I Love and I have Twins
Special Boys Identical Twins
Identical twins are the result of one egg splitting post-fertilization,
but the timing of the split determines if the twins will each have their
own sacs and placentas (split on day 1-3, called di/di twins), share a
placenta (split on day 4-8, called mo/di twins), share an amniotic sac
and placenta (split on day 8-13, called mo/mo twins) or be conjoined
(split on day 13-15) which is, of course, the most rare: only 1 in
200,000 births.
Learn more on Wikipedia