At what gestational age can a DNA paternity test be performed on a fetus?

DNA Paternity Testing on a Fetus: Timing and Methods

DNA paternity testing on a fetus can be performed as early as 7 weeks of gestation using non-invasive prenatal paternity testing (NIPPT) that analyzes cell-free fetal DNA in maternal blood, though optimal accuracy is achieved at 10 weeks or later. 1, 2, 3

Non-Invasive Prenatal Paternity Testing (NIPPT)

Earliest Feasible Timing

• NIPPT can detect paternally inherited fetal alleles starting at 7 weeks of gestation, though detection rates are lower at this early stage 2, 4, 3
• At 6-7 weeks, fetal DNA fraction in maternal blood is extremely low (often <1%), resulting in higher false negative rates of approximately 6% 2
• By 10 weeks of gestation, approximately 10-15% of cell-free DNA in maternal plasma is of placental origin, providing sufficient fetal fraction for reliable analysis 5

Optimal Testing Window

• The recommended timing for NIPPT is 10 weeks or later for optimal accuracy and reliability 5
• Fetal fraction increases progressively: median 0% at 4 weeks, 3.9% at 7 weeks, 5.1% at 12 weeks, and remains stable thereafter 4
• Detection of paternally inherited alleles improves significantly with gestational age: median of 3 alleles at 7 weeks versus 9 alleles at 12 weeks 4

Clinical Performance

• NIPPT using next-generation sequencing can accurately determine paternity from 7 weeks through delivery, with combined paternity indices ranging from 10^18 to 10^165 3
• In a large-scale trial of 349 cases (6-35 gestational weeks), biological fathers were successfully identified in 100% of cases with no false positives 3
• At 12 weeks, typical paternity indices reach 24, increasing to 199 by 20 weeks, providing conclusive results 4

Invasive Diagnostic Testing Options

Chorionic Villus Sampling (CVS)

• CVS can be performed from 10-13 weeks of gestation for definitive fetal DNA analysis 6
• Provides direct fetal tissue sampling with 100% accuracy for paternity determination 6
• Carries a small risk of pregnancy loss, particularly with transcervical approaches 6

Amniocentesis

• Amniocentesis can be performed starting at 15 weeks of gestation 6
• Risk of pregnancy loss is approximately 1 in 300-600 procedures with sonographic guidance 6
• Provides definitive results but requires waiting until second trimester 6

Clinical Algorithm for Paternity Testing

Before 7 Weeks

• NIPPT is not reliable due to insufficient fetal DNA fraction 2, 4
• Counsel patient to wait until at least 7 weeks, preferably 10 weeks 5, 2

7-9 Weeks

• NIPPT is technically feasible but has higher failure rates (up to 6% false negatives) 2
• If testing is urgent, proceed with NIPPT but counsel about potential need for repeat testing 2
• Consider waiting until 10 weeks for optimal accuracy 5

10-13 Weeks (Optimal Window)

• NIPPT is the preferred method with high accuracy and no procedural risk 5, 3
• CVS is available as an invasive alternative if definitive results are required immediately 6

15 Weeks and Beyond

• Both NIPPT and amniocentesis are available 6, 3
• NIPPT remains non-invasive and highly accurate throughout pregnancy 3

Important Caveats

Factors Affecting NIPPT Accuracy

• High maternal BMI strongly reduces fetal fraction and may cause test failure in up to 20% of obese women 5
• Multiple gestations or vanishing twin syndrome can affect interpretation 5
• Certain fetal chromosomal abnormalities (trisomies 13,18) may be associated with low fetal fraction 5

Critical Counseling Points

• NIPPT requires appropriate pre-test and post-test genetic counseling 5
• Testing should be performed in the context of maternal autonomy and informed consent 7
• Results should be interpreted by laboratories experienced in forensic genetic analysis 1, 2