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Preterm birth is defined as any birth occurring before the end of 37 weeks of gestation. The incidence is between 8% and 10% of all births, and preterm births account for more than 60% of non-anomalous-related neonatal mortality and morbidity. Most neonatal mortality occurs in those preterm deliveries that occur between 20 and 30 weeks of gestation (or in infants weighing less than 1,500 g). Survival of neonates delivered at tertiary care centers has improved yearly, particularly for those pregnancies.
The cause of preterm labor is unknown in most patients. Multiple risk factors, however, have been reported to be associated with an increased likelihood of subsequent preterm labor: multiple gestation (40-50%), previous preterm labor or delivery (20-50% recurrence), diethylstilbestrol exposure, hydramnios, uterine anomalies, previous cone biopsy, previous second-trimester losses, cervical dilatation and effacement.
Subclinical chorioamnionitis has emerged as a possible significant cause of preterm birth. Many cases of preterm PROM, as well as up to one fourth of cases of idiopathic preterm birth, may be due to subclinical intraamniotic infection. Bacterial products such as lipopolysaccharide can be identified in the amniotic fluid.
Management
Initial evaluation of patients with suspected preterm labor should include asessment of the presence and frequency of uterine contractions, the cervical status, and an assessment of gestational age. Before considering whether to use tocolysis, a search should be made for treatable factors of preterm labor, such as pyelonephritis, and an evaluation should be made to determine whether there are any maternal or fetal.
TOCOLYSIS
Methods to arrest preterm labor include bed rest and tocolytics. The most widely used tocolytic drugs are magnesium sulfate, beta-mimetics, calcium channel blockers, and prostaglandin synthetase inhibitors. Recently, atosiban (an oxytocin analog) and nitric oxide donor drugs have been used.
Magnesium sulfate is frequently used as a first-line drug for tocolysis, particularly in patients with diabetes. It is initiated by a loading dose of 4-6 g intravenously, followed by a continuous maintenance dose of 2-4 g per.
Calcium channel blocking drugs are used because of their ability to cause a decrease in intracellular free calcium and, hence, inhibition of myometrial contractility. In all studies, nifedipine was more successful or as good as ritodrine in stopping contractions. These adverse effects include hypercapnia, hypoxia.
Beta-adrenergic tocolysis is usually initiated by the parenteral route either by a continuous intravenous infusion, titrating the infusion rate against contractions and side effects, or the intermittent intramuscular.
Maternal side effects, such as hypotension, excessive tachycardia or cardiac arrhythmias, myocardial ischemia, and pulmonary edema, may be serious. Strict intake and output of fluids are necessary while the patient is on intravenous therapy and for 24 hours thereafter. Fluid restriction to less than 2,500 mL/d is recommended. Colloid osmotic pressure determinations may be useful because pulmonary edema is rare.
The benefits of pregnancy prolongation with the use of beta-adrenergic receptor antagonists are not clearly proven beyond the initial 24-48 hours. Several meta-analyses show that beta-mimetics given to prevent preterm birth delay delivery no more than 36-48 hours.
Prostaglandin synthetase inhibitors have been reported to be effective tocolytic agents in isolated reports. Concern related to adverse fetal effects, however, has limited their use to patients who are early in gestation and are showing signs of difficulty. Narrowing of the ductus arteriosus has been observed in some pregnancies during their use, and oligohydramnios may be induced after a few days.
CORTICOSTEROID THERAPY
Maternal corticosteroid treatment with betamethasone and dexamethasone has been shown in several controlled trials to decrease the incidence of respiratory distress syndrome. Antenatal corticosteroid therapy decreased the risk of respiratory distress syndrome, intraventricular hemorrhage, and mortality in infants born prematurely. Antenatal therapy with corticosteroids should be considered for all fetuses at risk.
There has been hesitation to use corticosteroids in some patients because of the impression that delivery will occur before a full course of therapy (ie, less than 24 hours). However, treatment with corticosteroids for less than 24 hours still appears to be associated with a significant reduction in neonatal mortality.
METHOD OF DELIVERY
Retrospective studies show that the singleton breech of less than 32-34 weeks of gestation or weighing less than 1,500 g has less morbidity and mortality if delivered by the cesarean method, particularly if the breech.