@inbook{Gray1979,
author = {Gray, R.},
title= {Biological factors other than nutrition and lactation which may influence natural fertility: A review},
year = {1979},
editor = {Leridon, H.},
editor = {Menken, J.},
booktitle = {Patterns and Determinants of Natural Fertility},
address = {Liège},
publisher = {Ordina editions},
pages = {217--251},
}