Recent research has shown that children born before and after short birth intervals run a considerably greater risk of dying in infancy or childhood than do others. This report investigates which women have short interbirth intervals, under what circumstances, and for what reasons. The analysis uses data from the Malaysian Family Life Survey to examine influences on the two main behaviors--breastfeeding and contraceptive use--that affect birth interval length, and assesses the the impact of these same variables on the probability of having a birth interval of less than 15 months. The analysis shows that many of the independent variables affect breastfeeding and contraceptive use in opposite directions, with no significant net effect on the likelihood of a short interval. For example, a woman's education is negatively related to the probability that she breastfeeds, positively related to the probability that she uses contraceptives, and has no significant effect on the likelihood that the interpregnancy interval is less than 15 months. Having a family planning clinic nearby is associated with less breastfeeding, offsetting whatever positive effects family planning clinics have on contraceptive use in terms of the percentage of birth intervals that are so short as to be detrimental to infant and child health. Hence, factors that increase contraceptive use do not necessarily reduce the incidence of short interbirth intervals, because they are also associated with reduced breastfeeding. We simulate the proportion of intervals that would be short for alternative combinations of breastfeeding and contraceptive use in the population and show that over the period covered by the data (1961-75), breastfeeding had a considerably greater effect on preventing short interbirth intervals than did contraceptive use.
Several new concepts are used to describe contraceptive use histories for nearly 1200 women in Peninsular Malaysia. These histories are summarized by 81 episode histories. Transition matrices provide useful summaries of the changes women make in their contraceptive practice from one pregnancy interval to the next. Data from the mid-1940s to mid-1970s, during which period there was a dramatic increase in contraceptive use, reveal considerable inertia in individual couples' contraceptive practice. Persistence with a method was greater the less effective the method: while 86% of couples using no method in one interval used no method in the next, only 56% of couples using the pill in one interval also used it in the next. Virtually all transitions are of three types: continuation with the same method, a change from no method to some method, or a change from some method to no method. For only 1% of all pregnancies did couples use one contraceptive method before a pregnancy and a different method after the pregnancy. Differences are examined by calendar year and education.