Doctoral Dissertation Research: Seasonality of Fertility in the Laguna de Sinamaica, Venezuela

Seasonal birth patterns have been observed in virtually all human

populations. While such patterns are known and well understood in many

animal species, they are difficult to analyze in humans, since biological

factors contributing to birth rates are confounded with cultural factors.

Taken as a whole, the conclusion of the many studies on birth seasonality

in humans is that while such seasonal variation is universal, the causes of

variation are not universal. Most of the research that has attempted to

explain observed seasonal patterns in specific populations has focused on

only a few possible causal factors. In many cases factors such as diet,

workload, and climate are shown to be correlated with fertility, but the

mechanisms linking them with fertility remain unexplained.

The proposed study will analyze seasonal birth patterns in an indigenous

American population (the Anu of northwestern Venezuela). It will describe

seasonal variation in proximate determinants of fertility, including

characteristics of ovarian cycles and fetal loss. These data will be used

in models of fecundability and fetal loss that together describe

seasonality of fertility. The study includes analysis of 1346 urine

samples that were collected twice weekly during the seasons of most and

fewest conceptions. The samples will be assayed to determine levels of

human chorionic gonadotropin (the hormone that indicates pregnancy) and of

urinary metabolites of the hormones estradiol and progesterone; the assay

data then will be used with interview data to determine proportion of

cycles ovulatory, length of cycles, rates of conception, rates of fetal

loss, and levels of progesterone, as an indicator of luteal sufficiency.

Previous investigations of seasonality of fertility have been based

largely on factors specific to the population being studied. The proposed

study uses a model of seasonality of fertility that can be applied to any

population. By combining existing models of fecundability and fetal loss,

the impact of individual proximate determinants of fertility on seasonal

fertility patterns can be more thoroughly analyzed than in previous

studies. Seasonal data on more distal determinants, such as diet,

workload, spouse absence, and nutritional status, can then be evaluated in

terms of the information on the proximate determinants, shedding light on

possible linking mechanisms and opening avenues for further research.