Twins are commonly understood to be either identical, originating from a single fertilized egg that splits, or fraternal, developing from two separate eggs fertilized by two different sperm. However, a less known biological phenomenon allows twins to share the same mother but have different biological fathers. This occurrence, though rare, is scientifically possible.
The Phenomenon Explained
Twins having different fathers is a biological event termed heteropaternal superfecundation. This term combines “heteropaternal,” signifying different fathers, and “superfecundation,” meaning the fertilization of two ova during the same menstrual cycle by separate mating actions. It describes a situation where a woman releases two eggs within the same ovulatory cycle, and these eggs are then fertilized by sperm from two different partners during separate instances of sexual intercourse within a short timeframe. These twins are always fraternal, originating from two distinct eggs fertilized by two distinct sperm cells.
The Biological Basis
The possibility of heteropaternal superfecundation stems from specific processes within the female reproductive cycle. Typically, during ovulation, a woman’s ovary releases one egg, which remains viable for fertilization for 12 to 24 hours. However, some women can experience “hyperovulation,” where two or more eggs are released during the same menstrual cycle. Sperm cells can survive within the female reproductive tract for several days, creating a fertile window. If a woman releases two eggs and engages in sexual intercourse with different partners within this narrow fertile window, each egg can be fertilized by sperm from a different father.
Incidence and Identification
Heteropaternal superfecundation is exceptionally rare in humans, and its true incidence is challenging to determine because it often goes undiscovered unless paternity is questioned or challenged. The rarity is due to the precise timing required: multiple ovulations, sexual encounters with different partners, and successful fertilization of both eggs within a short period. Confirmation relies on genetic testing after the twins are born. DNA paternity tests compare specific genetic markers from each child with those of the alleged fathers. These tests can determine with high accuracy whether a man is the biological father of each twin, with a result of 0% probability ruling out paternity and 99.9% or higher confirming it.