the relationship between number of pregnancies and

introduction : osteoporosis is characterized by low bone density and increase in the risk of fracture. because in pregnancy and lactation there are many factors which influence bone mineral density, we sought to assess whether the accumulation of multiple pregnancies and the accompanying extended periods of lactation were a risky factor for low bone mineral density and osteoporosis. materials & methods : this study was a cohort  survey. the population consisted of 92 subjects. the experimental group consisted of 53 grand multiparous women who had born 3 or more children as well as 39 women with 0-2 children as the control group. all of them referred to loghman hospital during 2001-2002,to be assessed in terms of bone mineral density (bmd) and none of them had any risky factor for osteoporosis. bmd of the lumbar spine and femural neck in two groups was measured by means of lunar and dexa method. their body mass index(bmi) were calculated. acceptable level of significant for rejecting null hypothesis was p<0.05. in this study x 2 , independent t-test and mann withney were employed.  results : the results showed that there wasn’t any statistically significant difference between age , menarche age , bmi and bmd of spine in both groups , whereas there was statistically significant difference between lactation period, number of children and femoral bmd in the both groups. conclusion: this study suggests that bone mineral density levels can be sustained in multiple pregnancies.