Biology



In a woman who has conceived identical twins, one
egg joins with one sperm and form the zygote and
the zygote splits. Both parts of the zygote travel
down the Fallopian tube and attaches in the womb
together to separately.


The developing embryo is surrounded by a sack of
two layers of thin tissue called membranes: an
amniotic membrane (amnion) and around that a
chorionic membrane (chorion). If the zygote
happens to split early (1 to 3 days after
fertilization), each embryo has a separate chorion
and amnion. The later the zygote splits after
ovulation, the higher the chance is of the embryos
sharing one chorion. If after 8 days the zygote
has split, the amnion may also be shared. If after
14 days the zygote has split, there is a
possibility the twins may be joined together at
some part of their bodies and are called conjoined
twins (Siamese twins).
In a woman who has conceived fraternal twins, two
separate fertilizations occur at the same time
between two eggs and two sperm. Each embryo has
its own amnion and chorion.