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Identical twins (monozygotic twins)
Two offspring created when a single fertilized egg
divides to form two separate embryos during the
first 2 weeks after conception. Identical twins account for about 30% of
naturally occurring twins in the United States.
Implantation (nidation)
Penetration into the uterine
mucosa by the embryo. Implantation occurs approximately 6 days after conception.
Abnormal implantation of the embryo outside of the uterus is called an ectopic
pregnancy.
Incompetent Cervix
See cervical incompetence
Infant
A child under one year of age.
Infertility
Inability to conceive after one full year of
regular sexual intercourse without the use of contraception.
Intrauterine fetal death
A fetus with a crown-rump length more than 15 mm long without cardiac
activity.
In utero
Inside the uterus (womb).
Inversion
A chromosomal rearrangement in which a segment of the chromosome
breaks away from the chromosome and re-inserts into the chromosome 180 degrees
relative to its previous orientation.
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Pericentric inversion
An inversion in which
the breakpoints occur on both arms of a chromosome. The inverted segment
includes the centromere
(
the constricted portion of the chromosome).
Inversion of chromosome 3 or 8 has been associated with an increased
risk for abnormal offspring . Pericentric inversion of chromosome 9 appears to be a normal
variant in humans, and occurs in approximately one percent of the
population. Inversion of chromosome 9 does not appear to
be associated with an increased risk of miscarriage or abnormal offspring.
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Paracentric inversion
An inversion in which
the breakpoints occur on one arm of a chromosome.
The inverted segment does NOT include the centromere
(
the constricted portion of the chromosome).
Because paracentric inversions do not change the arm ratio of the
chromosome they are difficult to detect on routine chromosome analysis.
The risk of
abnormal children for persons who have a paracentric inversion on one
chromosome is
low but increases with the finding of recurrent abortions or
abnormal children or both in other carriers in the family. |
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Isochromosome
A chromosome with two identical arms
due to abnormal division of the chromosome in the transverse plane instead of
longitudinally.
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