (5.37%) and trisomy 21 also occur with notable frequency in early pregnancy losses 3

Why Other Options Are Less Common

While the other answer choices do occur in spontaneous abortions, they are significantly less frequent than autosomal trisomies:

• Monosomy X (Turner syndrome) accounts for only 6-13% of chromosomally abnormal first-trimester losses, making it substantially less common than the collective category of autosomal trisomies 2, 3
• Triploidy represents approximately 9% of chromosomal abnormalities in first-trimester miscarriages, placing it third after autosomal trisomies and monosomy X 2
• Aneuploidy (Option D) is not a specific diagnosis but rather an umbrella term that includes trisomies, monosomies, and other numerical abnormalities—making it too broad to be the "most common" specific abnormality 1

Clinical Context and Maternal Age Correlation

Understanding the relationship between maternal age and chromosomal abnormalities is critical:

• Autosomal trisomies show strong association with advanced maternal age, with the mean maternal age being significantly higher in the trisomy group compared to other chromosomal abnormalities (p < 0.001) 1
• The incidence of chromosomal abnormalities peaks in mothers aged 31-35 years, with 30% of cases occurring in this age group 4
• Monosomy X and polyploidy appear inversely related to maternal age, distinguishing them from autosomal trisomies 2

Important Clinical Caveats

• Overall, 50-61% of first-trimester spontaneous abortions have chromosomal abnormalities, meaning that nearly half of early pregnancy losses occur despite normal fetal karyotype 5, 1, 2
• The success rate of karyotyping varies, with culture failure occurring in approximately 16-20% of specimens, which may underestimate the true prevalence of chromosomal abnormalities 1
• Fetal chromosomal abnormalities account for the majority of early pregnancy losses, with advanced maternal age (≥35 years) significantly increasing this risk through age-related aneuploidy 5