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 will try to imitate this naturally evolved method of fertility control (McNeilly, 1979).