The good news is that eliminating these factors before the 24th week of pregnancy can bring the odds down to those of unexposed women, the report indicates.

A so-called "small-for-gestational" or SGA, infant is one that is smaller than normal in relation to the stage of pregnancy. Typically, their birth weights fall below the 10th percentile. SGA infants are at increased risk for a number of problems, including low levels of oxygen and blood sugar at birth.

The present study is one of several in recent years to examine the link between work conditions and the effect on pregnancy outcomes. While many have reported findings supporting an association, others have not.

In the current study, Dr. Agathe Croteau, from Universite Laval in Quebec, Canada, and colleagues assessed the impact of occupational conditions on the risk of having an SGA infant by analyzing data from 1,536 mothers with SGA babies and 4,441 mothers with normal babies. All of the subjects were interviewed by telephone following delivery.

Factors that had a cumulative effect on risk included working night hours, irregular or shift-work schedule, standing, lifting loads, noise, and high psychological demand coupled with low social support. Compared with the complete absence of these conditions, the risk of having an SGA infant ranged from 8 to 129 percent when one to all six of these conditions were present.

However, as noted, when preventive measures were taken before the 24th week of pregnancy, such as reassignment to a safer job or withdrawal from work, the elevated risks were largely eliminated.