Pregnancy Terms Glossary
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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

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


A child under one year of age.


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).


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.

  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).

Milunsky JM In Milunsky, A ed , Genetic Disorders of the Fetus, 5th edition Baltimore: The Johns Hopkins University Press 2004. P 322


  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.


A chromosome with two identical arms due to abnormal division of the chromosome in the transverse plane instead of longitudinally.


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