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P. W. Howie and A. S. McNeilly

An adequate inter-birth interval is of great importance because it enables a mother to recover her physical and emotional strength between pregnancies and confers upon the child advantages of better health and development (Morley, 1977). Breast feeding is the naturally evolved method of ensuring an adequate inter-birth interval but the contraceptive effect of breast feeding has been largely dismissed in developed countries on the grounds that it is an unreliable method of family planning for individual mothers (Kamal et al., 1969). The use of artificial contraception varies very widely in different countries, being much higher in developed nations than in the poorer developing countries (Text-fig. 1). Where the use of artificial contraceptives is low, breast feeding is the most widely used method of birth spacing and assumes major demographic importance. It is important, therefore, that the factors controlling lactational infertility should be understood so that guidelines can be developed which

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A. S. McNeilly, Anna Glasier, Julie Jonassen, and P. W. Howie

Introduction

There is no doubt that in breast-feeding women, suckling with its associated hyperprolactinaemia prevents the resumption of ovarian activity for prolonged periods (see McNeilly, 1979). The extent of this suppression varies greatly among species but in all for which there are adequate data it appears to depend critically upon the intensity of the suckling stimulus (Lamming, 1978). Our recent data from women show that in the pattern of suckling, frequency and duration, throughout the day, are both key factors in maintaining the elevation of basal levels of prolactin associated with lactation (McNeilly, Howie & Houston, 1980a; Howie & McNeilly, 1982). Suckling also releases large quantities of prolactin, maintaining a physiological hyperprolactinaemic state which is directly associated with the duration of lactational amenorrhoea (Delvoye, Badawi, Demaegd & Robyn, 1978; Duchen & McNeilly, 1980). The question remains, how does suckling suppress ovarian activity?

The levels of prolactin in blood during peak