Our ancestors were unique in having the lowest rate of reproduction of any living mammal. This was achieved by the postponement of puberty until well into the second decade of life, a maximal probability of conception of only about 25% per menstrual cycle even when ovulation had commenced, a 4-year birth interval as a result of the contraceptive effects of breast-feeding, and sharply declining fertility during the fourth decade of life, leading to complete sterility at the menopause (Short, 1976, 1983).

Longitudinal studies involving weekly measurement of urinary oestrogen and pregnanediol excretion were performed in 55 post-partum women to identify the patterns of returning fertility. The women kept diaries of breast-feeding episodes, mucus symptoms and times of bleeding; some recorded basal body temperature and acts of intercourse. The study represented a total time of 36 years (7·8 months per woman). The majority of the women were breast-feeding throughout the study. During lactation amenorrhoea, there was complete ovarian quiescence in two-thirds of the subjects and minimal cyclic activity in the others. The first bleeds were associated with anovulatory ovarian activity in 40% of the women, with normal ovulation in 19%, with ovulation but short luteal phases in 25% and with ovulation but deficient luteal phases in 16%. The incidence of normal ovulatory cycles increased with time after delivery and with the number of cycles after delivery and eventually reached 85% even though many women were still breast-feeding at the time. Twenty-two pregnancies occurred, fourteen being unplanned: eight women were fully breast-feeding at the time of conception. Mucus symptoms correlated with the hormone values in approximately 60% of cycles. However, some women had symptoms which were difficult to interpret during their early cycles or potential fertility was not indicated by raised mucus scores or the presence of the peak mucus symptom. These problems together with the resulting confusion appeared to be the major causes for the unplanned pregnancies. Methods for coping with these problems are discussed.

Cross-sectional studies in Australia and the Philippines and a longitudinal prospective study in a selected Australian sample of breast-feeding mothers have shown that basal serum prolactin (PRL) concentrations are elevated during 15–21 months of lactational amenorrhoea.

A predictive model of serum PRL levels and return of cyclic ovarian activity during full breast-feeding, partial breast-feeding and weaning has been developed from the results of breast-feeding behaviour and serum PRL, gonadotrophin and oestradiol measurements in 34 mothers breast-feeding on demand for a mean of 67 weeks.

Breast-feeding patterns influence serum PRL levels. Important factors during full breast-feeding are the age of the baby, the longest interval between feeds at night and total 24-hr suckling time, and following the introduction of supplements, the mean interval between feeds, together with the total 24-hr suckling time and the number of solid supplements per day.

The precise mechanisms whereby breast-feeding regulates cyclic ovarian activity remain unknown. Gonadotrophin secretion appears to be quantitatively normal, but qualitative changes, secondary to altered hypothalamic activity, may be the most important factor. A direct inhibitory effect of PRL on ovarian follicular development and steroidogenesis remains possible.

Ovulation with a normal luteal phase is probable for 30% of breast-feeding mothers before the first menses, but is unlikely before 6 months, provided breast-feeding is frequent day and night.

Measurement of serum PRL is a sensitive index of the return of menstruation and fertility during lactation in the population studied.

Breast-feeding is an integral part of the reproductive process. This paper reviews the consequences of breast-feeding and bottle-feeding for the child and the mother with emphasis on health parameters and child spacing. Controversies that have arisen in this field are considered. Methodological problems in study design and areas for further research are discussed.

Lactating women are at reduced risk of conception, but the individual woman may not wish to accept even a reduced level of risk. The risk increases when infant feeding supplementation begins and an alternative form of contraception is then required for the lactating mother. The relationship between lactation and fertility, trends in prolonged lactation, factors affecting these trends, and contraceptive use during lactation are reviewed. Suggestions for advising individuals and formulating programme guidelines are discussed.

At a time when women's participation in developing countries and their breast-feeding practice are the objects of promotional campaigns, findings on the relationship between the two are required for formulating policies. Empirical findings from the 1973 National Demographic Survey in the Philippines strongly suggest the adverse effects of modernization on both the initiation and duration of breast-feeding. In this study a high level of education and working outside the house are found to be the modernization variables that are most effective in depressing the level and duration of breast-feeding practice, independent of the influences of other socioeconomic and demographic factors.

