Why High Birth Rates are so Low by Bongarts?

The highest fertility ever recorded reliably in a society is founded among Hutterities, a religious set living in a small community near the united stars and Canadian border. In 1950, Hutterities women who had reached the end of their reproductive years had given birth to an average of 89 children.

Although Hutterities fertility is very high compared with that of many of human population, it is surprisingly low from the point of view of biological reproductive efficiency. Clearly a women is able to bear more than about nine children during her 30 year or so of reproductive life (age 15-45) as is evident from the numerous examples of women who have given birth more than 15 children in their reproductive years.

But the question arises, why then are women in such societies such as the Hutterities, where deliberate birth control is absent, reproducing at rates substantially below biological limits? In this particular case two factors are responsible—

1. Demographic Factors and natural fertility.
2. Biological factors and natural fertility.

A. Demographic Factors and Natural Fertility: Marriage is the principle demographic factor affecting the birth rate in population with natural fertility. A women’s reproductive career also being with marriage but not the case in every society. For example, in Indian young bride often stay with their parents for sometimes after marriage and in part of Latin America child bearing is legitimate in consensual unions. Before the official marriage ceremony. Such as Maradona, Messi.

Among young women the proportions of married in different age groups depends mainly on the average age at first marriage while over age 30 the incidence of 

• Celibacy.
• Widowhood.
• Divorce and 
• Remarriage is the chief determinant.

In a study of Bourgeoies and Pichat 1950 found that with average of respectively 10 and 40 percent of potential marital life remaining unused between the ages of 15 and 45. Among high fertility counties value for lost reproductive years at about 25 percent was common, so that of  in the 30 years of potential reproductive life 22.5 years were spent in the married state. This implies that, if the total fertility rate equals to 7 births per women, the rate of child bearing is 0.31 births (7/ 22.5) per married women per year.

It should emphasized that a marital fertility rate of 0.31 is an average, higher value prevail in the age group bellow 30 ages, whereas near the end of the reproductive period fertility is much lower. One of the first analyses of natural marital fertility rates was made by Louis Henry, who collected data from 13 societies with widely varying cultural characteristics in which deliberate birth control could be assumed absent. Averaging the fertility rates in these 13 societies resulted in the following values----

0.42 for women aged 20-30 years.

0.33 for women aged 30-40 years.

0.15 for women aged 40-45 years.

There is a clear age pattern, but the value of 0.31 for all ages, arrived at by the above crude calculation, appears to be a representative average of observed natural age specific marital fertility rates.

B. Biological Factors and Natural Fertility: Rates sterility is the biological fact primarily responsible for the age pattern in natural marital fertility. A distinction is usually made among the following three types—

• Adolescent Sterility: Present among teen agers who have not yet reached menarche or who ovulate irregularly after menarche.
• Primary Sterility: Present among women who are never able to reproduce.
• Secondary Sterility: Which sets in after having been fertility little is known about the determinants of the prevalence of each of the forms of sterility, but presumably health and nutrition are important. Ex: Because of accident. 

From the available measurement, it is estimated that in health population, the loss of fertility years among all women between the ages 15-45 is probably less than 20 percent, but in areas where malnutrition and poor health conditions are common a higher sterility incidence may be observed.

One could find that in a natural fertility population only 17 or 18 out of the 22.5 years married life are non-sterility and thus available for reproduction. If the total fertility rate is 7 births per women, this would mean a rate of birth 0.4 (7/17.5) per non-sterile married women per-year and an average interval between births of about 2.5 (17.5/7) years.

Immediately, following a birth women experience “ post-partum amenorrhea” a period during which the normal eyelike pattern of menstruation and ovulation is absent. But after the regular ovulation function is restored married women are subject to the rise of conception. And once an ovum has been fertilized, it has about 50% chance to survive 9 months and yield a live birth, with approximately one- third of fertilized ova being rejected during the first two weeks after conception. (In this regards Bongarts divided the segments of interval between two births, a typical thirty months birth interval in a natural fertility population as follows-

•  12 month- postpartum amenorrhea.
• 4 month- waiting time to conception before an intrauterine death. 
• 1 month- non-susceptible period associated with the intrauterine death.
• Another 4 month- conception waiting time a live birth.
• And finally 9 month full time 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. As a result birth rates rarely exceed 50 in population in which no deliberate actions are taken to affect the biological process of reproduction voluntary.