The choice of first-line drugs to avert preterm labor is controversial "because of inconclusive information on the relative safety of the various agents," Dr. Roel de Heus at University Medical Centre Utrecht and associates explain in the British Medical Journal.

Most clinical trials of these drugs have been restricted to women with low-risk pregnancies and therefore don't reflect real-world situations.

The current study involved 1920 women treated with various drugs to stop preterm labor. There were 14 serious maternal adverse events associated with treatment, including breathing difficulties, low blood pressure, heart failure, and fluid in the lungs.

When combinations of drugs were used, the occurrence of serious adverse reactions was high at 16 percent. Four of these patients required intensive care.

"As there is no evidence that treatment with combined (drugs) is superior to single or sequential treatment, we believe that combined treatment should be discouraged," the investigators write.

"The real dilemma," state the authors of a related editorial, is "whether to treat or not..., not which drug to use." They say that even after more than 30 years of research, it is still not known if attempting to stop preterm labor benefits the baby, or indeed, whether the practice actually causes harm.