Embryo implantation into a receptive uterine environment is necessary
for the development of a successful pregnancy. Barriers to implantation
certainly can exist for the preimplantation embryo (fertilized
egg) that normally enters the uterine cavity following fertilization
in the fallopian tube.
At the time of In Vitro Fertilization, 4 or 5 good quality embryos
are often placed into the uterine cavity and there is usually
a dramatic reduction in the number of embryos that actually will
implant and develop into a pregnancy.
Research on natural conception suggests that a large percentage
of fertilized eggs are lost both prior to implantation and also
following implantation prior to the clinical detection of pregnancy.
The reason for such a high loss rate remains unclear. Most researchers
are just now beginning to study the molecular events that occur
at the time of embryo implantation. There is an enormous complexity
to these molecular interactions.
Research results on cell adhesion molecules (CAMs, sometimes referred
to as "molecular glue" that allows the embryo to adhere
to the uterine wall) that may be involved in the process of human
embryo implantation include the "beta 3 integrin subunit"
and "osteopontin." This research has been publicized,
is fascinating, and yet none of the results have a widely accepted
clinical role. Therefore, this testing is experimental.
Clinical attention has been predominantly directed at detection
and correction of anatomic abnormalities associated with unsuccessful
implantation. "Congenital abnormalities of the Mullerian
ducts" are a relatively common group of uterine defects associated
with reproductive difficulty. These abnormalities result from
problems in development in female offspring that occur at an embryonic
stage. Normally two "tubes" known as Mullerian ducts
fuse at about 8 weeks of gestation (when the mother is in the
8th week of pregnancy) in embryos that are destined to become
girls, and these fused ducts then go through changes that result
in the formation of the uterus and fallopian tubes. This process
is complete by about the 20th week of pregnancy. When there is
a problem in the normal development of the uterus and tubes then
anatomic abnormalities with reproductive impact can occur.
Mullerian duct abnormalities include
* (1) septate uterus:
this is a partial lack of resorption of the poorly vascularized
tissue within the uterine cavity. The remaining tissue creates
a wedge shaped septum in the fundus (top) of the uterine cavity.
Implantation of an embryo onto this septum, and within a septate
uterus, has a markedly greater chance of spontaneous abortion
(miscarriage) compared with a normally developed uterus. There
is also an increase in preterm labor and delivery as well as abnormal
fetal lie or presentation (such as breech).
Fortunately, the repair of the septate uterus is fairly straightforward
and usually very effective, requiring only an outpatient surgical
procedure. Reportedly, repair of a septate uterus will improve
the miscarriage rate from about 90% if untreated to about 10-15%
if treated.
* (2) unicornuate uterus:
this is the failure in development of 1 of the Mullerian ducts,
most likely due to a failure in migration (movement) of the duct
to its proper location resulting in its total loss. The resulting
unicornuate or half uterus has connection to only 1 fallopian
tube since the other tube was to be formed from the "lost"
Mullerian duct.
The caliber (size) of the cavity in the unicornuate uterus is
very important in determining the likelihood of reproductive success.
Unfortunately there is no accepted benefit for the treatment of
these uterine defects.
The unicornuate uterus is associated with renal abnormalities
(renal agenesis or lack of a kidney on the side of the missing
Mullerian structures) and reproductive problems (abnormal lie
or presentation, intrauterine growth retardation, preterm labor
and delivery, incompetent cervix).
* (3) bicornuate uterus:
occurs with a partial lack of fusion of the Mullerian ducts, resulting
in a single cervix and two uterine cavities in a heart shaped
partially unified uterus. Reproductive outcome may be normal so
no treatment is indicated unless reproductive problems are identified.
Reports suggest an increase in spontaneous abortion (miscarriage),
preterm labor and delivery, and abnormal presentation (breech).
The patient with recurrent pregnancy loss, a bicornuate uterus,
and treatment for all other identified causes for the losses may
reasonably consider repair of the uterus.
The treatment is surgical repair requiring a laparotomy with unification
of the uterine cavities. The laparotomy requires a lengthy postoperative
recovery period.
Success with this surgery is generally quite good, improving the
miscarriage rate from about 90-95% if untreated (and the cause
for the recurrent losses) to about 25-30% if treated.
* (4) didelphic uterus:
This results from a complete lack in fusion of the Mullerian ducts
with duplication of the uterus and cervix so that the patient
has 2 cervices and two uteruses (each smaller than normal). This
is commonly also associated with a vaginal septum so that there
are 2 vaginal canals at the top of the vaginal vault.
Occasionally, one of the sides will become obstructed and result
in pain as blood accumulates in the obstructed region.
These are associated with abnormal lie or presentation as well
as preterm labor and delivery.
* (5) rare abnormalities:
there are an entire host of intermediate or somewhat unique problems
associated with abnormal development of the Mullerian structures.
Isolated endometrial (lining of the uterus) or cervical (mouth
of the uterus) agenesis (lack of development) are rare. Communicating
and noncommunicating uterine horns that failed to fuse and canulate
properly are possible.
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