This cohort profile paper describes a large longitudinal prenatal birth cohort study with Rohingya refugee and host communities in Cox’s Bazar, Bangladesh. Over the course of 12 months, this study recruited 2889 pregnant women, a subsample of husbands (N=853), and, where present, an existing child aged 36 to 60 months (Early Years cohort / EY; n=527). At the time of writing this paper, 1967 women had had a live birth (Prenatal Cohort / PN; target child). This study started as part of a set of activities conducted under the LEGO Foundation funded Play to Learn initiative aimed at building the evidence base on early childhood development in humanitarian settings. To our knowledge, it is the first ever longitudinal cohort study starting prenatally with a forcibly displaced population residing in a low- or middle-income country (LMIC). This paper provides details on the study design and protocol, and reports on data from the completed recruitment wave.