RESUMO
The effect of lactation on menstrual cycles, ovulation and conception was studied in a group of non-contracepting Amerindian Mopan Mayan women. Anthropological observations of relevant events were made over a 21-month period. Blood samples were assayed to determine the plasma concentrations of prolactin, luteinising hormone, follicle stimulating hormone, human chorionic gonadotrophin, placental lactogen, oestrogen, progesterone and cortisol. The data show that: frequent and prolonged breast-feeding was associated with a marked increase in plasma prolactin concentrations to levels similar to those in lactating Gaing but higher than those in lactating Scottish women; ovulatory menstrual cycles and pregnancy occurred during frequent lactation; in lactating menstruating women there was an inverse correlation between fat weight and months post-partum. These data suggest that other factors as well as suckling account for the effects of lactation on fecundity.
PIP: Lactation, ovulation and conception were observed as part of an anthropological study of Amerindian Mopan Mayan women from the village of San Jose Hawaii in western Belize from March 1985-January 1987. Single blood samples from each subject were immunoassayed for prolactin, LH, FSH, hCG, placental lactogen, estrogen, progesterone and cortisol. Anthropomorphic data analyzed were body mass index (BMI), fat/weight percentage, total body water and lean body weight. 117 women had at least 1 child during the study; 91 were lactating; 51 reported no menstrual cycles. 50 submitted to blood testing. Almost all infants were breast fed for 18 months or longer, up to 3 years, typically at least 6 times per day and 3 times per night. Women averaged 9 live births and 8 surviving children, with a mean birth interval of 28 months. 25 of the 29 women known to be pregnant conceived while lactating. 16 lactating women were pregnant. Their culture requires them to have 3 menses before conception to nourish the fetus, yet forbids speaking about menstrual blood: women fabricated menstrual dates, but in confidence 51 of 81 stated that they did not menstruate before the last conception. Most often menses began 12 months postpartum. Lactating women had heightened prolactin levels even if supplementing their children's diet. Thus frequent lactation delayed onset of menses, but supplementation had no effect. Most of the women were within the normal range of BMI, but 13% were below normal. In lactating menstruating women there was a significant negative correlation between fat weight and postpartum month. The data suggest that the interval to conception or menstruation was inversely correlated with fat weight. Here suckling frequency rather than prolactin levels seems to postpone fertility. In this society, with 10-12 births and 9-10 children in the completed family, the largest in the world, prolonged frequent lactation has little effect on fertility. Instead, birth trauma, maternal mortality, fetal and infant mortality, and perhaps nutrition, have more effect on completed family size.