Doctors say too much weight during pregnancy can put mothers and their babies at risk of health complications.
The study appears in this month's issue of the journal of the American Dietetic Association.
Dieting before pregnancy may lead to excessive weight gain
The study involved 1,200 women who had just become pregnant.
At the end of the study, all of women who had tried to restrict their diets gained more weight during pregnancy than the women who did not diet before pregnancy.
