الفهرس 
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Abstract
Preterm birth is defined as that occurring before 37 weeks gestation and it remains the primary cause of perinatal mortality and morbidity. The incidence of preterm delivery varies between 5 and 10 % of prgnancies but 70-80 % of perinatal deaths occur in preterm infants. Despite advances in perinatal medicine in recent decades, the problem of preterm delivery continues to frustrate satisfactory reproductive outcome, with little progress having been made in reducing the frequency of preterm births. Four circumstances may lead to the birth of a preterm baby; these are uncomplicated spontaneous preterm labor, premature rupture of the membranes, complicated emergency delivery and elective preterm delivery. A large number of risk factors have been associated with preterm birth. Reproducible factors include demogrphic risk, behavioral risks, health care risks, medical conditions predating pregnancy, and current pregnancy complications. The pathophysiologic pathways that connect risk factors to preterm birth, however, are still poorly understood. Five factors that may be important are: cervical, vaginal and amniotic fluid infections, variations in uterine contractility, stress, failure of plasma volume expansion and fetal- placental toxins. The evidence that infections is a cause of prematurity falls into two broad categories. First, there is the increased risk of infection, manifest clinically, bacteriologically and histologically of neonates and mothers associated with preterm birth. Second is the association of various vaginal organisms with prematurity and premature rupture of the membranes.
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Summary of evidence linking infections and PTL: ( lamont and Fisk, 1993 ).
1-Products of arachidonic acid metablism are an essential part of normal parturition.
2-Bacteria produce the enzyme phosphlipose A2 .
3-Bacteria products increase prostaglandin E2 production by amnion cells.
4-Bacteria produce proteases and other enxymes which may facilitate
penetration of the cervical mucous plug and invasion of the membranes.
5-Endotoxins induce macrophages / monocytes to produce substances such as interleukin -1, tumor necrosis factor or platelet activating factor which stimulate synthesis of prostaglandins.
6-In the amniotic fluid of women with intra - amniotic infection and preterm labor there are significantly increased levels of prosta-glandins.
7-A wide range of microorganisms associated with preterm labor and chorio - amnionitis cause an increase in prostaglandin E2 production by amnion cells.
Much research has been directed at identification of women in whom preterm labor is likely to occur so that prevention could be addressed. It is imperative that such ascreening tool, for use in a random population of pregnant women, be sensitive and specific and have high predictive value in a population where the incidence of the condition is low. Such a screening test must also be safe and cost effective because it will be applied to a large number of individuals. Numerous methods of screening for preterm labor have been proposed but none has fulfilled all the necessary criteria. These methods include risk scoring, cervical assessment.