Drawing on both qualitative and quantitative data, the thesis describes and explains both the current patterns of natural protection against pregnancy and the use of modern contraception in the period following childbirth in two populations, one rural and one urban, in Bangladesh. First, quantitative data gathered through demographic surveillance systems of the International Centre for Diarrhoeal Disease Research, Bangladesh are used to explore the patterns and differentials in breastfeeding, postpartum amenorrhoea and risk of pregnancy in the months following birth in the two study populations. Next, the surveillance data are used to describe the patterns of adoption of contraception in relation to time postpartum, breastfeeding and menstrual status for the two study populations. Extensive use is made of life table methods, hazard models and logistic regression techniques in these analyses. Qualitative data gathered through in-depth interviews with users of contraception are then used to identify key themes of understanding that influence women's behaviour, including contraceptive uptake, in the period following childbirth. The current knowledge, attitudes and practices of family planning providers in the two study populations are next described using qualitative data collected through a series of in-depth interviews and group discussions. Findings from the quantitative and qualitative analyses are then integrated in order firstly, to explain the current patterns and recent trends in contraceptive use and lactational protection against pregnancy following childbirth, and secondly to explore their potential implications for birth intervals and fertility. Finally, important issues are identified and recommendations made for improvements to postpartum family planning programme approaches in Bangladesh.