The female reproductive axis in mammals is a highly complex dynamic system which goes through different transient or absorbent states during the course of a life-time. Little is known about the mechanisms controlling this system during fetal life and at birth, although it has been shown in numerous species, including primates, that the whole machinery is already functioning (Brooks et al., 1990; Plant, 1986). After a delay ranging from a few days to a few weeks, according to the species, the reproductive axis becomes quiescent and activity apparently resumes only at the time of puberty. Here again understanding of the phenomenon is still limited (Ojeda, 1991).

There is a wide range of duration of post-partum amenorrhoea and resumption of ovulation between individuals, within an individual or between populations. Several extraneous variables, such as parity, mother's age, sex of the breast-fed baby, socioeconomic status and cultural level of the family, can be controlled; then the remaining variables will probably explain a part of the total variability in post-partum amenorrhoea duration but say nothing about the physiological process. In attempting to question physiological aspects of the return of fertility several observational studies have tended to favour one of the different factors which are supposed to play a major role in the regulation and have compared different levels of it, such as body composition of the mother (Frisch & McArthur, 1974), breast-feeding pattern (Jones, 1989) or the life style of the women. Life style can be related to women's physical activity in normal life (Ellison, 1991), the difference between urban and rural life (Carael, 1981) or the environment (Laurenson et al., 1985). Prolactin as a possible mediator of the central regulation has been carefully considered (Lunn, Austin & Whitehead, 1984; Howie et al., 1982). These studies were mainly observational rather than experimental, supplementing mothers during the lactating period or during the pregnancy. If this information is added to what is known of other animal species (Loudon, 1987) or animal experimentation (Plant et al., 1989; Williams et al., 1990a; Williams et al., 1990b), the combination of several of the main factors believed to have a major role in the human species can be clarified and the aetiology of the resumption of fertility in nursing women investigated.

Whilst it is generally accepted that breast-feeding lowers the likelihood of conception, this relationship is not straightforward and there appears to be a wide variation in the effectiveness of the association between individual mother-infant pairs. Up to about 6 months post-partum breast-feeding probably can be used as a family planning method, with up to 98% effectiveness if behavioural guidelines are adhered to (Consensus, 1988). But beyond this time significant variations appear between different countries, and even different communities within countries, which make any overall recommendation impossible. To understand the reasons for variation in the duration of lactational infecundity it is necessary to understand the mechanisms involved, but knowledge of these processes is still far from complete. There are, nevertheless, enough data to indicate that suckling and maternal milk output are intimately linked with the return of fecundity.

The paper reviews methods, and their difficulties, in the measurement of the daily energy expenditure of rural women under field conditions in developing countries. Since all methods need to be validated against a reference method which is usually based on indirect calorimetry, examples of the use of this technique are given. The energy costs of most agricultural and daily tasks of rural women in developing countries have been measured. Large intra- and inter-individual variations in the cost of a single activity occur, so repeated measurements are needed to obtain a valid mean energy cost for a specific activity for a homogeneous group of individuals.

Much work remains to be done on the assessment of the duration and the intensity of the physical activity of the rural adolescent and adult female population. Studies indicate that the workload of most rural women in developing countries is excessive and frequently associated with acute poverty.

Two methods of dietary recording, the 24-hr recall and the weighed dietary intake methods, are considered appropriate for estimating energy and nutrient intakes in studies of human fertility. The former method gives lower estimates than the latter, although weighed intakes may underestimate true intakes. Examination of food intakes of pregnant, lactating, and non-pregnant, non-lactating New Guinean women shows their diet to be less homogeneous than is generally assumed for groups in developing countries. As a result direct observations of food intake for a limited number of days are not sufficiently accurate for the estimation of intake of most of the nutrients examined. Rather the study design should reflect the variability of intakes of the nutrients and groups under consideration.

The measurement of food intake has long been used to describe ‘adaptation’ to low energy intakes in certain tropical peoples. However, the methods available to quantify food intake are unlikely to reflect accurately real energy intakes in free living peoples. Alternatively, estimating energy expenditure shows some promise—particularly the measurement of basal metabolic rate (BMR). The BMR may be measured effectively in males, but females show wide intra-individual variation in BMR during their menstrual cycle, which makes BMR measurements more difficult to interpret in the context of adaptation. The use of double-labelled water may be the only method suitable to quantify and define ‘adaptation’ to low intakes in women.

The two main ways in which disease and nutrition can influence fertility are by reducing fecundity or by extending the birth interval. Fecundity refers to reproductive ability, that is the potential to breed, as compared to fertility which denotes actual childbearing (McFalls & McFalls, 1984). Reduced fecundity, which is usually referred to as subfecundity, results from impairment of any of the biological aspects of reproduction, including coital inability, conceptive failure as well as pregnancy loss. Subfecundity is only one factor operating to reduce fertility; other factors include those governing mate exposure (both formation and dissolution of unions as well as exposure to intercourse within unions) and birth control.

The measurement of short-term changes in maternal body composition during the post-partum period under field conditions poses many problems: (1) body composition techniques depend on the constancy of the proportions of components or their physical properties and are less suitable for measuring changes; (2) many of the techniques require expensive, technically sophisticated apparatus that is inappropriate to field conditions in many countries; (3) changes in body composition affect some areas of the body more than others so regional as well as whole body approaches are required. The measurements of body weight, triceps and subscapular skinfold thicknesses and upper arm circumference are essential measurements. These can be supplemented with further skinfold thicknesses and circumferences, and possibly body density and body water measurements. There is little to be gained by transforming anthropometric variables into whole body composition indices in these circumstances.

The factors responsible for the return of fertility post-partum have not been very clearly defined. Several previous studies (Gross & Eastman, 1979; Diaz et al., 1988) have reported apparently major differences between populations in the duration of infertility associated with lactation. However, these differences may be related to the differences in study designs.

The Lese are subsistence farmers living in the Ituri Forest of north-east Zaïre. They exhibit significant birth seasonality, with lowest frequencies of conception when food production is least, nutritional status is low and ovarian function, as measured by salivary steroid hormone levels, is reduced. Efe pygmy foragers, who live in the same geographical area but are less dependent on cultivated foods and have a more flexible life style, do not exhibit frequent fluctuations in nutritional status nor significant birth seasonality. These findings support a model of birth seasonality relating climatic variables to variation in fertility through a causal chain linking rainfall to food production to energy balance to ovarian function to fertility. The model, which emphasises an ecological approach to the study of human reproduction, should have broad applicability since seasonality of food production and energy balance is widespread geographically and across a wide variety of economies and cultures.

Fifty-one mother-infant pairs were followed prospectively by home visits and telephone contacts during the first 6 months post-partum. Comparisons between mothers who experienced lactation crises because of perceived breast-milk insufficiency (crisis group) and those who did not (non-crisis group) revealed differences in attitudes to breast-feeding, breast-feeding behaviour and sexual life. The crisis group tended to initiate breast-feeding for infant-related reasons more frequently than the non-crisis group, which more frequently gave mother-related reasons. During the course of breastfeeding attitude changes in a negative direction were significantly more common in the crisis group. No difference was found between the groups in the resumption of sexual life, but 59% of all the women claimed that they experienced less or no desire compared to before pregnancy and, of these, a significantly higher proportion belonged to the crisis group.