Article Sections

Introduction

Term pregnancies are subdivided into the following groups:

  • Early term:  37-38.6 weeks gestation.
  • Full term:  39.0-41.6 weeks gestation.
  • Late term:  41.0-41.6 weeks gestation.
  • Postterm:  ≥42 weeks gestation.

With increasing gestational age past the expected due date, there is increasing perinatal morbidity and mortality.  Postterm pregnancies account for fewer than 1% of pregnancies; however, they are associated with the most significant perinatal morbidity and mortality due to placental aging and subsequent dysfunction.

Pathogenesis and risk factors

Although the exact mechanism of labor onset is poorly understood, it is believed that term labor is triggered by a cascade of sequential changes in the uterine myometrium, decidua, and cervix.

The myometrium is inherently contractile at baseline, which serves to extrude cyclic menstrual blood and contract uterine radial arteries to minimize blood loss.  However, during pregnancy, the myometrium is held in a relaxed, quiescent state until term through multiple hormones and mediators (eg, progesterone, relaxin, nitric oxide) that allow for fetal and placental growth and development.

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