Introduction

The absence of mechanization and transport facilities in the Third World requires subsistence farmers, horticulturalists and hunter-gatherers to sustain moderate to high energy expenditure (Lawrence et al, 1985; Bleiberg et al, 1980; Bentley, 1985). Their diet is mainly derived from noncultivated or home-grown food. Marked seasonal food shortage and chronic malnutrition are common occurrences (Bahuchet, 1988; Pagezy, 1984; Swinton, 1988; Bailey & Peacock, 1988; Hill et aL, 1984), and a varying form of adaptation might be needed to achieve a ‘long-term energy equilibrium’ (Ferro-Luzzi, 1988).

This is particularly relevant to rural areas where the phenomena concerning fertility are more acute than those observed in urban areas in the same countries. Recent immigrants to large urban areas have generally changed their diet to a more diversified one. At the same time women seem to reduce their physical workload, as many of them work as maids or sell home-baked food in the market, which is a quite different pattern of energy expenditure from that of the daily activity of women living in the bush. Another difference between urban and rural areas is the more widespread use of modern contraceptives in towns, including sterilization for men, or the use of long-duration contraceptives for women, such as six monthly injections of Depo-Provera.

In Third World populations, breastfeeding traditionally continues for long periods of time, and this is accompanied by a long duration of postpartum amenorrhoea with long birth intervals. Health care facilities are limited and the child mortality rate up to the age of five years is generally high.