Can Fraternal Twins Have Different Fathers?
Yes, fraternal (dizygotic) twins can absolutely have different biological fathers through a rare phenomenon called heteropaternal superfecundation, which occurs when two eggs released during the same ovulatory cycle are fertilized by sperm from different men during separate acts of sexual intercourse.
Biological Mechanism
Heteropaternal superfecundation requires the following conditions:
- Multiple ovulation: Two or more eggs must be released during the same menstrual cycle 1
- Separate sexual encounters: The woman must have intercourse with different male partners within a short timeframe during the fertile window 2, 3
- Successful fertilization: Each egg must be fertilized by sperm from a different man 4
This results in dizygotic twins who are genetically half-siblings rather than full siblings, sharing only their maternal genetic contribution 5.
Clinical Frequency and Detection
The actual incidence of heteropaternal superfecundation is likely underestimated:
- General population estimate: Among dizygotic twins born to married white women in the U.S., approximately 1 in 400 twin pairs may be bipaternal 6
- Overall superfecundation rate: At least 1 in 12 dizygotic twin pregnancies may be preceded by superfecundation (fertilization from different coital acts, whether from the same or different fathers) 6
- Detection bias: The true frequency is probably higher than reported because most cases go undetected unless paternity testing is performed 5, 6
Diagnostic Confirmation
When heteropaternal superfecundation is suspected (typically in paternity disputes):
- DNA testing is mandatory for both twins: Testing only one twin in a paternity case can lead to incorrect conclusions 3
- Multiple genetic markers: Standard short tandem repeat (STR) analysis, Y-chromosome markers, and newer microhaplotype sequencing can definitively establish different paternity 2, 3, 5
- Early detection possible: Modern techniques including microhap sequencing of maternal plasma cell-free DNA can identify heteropaternal superfecundation as early as 10 weeks gestation, noninvasively 2
Clinical Pitfall
Critical warning: In any paternity dispute involving dizygotic twins, both twins must be tested. Testing only one twin and assuming the result applies to both can result in serious legal and medical errors, as the twins may have different biological fathers 3.