The objective was to evaluate whether including pregnant women in a preventive dental program prevented the appearance of caries in their children up to the age of 6, and whether the effect was similar in children of immigrant and non-immigrant women. In phase I, 90 pregnant women, 45 immigrants and 45 natives, were taught about the development and prevention of caries. In phase II the oral health of their children at the age of 6 (n = 90) was evaluated, along with a control group of children of natives and immigrants of the same age (n = 90). A survey was used to determine participants’ backgrounds and habits. A multivariate study of the results was performed using R-core software. The number of children without caries was 128 (71.1%), whereas 52 (28.9%) had caries, 15 from the protocol (16.67%) and 37 from the control group (41.11%), with statistically significant differences (p < 0.001). The mean number of caries for the children in the protocol was 0.62 ± 2 and in the control group it was 1.88 ± 2.9 (p = 0.001). In the multivariate analysis the risk of developing caries was higher for the condition of being the child of an immigrant (OR = 11.137), inadequate oral health (OR = 4.993), the children being overweight at the age of 6 (OR = 10.680), and the consumption of candies (OR = 5.042). In conclusion, the preventive protocols started during pregnancy reduced caries in participants’ children, which suggests that these protocols should be encouraged. Because immigrant children are more vulnerable to caries, they and their parents should be included in preventive programs once they arrive in the host country.