It is biologically possible for twins to have different biological fathers. This rare occurrence involves two distinct fertilization events during the same reproductive cycle.
How Twins Can Have Different Fathers
The biological process enabling twins to have different fathers is known as heteropaternal superfecundation. This occurs when a woman releases two separate eggs during a single ovulatory cycle, and these eggs are then fertilized by sperm from two different male partners within a short timeframe. Sperm can remain viable in the female reproductive tract for up to five days, while an egg is typically viable for 12 to 48 hours after ovulation. This window allows for two separate acts of intercourse with different partners to result in the fertilization of each egg.
For this to happen, the woman must first undergo hyperovulation, which is the release of more than one egg in a single menstrual cycle. While typically only one egg is released, some women naturally release multiple eggs, and this can also be influenced by factors like fertility treatments. Each released egg must then be fertilized by sperm from a different male, creating two genetically distinct embryos. These embryos then develop concurrently, leading to the birth of twins who share a mother but have different biological fathers.
Understanding Twin Varieties
To understand this phenomenon, it helps to distinguish between identical and fraternal twins. Identical twins, also known as monozygotic twins, result from a single fertilized egg that splits into two embryos early in development. Because they originate from the same egg and sperm, identical twins share nearly 100% of their genetic material and are always the same sex.
Fraternal twins, or dizygotic twins, develop from two separate eggs fertilized by two separate sperm cells. These twins are genetically as similar as any other siblings, sharing about 50% of their DNA, and they can be the same or different sexes. Twins with different fathers are always fraternal because they arise from two distinct eggs, each fertilized independently. These fraternal twins, while sharing a mother, are biological half-siblings due to their differing paternal genetic contributions.
Uncovering Such Cases
Heteropaternal superfecundation is considered extremely rare in humans, though its exact prevalence is difficult to determine because many cases may go unnoticed. Reports indicate that only about 19 cases have been documented worldwide, though some experts suggest it might occur more often than confirmed. These cases typically come to light when paternity is questioned, often due to differences in the twins’ appearances or other familial circumstances.
Confirmation of heteropaternal superfecundation primarily relies on DNA paternity testing. After the twins are born, DNA samples are collected from the twins and the potential fathers. Genetic analysis then definitively determines the biological father of each child. While less common, paternity testing can also be performed during pregnancy through non-invasive prenatal paternity tests.