Although a fuller review of the relationship between infant mortality and breastfeeding is presented below, it seems appropriate to mention its relationship to maternal age here. For example, teenagers in the United States were found to breastfeed less than older women. For 1989, Ryan et al. (1991:723) reported that only 30 per cent of the US teenage mothers initiated breastfeeding for their infants in hospital compared to 45 per cent of mothers aged 20-24 and 59 per cent among those aged 25-29. At six months, the percentage still breastfeeding their infants was only six among teenagers, compared to 12 among mothers aged 20-24 and 21 among those aged 25-29 (Ryan et al., 1991:723). In addition, teenage mothers have been found more likely to have complications at delivery which delayed the initiation of breastfeeding (Peterson and DaVanzo, 1992:444). Delayed initiation of breastfeeding tends to discourage breastfeeding (Feinstein et al., 1986:213), as does media advertising of infant formula (Lawrence, 1991:868).
Birth interval length
Birth interval length has also been found to have some impact on infant mortality (Wolfers and Scrimshaw, 1975; Winikoff, 1983; Cleland and Sathar, 1984; Hobcraft, McDonald and Rutstein, 1985; Pebley and Millman, 1986; Tu, 1989; Koenig et al., 1990). For example, Pebley and Millman (1986) analysed WFS data for 39 countries and found that the mean relative risk of death for children born within two years of a preceding birth was 58 per cent higher in the neonatal period, 96 per cent higher in the post-neonatal period and 45 per cent higher in the second year of life than for children bom with intervals of more than two years. Hobcraft (1991) analysed DHS data for 25 countries and found that children born within 18 months of an earlier birth were twice as likely to die as those bom with a preceding interval of 24-47 months. Boerma and Bicego (1991) also analysed DHS data for 17 countries and found that children born within two years of a preceding birth had 89 per cent higher risk of death than those with an interval of two years or longer.
Birth interval lengths have been measured in various ways, depending on the theoretical persuasion of the researcher. These range from interbirth intervals (the interval between two consecutive births), to interpregnancy intervals (between one pregnancy outcome and the next conception) and average number of births within a specified period (Bogue and Bogue, 1980:12-13). In this thesis, birth interval is defined as the interval between two successive live births to the same woman.
According to Potter (1963:155-156), assuming that there is no pregnancy wastage, birth interval length has four basic components. These are postpartum amenorrhoea, anovulatory cycles, ovulatory exposure, and the pregnancy that closes the open birth interval. These basic components of birth interval have been reduced to three by later researchers by combining the anovulatory cycle and ovulatory exposure as an 'at risk period' (see DaVanzo and Starbird, 1991:241-242). There are at least two major mechanisms through which birth interval length affects child survival: one is biological and the other is behavioural. The biological mechanism is the 'maternal depletion syndrome' and its associated consequences such as intra-uterine growth retardation (Lieberman et al., 1989) and premature delivery (Miller et al., 1992). Close birth spacing (less than 24 months) tends to hamper the mother's full recuperation before the next pregnancy and hence weakens her biological capacity to provide a wholesome prenatal environment, leading to low birthweight or even congenital anomalies (Hobcraft et al., 1985:376-377). The behavioural mechanism is measurable in the form of ineffective use of prenatal services, impaired breastfeeding and sibling competition for scarce childcare resources (Pebley and Millman, 1986; Majumder, 1991), and intrafamilial disease infestations, especially measles (Aaby, 1988). Caring for children of very close ages drains the mother's energy and may reduce the quality of care available to each child. In such cases, the older sibling tends to be at a disadvantage, according to one study in rural Bangladesh (Koenig et al., 1990).
Long birth intervals may act in at least two ways. They may afford the mother more time to prepare biologically and materially for the next pregnancy. Alternatively, they
may also result from health or economic problems or a desire to terminate childbearing, and so make the next birth less welcome (Rutstein, 1983:30-31).
The duration of breastfeeding is one major factor that research findings have indicated as closely related to birth intervals (Wood et al., 1985; Labbok, 1985; Millman and Cooksey, 1987; Gray et al., 1990; Majumder, 1991). Breastfeeding duration affects the return of ovulation in women, which in turn determines the risk of conception if a woman is sexually active. However, the relationship between breastfeeding and birth interval is not unidirectional. This is what makes it somewhat difficult to determine the effect of each on child survival.
