Fetus in
fetu
Fetus in
fetu (or
fœtus in
fœtu) is a
developmental
abnormality
containing
entire organ
systems,
even major
body parts
such as
torso or
limbs. The
abnormality
occurs in 1
in 500,000
live
births.
Theories of
development
There are
two main
theories
about the
development
of fetus in
fetu; one
simple, the
other
complex.
Teratoma
theory
Fetus in
fetu may be
a very
highly
differentiated
form of
dermoid
cyst, itself
a highly
differentiated
form of
mature
teratoma.
Parasitic
twin theory
Fetus in
fetu may be
a parasitic
twin fetus
growing
within its
host twin.
Very early
in a
monozygotic
twin
pregnancy,
in which
both fetuses
share a
common
placenta,
one fetus
wraps around
and envelops
the other.
The
enveloped
twin becomes
a parasite,
in that its
survival
depends on
the survival
of its host
twin, by
drawing on
the host
twin's blood
supply. The
parasitic
twin is
anencephalic
(without a
brain) and
lacks some
internal
organs, and
as such is
almost
always
unable to
survive on
its own.
Sometimes,
however, the
host twin
survives and
is
delivered.
The
parasitic
twin grows
so large
that it
starts to
harm the
host, at
which point
doctors
usually
intervene.
The
condition
causes the
host to look
pregnant,
and can
occur in
both males
and females.
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