Monozygotic twins,, or more commonly referred to as identical twins, begin life as one egg, which becomes fertilized by one sperm and splits in to two eggs early in the gestational period. Identical twins are very rare, have no hereditary link and are almost always of the same gender (boy/boy or girl/girl). Less than one-third of live twin births are identical.
A little known fact is the physical similarities of identical twins depend on how soon the embryo splits after fertilization. The embryo splits very early on during pregnancy, most often within the first 12 days. An embryo that splits before day 4 will result in identical twins who appear much in the same way as fraternal twins, there could be very little physical similarity betw. In this case, a zygosity test may be needed to confirm the babies' identical status. A split between days four to eight will result in the babies sharing the same sac, making it easier to determine zygosity. Once the embryo reaches day 8 to 12 and splits during this timeframe, then the 'seeing double' effect takes place, resulting in twins who are very similar in appearance. Embryos splitting on or after the 12th day will most likely result in conjoined twins.
Dizygotic twins, or fraternal twins, begin life as two eggs fertilized by two separate sperm. The eggs are usually fertilized at 2 separate times during ovulation, but the time interval is very close. Dizygotic twins are the most common type of twin birth, with over two-thirds of live twin births being fraternal, especially with the increased use of fertility drugs, which almost always result in a fraternal twin pregnancy. Fraternal twins may be of the same sex or mixed-gendered, have strikingly similar appearance or look nothing alike, as in the case of my fraternal twins. And, yes, it is possible that fraternal twins can be fathered by sperm of two different men.
By the time an expectant mother has had her first ultrasound, her doctor should be able to discern whether the twins are in separate sacs or share a sac
However, I doubt it, but it's a nice thought.
. I doubt that would really happen though. 
