The Food and Drug Administration reviewers said the gel's efficacy was "a major concern" and asked an advisory panel to consider whether the company should conduct more U.S. clinical trials.

The vaginal gel, which contains the hormone progesterone, is meant for women with a short cervix, who have a higher risk of premature birth.

Preterm birth affects 10 to 12 percent of all pregnancies in the United States, and babies born early have a higher risk of early death and long-term health and developmental problems.

The reviewers' comments were posted on the FDA's website on Tuesday. On Friday, an FDA panel of outside experts will vote on whether the agency should approve the drug, and the FDA will make a final decision by February 26.

Last year, Columbia and Watson said that in trials, their gel helped cut the risk of premature birth by almost 45 percent for women worldwide.

But the FDA staff said the drug's efficacy for U.S. women was not statistically significant. The staff also said the overall trial results may have been influenced by the drug's success in countries such as Belarus and South Africa.

The FDA staff said patients treated with the gel did not have a higher rate of side effects than patients using a placebo.

If approved, the Prochieve gel would compete with a similar drug called Makena from K-V Pharmaceutical Co.

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