Paternity Assessment Based on Clinical Timeline
Based on the ultrasound dating (9 weeks gestation, CRL 2.56 cm) that aligns precisely with your LMP, conception occurred during the ovulation window indicated by your egg-white cervical mucus observation, making Partner A overwhelmingly likely to be the biological father—the Plan B taken after the first unprotected encounter with Partner A does not eliminate pregnancy risk, and the condom-protected intercourse with Partner B occurred outside the fertile window.
Clinical Dating Analysis
Your ultrasound findings provide the most reliable gestational age assessment:
- Crown-rump length measurement of 2.56 cm at 9 weeks gestation is the gold standard for pregnancy dating, with an accuracy of ±7.3 days when measured between 8-12.5 weeks 1, 2.
- The ultrasound-derived estimated date of delivery matches your LMP-based calculation, confirming that ovulation and conception occurred during the expected fertile window 3.
- In a typical 28-day cycle, ovulation generally occurs between days 9-20, and your observation of egg-white cervical mucus is a reliable indicator that you were in the 6-day fertile window (the 5 days before ovulation plus ovulation day itself) 3, 4.
Contraceptive Efficacy Considerations
Plan B (Levonorgestrel) Limitations
- Levonorgestrel 1.5 mg prevents approximately 80% of pregnancies, meaning it has a 20% failure rate even when taken promptly 5, 6.
- The mechanism works primarily by blocking or delaying ovulation when given before the preovulatory LH peak—if ovulation had already begun or occurred shortly after administration, efficacy is substantially reduced 5.
- Plan B does NOT prevent pregnancy from intercourse that occurs after ovulation has already been triggered, and your cervical mucus observation suggests you were entering or in your fertile window during the unprotected encounters with Partner A 7, 5.
Condom Efficacy
- Male condoms have an 18% typical-use failure rate and 2% perfect-use failure rate over one year of use 3.
- However, a single act of intercourse with intact condom use by an experienced user who does not want more children carries extremely low pregnancy risk, particularly when occurring outside the fertile window 3.
Probability Assessment
Partner A (Unprotected Intercourse During Fertile Window)
- Multiple episodes of unprotected intercourse occurred during or immediately before your documented fertile window (egg-white cervical mucus observation) 4.
- The ultrasound dating directly supports conception during this timeframe 1, 2.
- Even with Plan B administration, the 20% failure rate combined with timing near ovulation makes pregnancy from these encounters highly plausible 5, 6.
Partner B (Condom-Protected Intercourse Outside Fertile Window)
- The encounter occurred outside the 6-day fertile window based on your cervical mucus observation and ultrasound dating 3, 4.
- No evidence of condom failure was noted, and the user was experienced 3.
- For pregnancy to have resulted from this encounter, both condom failure AND late ovulation (significantly beyond the typical window) would need to have occurred simultaneously—this represents a compound improbability 3, 4.
Clinical Reassurance
Your clinical data are internally consistent and highly reliable:
- The ultrasound CRL measurement provides ±7.3 days accuracy for dating, making it the most precise method available in early pregnancy 1, 2.
- The concordance between LMP-based dating and ultrasound dating confirms regular ovulation during the expected window 1, 2.
- Your observation of egg-white cervical mucus is a validated marker of the fertile window, further supporting the timeline 4.
Prenatal Paternity Testing
- Non-invasive prenatal paternity testing using cell-free fetal DNA from maternal blood is available as early as 8-9 weeks gestation and carries no procedural risk to the pregnancy 8.
- Invasive procedures (amniocentesis, CVS) carry a 0.5-1.0% pregnancy loss risk and are not necessary for paternity determination 8.
- As a self-pay patient, discuss non-invasive prenatal paternity testing options with your provider for definitive confirmation without procedural risk 8.
Summary of Evidence
The convergence of multiple data points—ultrasound dating matching LMP, observed fertile window signs, unprotected intercourse timing, Plan B's known failure rate, and absence of condom failure—strongly indicates Partner A as the biological father. The scenario requiring Partner B to be the father would necessitate both condom failure and ovulation occurring well outside the typical window, neither of which is supported by your clinical findings 3, 4, 1, 2.