Lactation and natural fertility

Abstract

Data for this study were taken from a retrospective survey conducted in 1967 on 500 Taiwanese women, married and aged 20-39, living mostly in rural areas. Purpose of the study was to observe the relationship between length of lactation and the period of postpartum amenorrhea. Results from this study confirmed previous findings by different authors as to influence of prolonged lactation on menstruation resumption, the zero order correlation coefficient between the 2 being 0.43; on the average, 1 month of lactation delays resumption of menstruation by about 0.4 month. In the present sample 6% of women became pregnant before resuming menstruation; with an average duration of postpartum amenorrhea of 10.2 month, pregnancy rate was 7.5/100 woman years of amenorrhea, which is much lower than the rate in absence of contraception. The study also shows, by regression analysis and by estimating probabilities of conception following resumption of menstruation, that there is a notable influence of lactation on the birth interval beyond resumption of menstruation. On average each month of lactation beyond resumption of menstruation adds 0.6 months to the birth interval; 45% of women who breastfed beyond this time became pregnant within 6 months, as compared to 68% among women who did not breastfeed. This birth interval is not affected by age, parity, education, ownership of modern objects, or place of residence. The chapter includes an extensive review of similar studies, and a brief examination of certain behavioral and biological mechanisms which may influence such data.