Objective: To determine maternal bone mineral density changes during pregnancy in healthy women using REMS. Design: Prospective cohort study. Setting: UMAE Hospital de Gineco Obstetricia No.4 “Luis Castelazo Ayala,” IMSS, Mexico City, Mexico. Population: Women with singleton pregnancy. Methods: Bone mineral density was determined in all patients once in each trimester by REMS densitometry. Lifestyle and demographic data were collected by interview. Main outcome measures: Bone mineral density. Results: BMD was significantly lower in the second and third trimesters compared with the first trimester. A wide interindividual variation in BMD change was observed; therefore, women were stratified into a bone loss or bone gain group according to the BMD loss or gain rate from the first to third trimester. Binary logistic regression showed that no calcium + vitamin D (Odds ratio [95% CI]; 21.62 [4.72-99.03]; p< 0.0001) and no multivitamin use (Odds ratio [95% CI]; 11.00 [1.34-90.11]; p=0.025) were independent factors significantly associated with bone loss at the end of pregnancy. According to previous reports in postmenopausal women, two subpopulations are identified when the rate of bone loss is measured annually; thus, the bone loss group was stratified into slow and fast bone loser groups, considering a bone loss >3% during pregnancy as a cutoff. Fisher’s exact test showed that gravidity was significantly associated with fast bone loss (p=0.003). Conclusions: Decreased BMD during pregnancy is associated with no multivitamin use and no calcium + vitamin D use. In addition, an association between fast bone loss and the number of pregnancies was shown.