The strengthening of reproductive and sexual health services for youth in low and middle income countries is a key priority in international frameworks for improving population health. Effective implementation of appropriate services requires an understanding of the population at risk and their patterns of behaviour. Previous research has suggested that sexual activity at younger ages is associated with greater likelihood of unprotected intercourse and multiple partners (Blanc & Way, 1998). At the same time, educational attainment has been found to be independently related to increased contraceptive use during adolescents' first sexual encounter (Gupta, 2000). Programme planners and decision-makers need to be aware of how the adoption of safer versus riskier behaviours is interrelated with other influences, such as schooling, local context or other clustering effects.
Seasonal clustering of the timing of sexual initiation among youth is a relatively unexplored domain, especially for developing countries. Studies in various contexts have examined the seasonality of other reproductive and sexual health-related outcomes, including conceptions, births and onset of menarche (for example, Becker,1994; Lam, Miron & Riley, 1994; Chompootaweep et al., 1997; Pascual et al., 2002). Less attention has been laid on seasonality of sexual activity, except in terms of frequency of intercourse within marriage. Many studies, particularly in developing countries, have simply assumed a rectangular distribution of first intercourse within the calendar year (Zaba et al., 2002; Ali, Cleland & Shah, 2003).
An examination of the patterns and correlates of seasonality of first sex can help further specify the determinants of early sex among different subgroups. In a study in the United States, Rodgers, Harris & Vickers (1993) observed summer peaks in first sex among adolescents and young adults, which the authors attributed in part to increased libido when temperatures are higher and sunlight longer, and when students are on vacation from school. Among developing countries, testing for seasonality is usually hampered by a lack of data on month of first intercourse.
The 1996 Brazil Pesquisa Nacional sobre Demografia e Saude (PNDS), conducted under the auspices of the Demographic and Health Surveys (DHS) programme, stands out as offering a valuable opportunity for examining seasonal patterns in sexual debut among boys and girls in a context of development. The survey included a number of questions on the attributes of first intercourse, including month of the event. The objective of this study was to assess whether significant seasonal variations existed in the timing of premarital first sex among boys and girls. The main hypothesis was that, rather than following a uniform pattern through the year, the timing of first sex would be concentrated in summer. A secondary hypothesis was that selected characteristics of a first sexual experience would be different according to season, notably that contraceptive use would be lower if first sex occurred during the (presumed) more spontaneous summer months.
Data and methods
The DHS programme has been producing quantitative data on reproductive health knowledge, attitudes and behaviours throughout the developing world since 1984 (Macro International, 2007). The surveys are carried out using standardised instruments, methods of training, data collection and data processing. Personal interviews are conducted among representative samples of women (and increasingly men) of reproductive age, drawing on model questionnaires that have been translated and otherwise adapted to the needs and conditions of the specific country.
The 1996 PNDS, the latest DHS implemented in Brazil, collected information via personal interviews with 12,612 women aged 15-49 and 2,949 men aged 15-59 (BEMFAM, 1997)....