The previous data indicated that the Hmong preferred having many children .Hmong women give birth to about 7 children on average with six new born babies surviving. (p. 13) It is understood that it is a socio-economic condition because the economy of the Hmong family depends on the shifting cultivation with basic technology so that a lot of labour is needed. They believe that the more labourers, the more wealthy they will be. (p.14) From the data study, it is found that there has been a change in ideas about fertility from preferring many children to the use of contraception. It is because of the economic change, the need for many children has decreased and the expense of raising children is higher. (p.23) It is also found that the Hmong communities in Chiang Mai are separated into 3 groups: a. the traditional villages where their cultivated land is quite fruitful; b. the changed villages where the cultivated land is of low quality and the utilization is limited and c. the urban community. (p.27) From the survey, it is found that some women plan for contraception and the number is increasing. (p.27) It is concluded that nowadays, the fertility of the Hmong is higher, but there is evidence showing that the rate of birth has decreased. Then the question is what the cause of the fertility change is. From the study, the answers are 1. during the past 10 years, the rate of the hilltribe population has constantly increased so that the government has limited the livelihood and the cash crops (p.29) 2. another reason is socio-economic development. (p.30).