In recent years both the prevalence and duration of breast-feeding have decreased, especially in urban areas of the developing world (McCann et al., 1981). While the majority of births in these areas continue to be attended by traditional birth attendants, the proportion of mothers giving birth in hospitals is increasing. The potential for hospital personnel and practices to affect infant feeding preferences is likely to increase as the trend towards hospital births continues.

A survey of the knowledge, attitudes, and practices of 1298 Filipino health professionals towards breast-feeding is analysed. Knowledge of the benefits of breast-feeding, the psychophysiology of lactation, contraindications to breast-feeding, and the effects of drugs and contraceptives on lactation and delivery are some of the issues examined. A low level of knowledge about breast-feeding, particularly of health contraindications, is found. Industry and government programmes have both positive and negative effects on breast-feeding knowledge and attitudes.

The relationship between the return of menstruation and ovulation and the pattern of breast-feeding was studied in a group of 29 women in a rural area of Mexico; eight non-breast-feeding post-partum women were also studied. The return of ovulation was assessed by weekly determinations of urinary pregnanediol. The number of suckling episodes and supplementary feeds was recorded. The cumulative probability of resumption of menses for the breast-feeders was 28% and 52% at 3 and 6 months, respectively, and for the non-breast-feeders was 75% and 88%. The cumulative probability of ovulation for the breast-feeders was 28%, 66%, 69% and 72% at 3, 6, 9 and 12 months, respectively, and for the non-breast-feeders was 50% and 100% at 3 and 6 months. The earlier the introduction of supplements the earlier the return of ovulation. The nonovulatory subjects maintained a higher frequency of suckling episodes than did the ovulatory subjects. The cumulative probability of pregnancy for the breast-feeders was 3%, 7%, 7% and 10% at 3, 6, 9 and 12 months, respectively, and for the non-breast-feeders was 13%, 38% and 50% at 3, 6 and 9 months.

Longitudinal data collected over a 2-year period (1982–84) on 152 first and second parity mothers who were delivered in a charity maternity hospital in Manila, Philippines, indicate the reasons for never brest-feeding and for early termination of brest-feeding. Socio-Cultural factors and beliefs are more important than physiological problems in minating breast-feeding.

Proper bottle-feeding is too costly for most low-income families. Bottle-fed babies have a higher incidence of diarrhoea. Mothers who change from breast- to bottle-feeding in the first 6 months are 1·7 times as likely to become pregnant in the first year post-partum as mothers Who brest-feed for 7 or more months.

In pre-transitional societies, contraceptive practice is usually low or absent and prolonged breast-feeding has been identified as the major factor in keeping marital fertility levels below the biological maximum (Bongaarts, 1978), so that the length of birth intervals is the strongest determinant of completed family size.

In transitional societies there is accumulating evidence that, with increasing modernization, the percentage of women who initiate breast-feeding and the duration of breast-feeding are declining (Jelliffe & Jelliffe, 1972; McCann et al., 1981; Rosa, 1976). Shorter durations of breast-feeding are observed for women who are more educated, belong to the upper socioeconomic class and live in urban areas (Jain & Sun, 1972; Lesthaeghe & Page, 1980).

The effects of infant suckling patterns on the post-partum resumption of ovulation and on birth-spacing are investigated among the Gainj of highland New Guinea. Based on hormonal evidence, the median duration of lactational anovulation is 20·4 months, accounting for about 75% of the median interval between live birth and next successful conception (i.e. resulting in live birth). Throughout lactation, suckling episodes are short and frequent, the interval changing slowly over time, from 24 minutes in newborns to 80 minutes in 3-year olds. Maternal serum prolactin concentrations decline in parallel with the changes in suckling patterns, approaching the level observed in non-nursing women by about 24 months post-partum. A path analysis indicates that the interval between suckling episodes is the principal determinant of maternal prolactin concentration, with time since parturition affecting prolactin secretion only in so far as it affects suckling frequency. The extremely prolonged contraceptive effect of breast-feeding in this population thus appears to be due to (i) a slow decline in suckling frequency with time since parturition and (ii) absence of a decline over time in hypothalamic–pituitary responsiveness to the suckling stimulus.