Preterm Labor

Labor is the occurrence of regular, frequent, uterine contractions, associated with progressive cervical dilatation and effacement. Most labor occurs within 2 weeks of the due date. Labor occurring prior to the 38th week of pregnancy is preterm labor, although definitions vary depending on the clinical circumstances. Some use the 37th week as the lower limit for term labor.

Delivering a little bit early usually poses no particular problem for the mother or the baby. More significant amounts of prematurity pose more significant risks for the infant. Of these, immaturity of the lungs is among the most hazardous, but other organs, including the brain, GI tract, and liver can also be affected

The cause of preterm labor is often unknown, but in about half the cases, it is associated with detectable intrauterine infection. other cases are associated with placental abruption, uterine overdistension (such as with twins)

Our instincts are to try to prevent preterm delivery to avoid the morbidity associated with it. This instinct is based on the premise that the problem is primarily one of prematurity. But in some patients, preterm labor is either a symptom of an underlying problem (infection, fetal stress, etc.), or part of the body's attempts to resolve a threat. In such cases, vigorous attempts to prevent delivery, even when successful, may only delay treatment of the underlying problem. Further, the medications commonly used to prevent premature delivery have significant side effects and risks. Third, the risk of prematurity, while significant, decreases rapidly with advancing gestational age. The following table shows approximate survival and handicap rates for premature infants born at various gestational ages in the United States.

Between 32 and 34 weeks, both the survival rate and long-term major handicap rates become nearly indistinguishable from the baseline rates of these adverse outcomes for full-term infants. (Learning disabilities are still somewhat increased in comparison to the 10% rate at full-term).

For these reasons, judgment is used to decide who should be treated for preterm labor and who should be allowed to deliver. In many hospitals, no attempt is made to arrest labor after the 34th week, unless it is to facilitate transfer to a high-risk center.

In forward military settings, it is usually better, depending on the gestational age, to move the patient to a high-risk center for management of her preterm labor. In this situation, use of tocolytic drugs (below), can be quite helpful in arresting the labor at least long enough to achieve a transfer.

Threatened preterm labor consists of regular, frequent contractions (every 10 minutes). If these contractions lead to progressive cervical change, then it is no longer threatened preterm labor...it is actual preterm labor. In many hospitals, it is customary to withhold any labor-stopping medication until cervical change is noted. These hospitals often have abundant resources to treat preterm labor and premature infants should labor unexpectedly progress rapidly. In  hospitals without these advanced resources, earlier treatment may be indicated.

It is often helpful to postpone delivery long enough to get the patient to a definitive care setting, even if the patient is more than 34 weeks gestation. It is best to coordinate the use of these medications with the receiving facility. Any of the following treatments may effectively disrupt the labor process for 24-48 hours, and this is usually long enough to move the patient to an area of greater resource.

• Magnesium sulfate, 4 gm loading dose over at least 5 minutes, followed by 2 gm/hour in a steady IV drip. Watch for magnesium toxicity with diminished reflexes and respiratory depression. If magnesium toxicity develops, it is treated with calcium. Common side effects include flushing, palpitations, headache and muscle weakness. Serum magnesium levels can be monitored (target level 6-8 mg/dL). Contraindicated with myasthenia gravis, recent myocardial infarction, or impaired renal function.

• Ritodrine (Yutapar) 100 µg/minute IV, increased every 15 minutes by 50 µg to a maximum of 350 µg/min. Titrate dosage to a maternal pulse of not less than 100 BPM and not greater than 120 BPM. Watch for pulmonary edema in the mother. Common side effects include shortness of breath, anxiety, tremor, palpitations, headache, nausea, vomiting, hyperglycemia and hypocalcemia. For these reasons, get baseline EKG, glucose, potassium and CBC before administering. Contraindicated with hypovolemia, hyperthyroidism, uncontrolled diabetes, maternal cardiac disease, multiple gestations and toxemia of pregnancy.

• Terbutaline 0.25 mg SQ, every 1-4 hours x 24 hours, total dose not to exceed 5 mg in 24 hours. May also be given PO in 2.5 - 7.5 mg doses, every 1.5 - 4 hours. Target maternal pulse rate is > 100 and < 120 BPM. Contraindicated with hemorrhage, hyperthyroidism, uncontrolled diabetes, and heart disease. Common side effects include shortness of breath, anxiety, tremor, palpitations, and pulmonary edema. For these reasons, get baseline EKG, potassium , potassium and CBC before administering.

• Indomethacin (Indocin), 50 mg PO (or 100 mg PR), followed by 25 mg PO every 4-6 hours for up to 48 hours. Watch for gastric bleeding, heartburn, nausea and asthma. Other common side effects include headache, itching, fluid retention and bowel changes. Contraindicated in peptic ulcer disease, NSAID sensitivity, after the 30th week of pregnancy, with renal disease or any coagulopathy.

• Nifedipine, 10 -20 mg PO every 4-6 hours (Watch for headache, flushing and nausea). Contraindicated with aortic stenosis or congestive heart failure.

Many fetuses less than 34 weeks gestation will benefit from administering steroids to the mother. The effect of the steroids on the fetus is to accelerate fetal pulmonary maturity, lessening the risk of respiratory distress syndrome of the newborn. Appropriate doses include:

• Betamethasone 12 mg IM, and repeated in 24 hours.

• Dexamethasone 6 mg IM Q 12 hours x 4 doses.

When transporting the mother to a definitive care setting, have her remain way over on her left or right side, with a pillow between her knees, and an IV securely in place.