Fertility at low and high altitude in Central Nepal.

Abstract

Since the 1930s, a number of different studies have tended to show that fertility is lower at high altitude. The present investigation attempts to provide some answers to this question by examining completed fertility rate (CFR) in Highland and Lowland villages in Central Nepal and relating rate differences to age at menarche, age at 1st childbirth, age at 1st marriage, incidence of venereal disease, birth control (vasectomy or hysterectomy), length of postpartum amenorrhea, and breastfeeding. Data was obtained by direct questioning, and under-reporting of births thus cannot be excluded. Fertility histories were taken from post-menopausal women over the age of 45 years. Results indicate no significant difference in reported menarcheal ages between highlanders and lowlanders. Age at 1st marriage and 1st childbirth were both significantly later in highlanders. CFR was significantly lower in highlanders. It would appear that the reduced fertility rate at high altitude can be partly attributable to later age at marriage and later 1st childbirth. Other factors, e.g., husband absenteeism and remarriage have also been suggested as possible contributors to the observed difference. This paper presents the results of a multiple regression analysis using 9 dependent variables: ages of marriage, 1st childbirth and menarch, the average gap between pregnancies, the average amount of time the husband was away, the number of marriages, presence or absence of venereal disease at some time, whether birth control was practiced and altitude status. Average pregnancy gap, age at 1st childbirth and presence or absence of venereal disease were the only variables that independently made a significcant contribution to CFR variance. The increase in pregnancy gap may be related to longer periods of breastfeeding in high altitude women and there would be a concomitant delay in recommencement of menstruation. In testing the hypothesis, no difference is found in reported duration of breastfeeding or in postpartum amenorrhea. The age at marriage and age at 1st childbirth accounted for over 16% of the explained variance in CFR. Some of the observed difference in CFR can be explained by the difference in marital age but not by the interval between marriage and 1st childbirth, as it was very similar in both groups. The lower CFR among the high altitude population could be due to lowering of biological fecundity at high altitude, or simply a matter of choice. The difference might reflect human reproductive hormone differences between high and low altitude populations. Further research will be needed to determine whether or not differences in CFR can be explained by variation in these factors.