What degree of parental consanguinity is suggested by a fetal chromosomal microarray showing long contiguous regions of homozygosity comprising approximately 3 percent of the autosomal genome?

Degree of Parental Relationship with 3.01% Homozygosity

A fetal chromosomal microarray showing 3.01% regions of homozygosity (ROH) is most consistent with parental consanguinity at the level of second cousins (or possibly between second and third cousins). 1

Interpretation Framework

According to the 2022 ACMG technical standards, the interpretation of ROH percentage directly correlates with degree of parental relationship 1:

• Second cousins: Average 1.56% IBD (identity by descent)
• First cousins: 6.25% IBD
• First- or second-degree relationships (parent-child, siblings): >10% ROH

Your finding of 3.01% ROH falls approximately twice the expected value for second-cousin offspring (1.56%), which suggests:

1. Second-cousin relationship with additional background homozygosity from population-specific ancestral haplotypes
2. A relationship slightly closer than second cousins but more distant than first cousins
3. Possible third-cousin relationship with accumulated homozygosity from prolonged inbreeding in the population

Clinical Significance and Next Steps

Immediate Implications

This finding warrants reporting to the ordering clinician and triggers specific clinical actions 1:

• Increased risk for autosomal recessive disorders: The fetus has elevated probability of inheriting two copies of deleterious recessive alleles
• Autozygosity mapping utility: If fetal phenotypic abnormalities are present, genes within the homozygous regions become priority candidates for sequencing
• Genetics consultation is mandatory to discuss implications with the family

Important Caveats

The 3.01% finding does NOT suggest first- or second-degree parental relationships (which would require >10% ROH), so concerns about incest or mandatory reporting to authorities are not applicable in this case 1.

However, you must consider:

• Population-specific context: Research shows that populations with prolonged consanguineous marriage traditions demonstrate ~5% higher baseline homozygosity than predicted by simple models 2. Brazilian and Korean population studies confirm that ancestral haplotypes commonly create ROH "hotspots" that inflate percentages 3, 4

• Size of individual ROH segments matters: The ACMG recommends focusing on segments >3-5 Mb, as smaller segments likely represent linkage disequilibrium rather than true IBD 1. Verify that your 3.01% calculation uses only large contiguous segments

Reporting Threshold Context

The ACMG specifically recommends a 2-3% cutoff for reporting ROH in postnatal cases based on second-cousin progeny 1. However, for fetal testing, a higher threshold of >5% is recommended because limited phenotypic information is available prenatally 1.

Despite this, your 3.01% finding has clinical utility and should be reported because:

• It exceeds the 2-3% postnatal threshold
• It facilitates consideration of recessive mechanisms
• It enables targeted genetic testing if ultrasound abnormalities are present

Practical Management Algorithm

1. Report the finding using ACMG-recommended language emphasizing possible consanguinity and increased AR disorder risk 1

2. Review fetal ultrasound findings carefully: Research shows 59% of ROH cases have ultrasound abnormalities, most commonly soft markers 5, 6

3. If ultrasound abnormalities present: Consider whole-exome sequencing focused on genes within the homozygous regions 1, 5

4. Obtain detailed family history: Confirm or exclude known consanguinity; assess for ethnic backgrounds with cultural norms of cousin marriage 1

5. Counsel parents about:

   • Implications for current pregnancy
   • Recurrence risk in future pregnancies
   • Potential need for carrier screening for specific AR conditions if family history or ethnicity suggests particular risks

Key Pitfall to Avoid

Do not automatically assume pathology from ROH alone 1. The detection of homozygous regions is not diagnostic of any specific disorder—it simply increases the probability of AR conditions. Most fetuses with 3% ROH and normal ultrasounds have normal postnatal phenotypes 6, 7.