Twin Causes



The cause of monozygotic twinning is unknown, but
it is not usually thought to be genetic. Less than
20 families have been described with an inherited
tendency towards monozygotic twinning. (People in
these families have a nearly 50 percent chance of
delivering monozygotic twins!) Some evidence
suggests that the environment of the womb causes
the zygote to split in most cases.
Dizygotic twins are slightly more likely in
African American females, in women between the
ages 30 and 40, in tall and heavy women, in women
who conceive soon after stopping their
consummation of birth control pills, and in women
who have had many previous pregnancies. Those who
have been taking fertility drugs have the greatest
chance of having multiple births.