The nature of the open birth interval distribution

Background: The open birth interval -- the time since the woman's latest birth -- is closely correlated to the usual fertility measures, but it adds important information from the age of the woman's youngest child, with its implications for her freedom from domestic roles.  Studies of the open interval by age and parity can elucidate the transitions in reproductive behavior that women experience over time. Methods:  249 surveys of married women in 75 countries in the DHS series provide information on the open  interval by age and parity, and by the  fertility measures of the total fertility rate (TFR), the general fertility rate (GFR), and children ever born (CEB), with time trends.  Stata 15 and the "R" software were used, and a two-parameter equation was employed to model the distribution. Results:  The distribution of women by the open interval follows a downward curve from birth to 20 years; it varies across countries and over time only by its starting level and the steepness of the curve. Declines in the shortest intervals soon after birth reflect recent fertility declines. Variations are large by both age and parity, but in quite different patterns. Past modeling analyses demonstrate the effects of female and spouse mortality, declining fecundability, contraceptive use, and reduced sexual exposure. Both period and cohort effects can impact the curve. The open interval distribution is modelled in an equation with two parameters and calculated for the latest surveys in the 75 countries. Conclusions: The time since a woman's birth is easily captured with a single question in successive surveys. Changes in the open interval distribution serve as sensitive indicators of recent fertility changes, and the dynamics of reproductive behavior across women's life stages are captured in new ways, as gauged by age and parity trends in the distributions.