Biological and behavioral factors influence group differences in prolactin levels among breastfeeding Nepali women

This study of two natural fertility Nepali groups, the Tamang and Kami, identifies biological and behavioral factors associated with population differences in fertility. Previous research had established that Tamang experience longer interbirth intervals than Kami despite similarly intense nursing practices, and bear considerably higher energy expenditure due to workload. This cross-sectional study of 71 breastfeeding women includes prolactin determinations on three blood spot samples collected 5, 30, and 50 minutes following a timed nursing bout, and data on maternal age, BMI, menstrual status, previous birth interval, parity, infant age, nursing bout length, and durations of supplementation and postpartum amenorrhea. The findings show that Tamang breastfeeding mothers have higher average prolactin levels than Kami for as long as 22 months postpartum. Tamang mothers sustain average prolactin levels above those of nonpregnant, nonlactating women for nearly 2 years postpartum, whereas prolactin levels among Kami breastfeeding mothers are the same as this latter group after 1 year postpartum. Furthermore, the findings indicate that Tamang mothers have higher average prolactin levels regardless of maternal age, physical status (BMI, weight, or height), or infant age, and the rate of decline in prolactin from 5 to 50 minutes after suckling is significantly greater for Kami than Tamang. Since factors associated with prolactin levels differ by group, the findings also emphasize that populations vary not only in the strength of effects that proximate determinants have on fertility regulation, but also in the pathways by which they exert their effects (direct physiological versus indirect behavioral). Am. J. Hum. Biol. 10:191-210, 1998. © 1998 Wiley-Liss, Inc.