No wonder it took a long period of time for the world population to reach the first billion mark in 1820 AD. Further, Bongaarts (1975) argued that in the high fertility societies the birth rate too was not as high as what could have been under sheer biological reproductive efficiency. To quote:

…a typical thirty-month birth interval in a natural fertility population may be divided as follows: twelve months postpartum amenorrhea, four months waiting time to conception before an intrauterine death, a one-month nonsusceptible period associated with the intrauterine death, another four months' conception waiting time before a live birth, and finally a nine month full term pregnancy.

In summary, the highest observed birth rates are much lower than is biologically possible because women in natural fertility societies are pregnant during only about one-sixth of their reproductive years. The rest of these potential childbearing years is spent in the unmarried, sterile, postpartum anovulatory, nonsusceptible, or ovulatory states. As a result, the birth rate rarely exceeded 50 in populations in which no deliberate actions are taken to affect the biological process of reproduction. Voluntary fertility control efforts by couples during the childbearing years are required to reduce birth control below the natural level.

POPULATION IN MIDDLE AGES

It may be noted that in the beginning of our era, the world population is estimated to be around 326.5 million. The region-wise break up shows that 35 million people lived in Europe, 220 million in Asia, 21 million in North and Central America, 19 million in South America, 30 million in Africa and 15 million in Oceania. Thus nearly 21.8 percent of the world population lived in what may be called developed countries and 78.2 percent in the developing countries (Urlanis, 1978).