Twin Facts 
Twin Facts 
About one in 90 U.S. pregnancies include
multiple fetuses, primarily non-identical twins. African-American women have the highest
chances of giving birth to twins, followed by East Indian women, Caucasians and Asians.
In the past 15 years, births of sets of twins jumped 42 percent, to
97,064. Births of triplets or more rose 214 percent.
Nearly two-thirds of multiples were likely conceived because of
fertility procedures. Nearly one-third were born to mothers who got pregnant between ages
35 and 39. After age 40, multiple births tend to drop off. 
Identical twins result when a single fertilized egg splits after
conception. These twins are genetically alike. Identical twinning is a biological
phenomenon and the incidence remains fairly constant throughout the world. Identical twins
have the same chromosomes and usually have remarkable physical similarities.These
"monozygotic" twins are the same sex, the same blood type, and have the same
hair and eye color. They also have the same nose, ear lobe, and lip shapes. However, these
twins are not entirely identical as each one is an individual and has unique traits of his
or her own. 
Fraternal
twins (dizygotic) result from two fertilized eggs and are not necessarily any more alike
than any two singletons born to the same parents. They may or may not be of the same sex.
The tendency to bear fraternal twins may be inherited from either side of the family. The
incidence of fraternal twinning is affected by heredity, race, maternal age, and number of
children previously born.
Girl twins, especially identicals, are frequently more dependent than
either boy/girl or boy/boy twins. Identical girls are most likely to remain very close
throughout life, followed by fraternal girls, identical boys, fraternal boys and boy/girl
pairs. This is also the order of their dependence on each other.
Speech development in twins may be slower (by about three months) than
in singletons because twins are frequently premature, often develop their own language,
and mother may have less time to help them develop verbal skills. By the time twins are
eight years old, they have usually "caught up" with singletons.
The social development of identical twins is remarkably similar. They
tend to show about the same degree of competency in dealing with others, usually have
similar interest patterns, and have similar attitudes and feelings. Fraternal twins differ
from each other in social development to the same degree as ordinary siblings, especially
if they are of different sexes. Environment and parental guidance can have an effect on
children's similarities and differences.
The IQs of identical twins seldom varies more than five points,
while the IQs of fraternals may be as different as those for singletons born in the same
family.
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