Maternity migration and the recent normalization of the sex ratio at birth in Hong Kong.

Hong Kong is characterized by very low fertility. However, over a period from 2000 to 2015, both the total number of births and the sex ratio at birth (SRB) increased and then declined dramatically. We analysed the increases in a 2013 paper in this journal, where we largely ascribed them to a rapid growth in the number of 'transient' mothers from Mainland China disproportionately giving birth to boys in the territory. In 2012, policies were implemented to halt this 'maternity migration'. Here, we explore the impact of these policies, both on births and the SRB in Hong Kong. We conclude that the rises and falls in births and SRBs in Hong Kong can, indeed, be broadly ascribed to the reproductive behaviour of transient Mainland mothers. However, the role of the Hong Kong government's policy interventions is much less clearly defined.