Like single child conceptions identical twins are conceived through the ovulation and fertilisation of one egg, however this fertilised egg then splits into two parts and continues to grow as two seperate embryos.  Identical twins can share a placenta or each develop their own.  This depends on exactly when the fertilised egg split into two parts.  If the two parts move down the fallopian tube separately then each will separately attach itself to the womb lining.  This creates double embryos which will develop their own placentas. If the split happens after the fertilised egg has embedded in the womb lining then the embryos will share a placenta and occasionally they may also share a single sac of amniotic fluid.  Very rarely the fertilised egg can split later than this resulting in conjoined or siamese twins.

 

Fraternal twins are brought about by the production and fertilization of two separate eggs by two separate sperm. As both eggs are seperate they will always travel down the fallopian tubes seperately meaning they will always develop their own placentas and their own sacs of amniotic fluids, and can implant in quite different parts of the womb.