Fertilization of one egg by two sperm is possible, but the resulting zygote is almost never viable because such a zygote would have three sets of chromosomes instead of the normal two. This condition, called triploidy, is usually incompatible with life. Those rare triploid infants that do survive to term have severe and multiple birth defects and rarely live more than a few days.
Fertilization is the process where two gametes fuse to form a single zygote. Gametes, such as sperm and egg cells, are haploid, meaning they have only half the normal number of chromosomes. The combination of one sperm and one ovum creates a zygote with a full, diploid set of chromosomes. Triploid babies have 69 chromosomes instead of the normal 46. While triploidy is believed to occur in 1 to 2 percent of all conceptions, such pregnancies usually result in spontaneous miscarriage or still-birth before reaching full term. In 2007, Time Magazine reported on a pair of twins who were apparently the result of an egg fertilized by two different sperm. The resulting twins were identical on their mother's side, but only shared half of their father's DNA. While scientists are uncertain about the exact mechanism that occurred, they believe the triploid zygote must have divided into two separate zygotes after fertilization. This same division occurs in the development of identical twins, only in this extraordinary case, each new cell somehow managed to shed the superfluous genetic material, resulting in two diploid zygotes.