Breastfeeding
In the demographic and health literature, breastfeeding has been shown to have both health and fertility benefits. The health advantages of breastfeeding derive both from its contraceptive effects (Gray et al., 1990:25-29) and from the health benefits of the mother's milk to the child. Prolonged breastfeeding lengthens the interval between births, which in turn prevents sibling competition for food and maternal childcare time (VanLandingham et al., 1991:131). In non-contracepting societies, historical data have shown that the interbirth interval was associated with the survival status of the child bom at the start of the interval (Guz and Hobcraft, 1991:91).
The practical benefits of breastfeeding derive from its unique nutritional qualities, its immunological properties, its cleanness at source, and its birth spacing effects (Huffman and Lamphere, 1984:93; Palloni and Tienda, 1986:33). This is because the adequacy of nutrient intake and the strength of the immuno-defence system play important roles in the ability of children both to resist and to recover from diseases. Human milk contains anti-infective agents and powerful bacteriostatic compounds that protect the breastfed child from gastroenteritis and other infections (Gray, 1981:105). VanLandingham et al. (1991:133) have noted that breastmilk (especially colostrum) 'contains immunoglobulins, leukocytes and the biffidus factor, which help guard the newborn
against several types of bacteria and harmful organisms'. Wray (1978:205) concluded: 'For the world's "poorest billion", breastfeeding is the only economically feasible option'.
J Many scholars have studied various aspects of breastfeeding and child survival. Palloni and Tienda (1986:49) analysed Peruvian Fertility Survey data and observed that longer breastfeeding in the country could reduce infant mortality by 20 to 40 per cent. They also noted (1986:33) that a high mortality-reduction effect should be expected if breastfeeding is practised in an environment where there are intense exposure to infective agents, inadequate resources for safe supplementation and poor hygienic practices. Using Malaysian Family Life Survey data, Holland (1987:78) carried out a tightly controlled and rigorously specified hazard model analysis on breastfeeding and infant mortality. He observed that babies that were fully or partly breastfed were 12 times less likely to die than those that were never breastfed. Until the age of six months when the mortality differences were no longer significant, breastfed children had higher chances of survival than those that were not breastfed. The relationship between infant mortality and breastfeeding was also observed in the same country (Malaysia) by Habicht, DaVanzo and Butz (1988). They found that the effect of breastfeeding on mortality was modulated by environmental hygiene; the relative risk of mortality among children who were not breastfed was twice as high if there were no toilets and piped water in the household.
The relationship between breastfeeding, birth spacing and child survival has also been examined (Palloni and Millman, 1986; Retherford et al., 1989; Lantz, Partin and Palloni, 1992). These studies confirmed that breastfeeding duration had an independent effect on child survival, even though its role as a mechanism for birth spacing was equivocal. In Latin America, for example, it was observed that the effect of breastfeeding declined with the age of the child at the start of the interval and that the effect was strongest among the group having the highest mortality (Palloni and Millman, 1986). While Palloni and Millman (1986) observed that breastfeeding
performed poorly as a mechanism through which birth spacing affected child survival in Latin American countries, Retherford et al. (1989) noted the exact opposite in Nepal. A study of 13 countries using WFS data (Millman and Cooksey, 1987:211) found an attenuation of birth interval effect once breastfeeding was controlled.
Analysis of the effects of breastfeeding on infant mortality runs the risk of observing a spurious relationship for a number of reasons. Simultaneity bias confuses the direction of causality, such as when replacement effect and early pregnancy results in short breastfeeding duration. There is censoring bias in which the death of a child was the major cause of the observed short duration of breastfeeding (Palloni and Tienda, 1986:34-35). Potter (1988) calls attention to the need to consider the effect of data error and simultaneity bias in the observed effects of breastfeeding and contraceptive use on one hand, and child spacing on the other. Recently, Lantz et al. (1992:139) responded to this observation and examined WFS data from different countries to arrive at a tightly-qualified conclusion that the observed effects of breastfeeding and child-spacing are quite robust to errors of omission and of dating. Koenig et al. (1990:261) also believed that their findings in the Matlab study in Bangladesh were not affected by serious data error.
There is still considerable disagreement over whether there is a true age effect or whether age effects are only a proxy for socio-economic background. The types of data that have been available for analysis, mostly survey data, have set limits on the extent to which variables such as breastfeeding could be analysed. For example, the extent to which breastfeeding affects birth spacing depends on the intensity and frequency of feeds, and supplementation. Data on these are usually not available in surveys in the form in which they are needed. This is another reason why survey data need qualitative supplements to add context to observed patterns.