Natural fertility is a concept eveloped by French demographer Louis Henry to refer to the level of fertility that would prevail in a population that makes no conscious effort to limit, regulate, or control fertility, so that fertility depends only on physiological factors affecting fecundity. In contrast, populations that practice fertility control will have lower than "natural fertility" levels as a result of delaying first births (a lengthened interval between menarche and first pregnancy), spacing out the intervals between births, or stopping child-bearing at a certain age. Such control does not assume the use of artificial means of fertility regulation or modern contraceptive methods but can result from the use of traditional means of contraception or pregnancy prevention (e.g., coitus interruptus), or from social norms or practices regarding celibacy, the age at marriage and the timing and frequency of sexual intercourse, including periods of prescribed sexual abstinence. Ansley Coale and other demographers have developed several methods for measuring the extent of such fertility control, in which the idea of a natural level of fertility is an essential component.