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.
Induction of labor
Stimulation of uterine contractions before the spontaneous onset of labor in order to achieve a
vaginal delivery.
ACOG Practice Bulletin No. 10: Induction of Labor. American College of Obstetricians and
Gynecologists. 1999.
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.
46,X,inv(Y)(p11.2q11.2)
Pericentric inversion of the Y chromosome is usually a familial variant of no known clinical
significance occurring in 1 in 1000 males. A chromosome analysis of the father's blood may be
performed to determine if this Y chromosome is inherited (5 mL in a green-top sodium
heparin tube).
REFERENCES
Milunsky JM. In: Milunsky A, ed. Genetic Disorders of the Fetus, 5th edition. Baltimore:
The Johns Hopkins University Press; 2004. p.322.
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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.
