To put the problem in a historical perspective, consider the data, or more correctly estimates, shown in the Tables below.

Table: Historical Population Data

Continent   Persons     Persons   Persons  Persons  Persons  Persons
            BC 300,000  BC 2000   AD 1650  AD 1750  AD 1850  AD 1900 
Africa      0.1           ?       100       95        95      120
Asia        ---           ?       257      437       656      857
Oceania     ---           ?         2        2         2        6
Europe      ---           ?        96      117       214      303
U.S.S.R.    ---           ?         7       27        60      126
N. America  ---           ?         4        5        39      106
S. America  ---           ?         4        6        20       38
World       0.1         100       470      689      1086     1556

Table: Modern Population Data

Continent Persons  Persons  Persons  Persons   Increase  
          AD 1950  AD 1970  AD 1975  AD 2000   (% 1970-5)
Africa     206      345      401      517       2.6
Asia      1407     2055     2256     3870       2.1
Oceania     14       20       21       29       2.0
Europe     390      460      473      568       0.6
U.S.S.R.   201      245      255      379       1.0
N. America 216      320      343      510       0.9
S. America 209      190      218      394       2.7
World     2543     3625     3967     6280       1.9

There is no question that the human population of this planet is now higher than it has even been before and is continuing to increase rapidly. The source of this rapid growth is primarily twofold: a rapid decrease of the death rate from disease due to advances in medicine, and an increase in food production with increasing agricultural organization. The birth rate, contrary to popular impression, has not risen significantly in historical times and indeed has fallen but the decrease in death rate (especially that of infants) has been greater. Thus the net rate of population increase (birth rate less death rate) has been higher in recent decades than in all previous history. Continuous rapid increase in population is not indefinitely sustainable, since the resources and energy available on earth are limited. Since an increase in death rate is undesirable, the population problem can only be resolved by a decrease in the birth rate.

The causes of population growth are varied and appropriate sociological studies are beyond the scope of this work. For our purposes it suffices to deal with the gross birth rate (total live births/year) and gross death rate (total deaths/year), the difference between which given the net increase in population each year. Both of these factors are affected by technology. Medical technology is responsible for decreases in the death rate, and through prenatal care is responsible in small part for increasing the birth rate. Indirectly, medical technology is also responsible for decreasing the birth rate, since an older population has a smaller birth rate due solely to age, but this is a comparatively long-term factor.

In the short term, changes in birth rate can be achieved only by actions of individuals in their reproductive years (15-45) either not to begin pregnancies (contraception) or to terminate them prior to birth of the child (abortion). Methods currently available to prevent pregnancy are: abstention from sexual intercourse (effective), surgical sterilization (effective in male or female; permanent), physical contraceptive methods (male or female; effectiveness varies), chemical contraceptive methods (effective only in females; temporary), and intercourse scheduling (effectiveness limited). All have been and are being used, but only the chemical contraceptive methods will be discussed here. Statistics on the number of persons in the world using each of these techniques are not available, but in the developed countries chemical contraceptives are probably the most frequently used of the contraceptive methods.

Chemical Contraceptive Methods

The steroids are found in all plants and animals. The most abundant animal steroid, cholesterol, is both synthesized and absorbed by the human body.The concentration of cholesterol in blood serum is about one mmol/litre. Cholesterol has been linked to gallstone formation and arterial hardening. Biochemical alteration of cholesterol leads to other steroids such as cortisone. This adrenal cortex hormone has powerful effects as an antiinflammatory (arthritis) and in neurological diseases.

The sex hormones responsible for secondary sexual development in the male are testosterone and the related androgens, while those in the female are progesterone and the related estrogens such as estrone and estradiol. The concentrations of estradiol and other sex hormones in blood serum, perhaps one nanomole/litre, is far below the concentration of cholesterol.

In males, the levels of androgens remain constant during the reproductive years, while in females the levels change over the 28-day ovulatory cycle and in pregnancy. Oral chemical contraceptives are based upon the normal female hormonal level changes. At the beginning of an ovulatory cycle, the pituitary gland releases estradiol and progesterone which stimulate the ovary toxrelease an egg. After ovulation, both estrogens and progesterone are secreted by the ovary itself and the release from the pituitary gland then ceases. While the ovary is producing hormones further egg release is inhibited. If the egg is not fertilized and implanted, the secretion of hormones returns to normal after a few days and estrone and estradiol (in the absence of progesterone and other estrogens) initiate the deterioration of the old uterus wall and the formation of a new one. The cycle is repeated the following month. If the egg is fertilized and implanted on the uterus wall estrogens and progesterone continue to be produced and ovulation thus does not occur during the pregnancy.

The oral contraceptives prevent pregnancy by preventing ovulation. This is accomplished by mimicing the normal hormonal suppressive mechanism artificially. Usually one pill is taken each day for 21 days beginning on day 5 of the menstrual cycle; if pills are taken each day, for 28 days, 7 usually contain no hormones. One of the pills (Enovid; G.D. Searle and Co.) contains a mixture of norethindrone (the major constituent) and mestranol. Compare their structures with those of progesterone and estrogen.

Figure: Structures of norethindrone, mestranol, progesterone, and estrogen.

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