Fertility and it's Measurements

Fertility refers to the actual birth performance of a couple. Fertility is affected by number of socioeconomic,cultural and psychological factors on ne hand and number of biological factors on the other and the mechanism through which they influence fertility is quite complex. I It is also true that socioeconomic and other factors regulate fertility through the biological factors. The main biological factors are: (i) fecundability, 

(ii) incidence of sterility, 

(iii) chances of conception being terminated in a foetal death or in a live birth and 

(iv) the period of temporary sterility comprising the duration of gestation and the post partum amenorrhoea, associated with a pregnancy termination. The first two components are influenced the physiological and other characteristics of both partners, while the rest are concerned with female only. These factors may vary from female to female in a population and for a female, they may depend on time, age, duration of marriage, number of pregnancies or live births already experienced and such other maternal characteristics.

Fecundability- The fecundability is defined as the probability of conception in one unit of time of exposure when both the partners are biologically fit. Generally, the unit is taken as one lunar month which is the length of a menstrual cycle. Since the physiological status of both the partners as well as frequent of intercourse determines the value of this parameter, it is obviously considered as being couple dependent.

Fecundity- The physiological capacity of a woman (or a couple) to reproduce a child.

Sterility- The term sterility relate to the inability of a couple to procreate. A sterile couple is necessarily infertile but the converse is not always true. Even when a couple is fecund, it is possible that there is no inception, especially if fecundability is low. There are two types of sterility: primary and secondary. A female is primarily sterile if and only if she cannot change her status from parity zero to one. She is said to be secondary sterile if she becomes sterile after one or more births. Sterility may be attributed to the husband or to the wife or to both of them. Thus like fecundability, the sterility is also couple dependent.

Foetal wastage- A conception may not always result in a live birth. The outcome of a pregnancy that ends in a spontaneous or induced abortion or a still birth is known as foetal wastage.

Measures of Fertility

From the various sources of demographic data as mentioned in chapter 2, following types of fertility measures may be computed.

Just like the mortality analysis, fertility analysis also begins with good quality data on births and population. These data are conventionally obtained from vital registration systems and population censuses respectively. The crude birth rate and the specific birth rates (generally known as the fertility rates specific for age of woman, marital status of woman, parity of woman, birth order of child etc.) are simple measures of fertility. Based on the specific fertility rates, the measures of reproduction are calculated. Some measures of fertility are based on the census or survey data alone. Fertility studies involving effect of contraception, post-partum abstinence, and breastfeeding on fertility, and birth interval dynamics make use of many other population measures.

(i) Cohort measures of fertility- In this approach; fertility is computed indirectly on basis of data on the number of children ever born to women up to a certain point of time in the reproductive age group.

(ii) Period measures of fertility- In this approach, fertility is computed related to a particular period

based on data on the number of births for the referred period.

Crude Birth Rate

The crude birth rate is calculated by dividing the number of registered births in a year by the mid-year population for the same year. The rate is expressed as per 1,000 population.

General Fertility Rate

The general fertility rate is calculated by diving the number of registered births in a year by the mid-year population of females aged 15-44 years for the same year. The rate is expressed as per 1,000 female’spopulation in reproductive ages.

AGE SPECIFIC FERTILITY RATE

The age-specific fertility rates are calculated from births and female population both specific to each age or age group) or woman.

Total Fertility Rate

Mean or median age of the fertility schedule (a set of the age-specific fertility rates from the minimum age to the upper age of the woman's reproductive life span) is calculated from the age-specifi fertility rates at single or five year age groups of women. Note that these are better measures of the average age of mother giving a birth as these take into account the age distribution of the female population in the year.

Total fertility rate is a summary measure of the age-specific fertility rates. It is calculated as the sum of the age-specific fertility if ages rates are in single years of age. If the rates are for each five year age group of woman, the sum of the rates is multiplied by 5, on the assumption that the five year age group rate will hold at each single age within the age interval of five years. The total fertility rate can be calculated for each year ( or period) or for birth cohorts.