Day Ninety-four of Twinipedia Facts
Twins are defined by the Encyclopedia as "...two young who are simultaneously born from one mother." The father is not mentioned in the definition at all. Why are they not part of this definition? I guess at the time, science did not see the need for the father to be part of the term's clarification but as time goes on, the definition may need to be updated to make it more detailed.
One reason this definition may need updating is because "it has become more evident that twins can have two different fathers. (fraternal (dizygotic) twins that is, not identical (monozygotic) twins)"
In Multiples.about.com, one of my FAVORITE sites to find information about multiples, they noted that this occurrence, referred to as Superfecundation, "occurs when two or more of a woman's eggs are fertilized by different men within the same ovulation period."
It is crazy, but sperm can live inside a woman's fallopian tubes for 3-5 days. So the eggs released in her cycle can be fertilized by the two different men's sperm from the different encounters which then can produce babies from those two different fathers that are born at the same time.
The chances of this happening are very, very, VERY rare but it has occurred. There are between seven and ten documented cases in medical journals about these kinds of twins.
The first case ever noted in a science journal was in 1810. In this case, a white woman had a relationships with a black man and a white man. These relations were separated by only a few days. Fraternal twins, one white and the other, a combination of the two races, were the result. The latest edition of the same journal features a case from 1982, shown in the photograph on the left.
Non, multi-race twins make it harder to detect Superfecundation since the different skin colors don't happen but there have been some same sex, same race cases documented, including the one of the couple below from Texas.
Superfecundation can also occur when twins are conceived through fertility treatments. This is beyond rare but has also been documented. In the Dailymail.com, the story of "Koen and Tuen Stuart, Dutch boys who were the result of IVF (in vitro fertilization)", was shared. "In a mix up at the lab, equipment had been used twice, causing another man’s sperm to be mixed with the father's", resulting in the two eggs being fertilized by two men.
Cuties Koen and Tuen Stuart
Since the mid-1980s, documented cases of heteropaternal twins have been on the rise because technology has become more effective at assessing the twin's genetic backgrounds. Testing is easier to conduct as well. Plus more woman are having babies later in life or having fertility help and as women age or get in vitro, it becomes more likely for them to have multiple ova present.