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About
Twins
All babies are adorable, but twins
have that special something about them that makes heads turn every time
they pass. Twins are one of nature's wonders. It is amazing that there
can be two individuals born at the same time, who are the spitting images
of each other, and may even display similar personality traits. Twins have
been the subject of innumerable scientific studies and psychological experiments
and have provided invaluable data for the endless nature versus nurture
question. And of course, they have been the subject of innumerable books
and they have fascinated scriptwriters who have churned out hundreds of
films whose plots have turned on mistaken identity.
The biology of
twins
So how do we explain the twin phenomenon?
If you didn't know, there are two types of twins - identical and fraternal.
Identical twins are also called monozygotic, meaning one egg. They have
identical genes and are always of the same sex. Fraternal twins are called
dizygotic or two egg. They share approximately 25 percent of their genes,
as do any two brothers or sisters. One-half of fraternal twin pairs are
boy-girl, one-quarter are boy-boy, and one-quarter are girl-girl.
Identical twins are the result of
one egg joining with one sperm to form the zygote. The zygote then completely
splits. This two-cell mass passes down the Fallopian tube and attaches
separately or together in the womb. 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
splits early (1 to 3 days after fertilization), each embryo will have a
separate chorion and amnion. The later the zygote splits after ovulation,
that is at 4 to 8 days, the greater the likelihood of the embryos sharing
one chorion. If the zygote splits after 8 days, the amnion may also be
shared. But if the zygote splits after 14 days, the twins may be joined
together at some part of their bodies and are called conjoined or Siamese
twins.In the case of fraternal twins, two separate fertilizations occur
simultaneously between two sperm and two
separate eggs.
Fraternal or identical?
Vaginal delivery is often possible,
especially when the babies are in the head-down position. Sometimes one
monozygotic twin transfers too much blood to the other, leaving the first
twin anemic and small and the second ruddy and large. This can only happen
to monozygotic twins, and only when they share chorions. That's why the
term "identical" can be misleading, since monozygotic twins may differ
because one of them has a birth defect or there may be a difference in
their blood quantity. Bowed legs and bent ankles may occur in twins due
to the crowding in the womb. However, these problems usually disappear
completely as time passes.
It is important to determine whether
your twins are identical or fraternal right at the time of birth. The best
time to do this is right after the delivery. If the sexes differ, the twins
are dizygotic. If they're the same sex, the obstetrician, after carefully
examining the placenta, will send it to the pathologist for detailed study.
If there is only one shared chorion, they are monozygotic twins. If there
are two chorions, then they may be monozygotic or dizygotic, and a simple
blood test can be done to compare blood cell groups. If red blood cell
groups differ, then the twins are dizygotic.
Monozygotic twins who differ in size
may have birth defects. Identifying monozygotic twins at birth is important
because it can improve medical care throughout life. For example, if one
monozygotic twin develops a disease such as diabetes or epilepsy, the second
twin has a higher than average chance of getting the same disease and should
be carefully checked. Also, the behavioural development of monozygotic
and dizygotic twins differs, which is important in parenting. Generally,
identical twins have closer relationships than fraternal and they are more
cooperative. Fraternal twins, on the other hand, develop their individuality
easier because of clearer physical, emotional, and genetic differences.
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