In general,
tocolytic agents are effective in stopping uterine contractions and in temporarily delaying delivery. The benefit of stopping uterine contractions is dependent on the fetal status and gestational age. The rationale for stopping
preterm labor is to improve neonatal outcome. At this time, the best way to improve neonatal outcome would be to assure delivery in a center capable of caring for a preterm infant and prescription of
glucocorticoids to decrease the risk of
respiratory distress syndrome and other neonatal complications. Intravenous
tocolysis for
premature labor has found a prominent place in the obstetrician's armamentarium. We recommend the use of
magnesium sulfate as first-line
therapy. When comparing maternal and fetal risks, side effects, and the safety profile,
magnesium sulfate is superior to beta-mimetics; however, there are still significant problems with potential morbidity and mortality for both mother and fetus with any
tocolytics. Adjunctive use of
indomethacin with
magnesium sulfate may be used through 32 weeks for up to 48 hours at a time. Most
tocolytics are effective in stopping labor for 48-72 hours. None have been shown to decrease the rate of preterm delivery. Once the uterus is quiescent and intravenous
tocolytics are stopped, prolonged use of
tocolytics has not been shown to be effective in preventing
preterm birth.
Tocolytics have significant long-term side effects to the mother's cardiovascular system, carbohydrate metabolism, and the fetal cardiovascular system. Thus, the prolonged use of prophylactic
tocolytics after cessation of intravenous medications is not recommended.
Tocolytics may be an appropriate
therapy during
preterm labor vaginal bleeding, ruptured membranes, multiple gestation, or advanced cervical dilatation. In all situations, however, careful guidelines must be observed. These guidelines include: (1) maternal and fetal well-being must be established before
tocolytic therapy; (2) causes of
preterm labor should be evaluated and treated when possible; (3) the risk/benefit ratio for both the mother and fetus must be re-evaluated on an ongoing basis; (4) when
tocolytics are given before pulmonary maturity, then antenatal
corticosteroids also should be considered in every case; (5) long-term use of
tocolytics is difficult to justify at this time; (6) the safest
tocolytic should be used for the shortest amount of time possible. It is doubtful, because of the nature of
tocolytics, that newer
tocolytics will be developed that will eliminate the problems of preterm delivery. Preterm delivery is an end-stage symptom of a multifactorial disease.
Preterm labor is one of the last symptoms in a cascade of biochemical events that lead to preterm delivery. The most appropriate way to end preterm delivery would be to prevent the causes that initiate the cascade that ends in
preterm labor. Authors' Note: Literally hundreds of papers have been written in the last 30 years on tocoloysis. For the purposes of space, when studies are summarized in peer-reviewed articles, we have referenced the reviews instead of the individual studies.