Pregnant women who work more than 25 hours a week tend to have babies with smaller heads and lower birth weights, according to new research.

A new study published in the journal Occupational and Environmental Medicine also found that pregnant women who stand for long periods of time also had babies who had significantly smaller head circumferences compared to babies of mothers who were not on their feet for long periods.

Researchers noted that while the differences were statistically significant, they were relatively small.

Women who worked more than 25 hours while pregnant were more likely to have babies that weighed half a pound less than the average birth weight and had heads that were 0.4 inches smaller than average.

Women who were on their feet for long periods of time also had babies with heads that were also 0.4 inches smaller than average.

Researchers from the Erasmus University Medical Center's Department of Public Health in Rotterdam, the Netherlands analyzed fetal growth rates of 4,680 pregnant women from early pregnancy to delivery between 2002 and 2006.

Midway through their pregnancy, participants filled out questionnaires about their work conditions and the physical demands required at their jobs, including whether they included lifting, long periods of standing or walking, night shifts and long working hours.

Among the participants, about 38.5 percent reported spending a long time on their feet, 45.5 percent said they had to walk for long periods, 6 percent said heavy lifting was part of their job and 4 percent worked night shifts.

About 48 percent of the women reported working between 25 and 39 hours a week while 23 percent reported working more than 40 hours a week.

Researchers found that while physically demanding work and working for long periods of time were not consistently associated with smaller babies or premature birth, researchers found that women in jobs like sales, childcare and teaching tended to have babies whose heads were 3 percent smaller or less than half an inch smaller than average at birth, suggesting a slower growth rate.

While the changes in head circumference was similar in babies born to mother who worked more 25 hours a week to mothers who spent long periods of time standing, women who worked more than 25 hours a week also had babies who weighed 0.33 pounds to 0.44 pounds less than average.

Researchers noted that past studies have found that long working hours may increase the risk of birth defects, premature birth, stillbirth and low birth weight.

"Long periods of standing and long working hours per week during pregnancy seem to negatively influence intrauterine growth," the authors concluded.