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630
There are a number of measures which are used to calculate the frequency of births. The simplest is the birth rate ^{1}, more accurately the crude birth rate ^{1}, which is the fraction, the numerator of which is the total number of births in a population during a given period and the denominator is the total number of personyears lived by the population during that period. The latter expression is generally approximated by the size of the population at the midpoint of the period multiplied by the length of the period in years. The rate is usually stated per 1,000. Like other rates in which the population at the midpoint of the period is used as the denominator of the fraction this is sometimes called the central birth rate ^{1}. Where the term "birth rate" is used without qualification, the live birth rate ^{2} is generally meant and only live births (6014) appear in the denominator. The total birth rate ^{3} based on live births and late foetal deaths is sometimes calculated. Legitimate birth rates ^{4} and illegitimate birth rates ^{5} with legitimate and illegitimate births respectively in the denominator are computed and the illegitimacy ratio ^{6}, the number of illegitimate births per 1,000 total births, is frequently used. These rates and ratios are generally based on live births. To compare the fertility of different populations, standardized birth rates ^{7} are often used to eliminate the effect on the birth rate of certain differences in structure of the population (most commonly the age and sex structure). The childwoman ratio ^{8}, most commonly the number of children aged 0 to 4 per 1,000 women of childbearing age, i.e., 15 or 20 to 44 or 49, is used as an index of fertility when reliable birth statistics are not available.
 3. The stillbirth ratio or late foetal death ratio is obtained if the number of stillbirths or late foetal deaths is divided by the number of live births {of. 4114 for late foetal death rate or stillbirth rate.)
631
The term fertility rate ^{1} is often used when the denominator of the birth rate fraction is restricted to females (or males) of reproductive age. Female fertility rates ^{2}, which relate births to women of childbearing age, are more common but male fertility rates ^{3} are also computed sometimes. Legitimate fertility rates ^{4} relate the total number of legitimate births (6103) to the number of currently married women (or men); illegitimate fertility rates ^{5} relate the total number of illegitimate (6104) births to the total number of single, widowed and divorced women (or men). Fertility rates without distinction of marital status (5151) (called in French taux de fécondité générale) make use of legitimate and illegitimate births combined. General fertility rates ^{7} usually relate all births to all women of reproductive age regardless of marital status and in such cases are the same as female fertility rates (6312). Rates based on a narrower age range (usually one year or five years) are called agespecific fertility rates ^{8} or agespecific birth rates ^{8}. Here again, births arc usually taken to mean live births.
632
The term cohort fertility ^{1} refers to the reproductive performance of particular birth or marriage cohorts. (Cf. 1162 for "cohort"). When birth cohorts are considered, the term generation fertility ^{1} is also used. The cohorts used are more commonly female cohorts. When total reproductive performance from the beginning of the exposure to the risk until some later date is being considered, we speak of cumulative fertility ^{2}. Completed fertility ^{3} or lifetime fertility ^{3} is the cumulative fertility of a cohort at the time when all its living members (in the case of a birth cohort) or its youngest living members (in the case of a marriage cohort) reach the end of the reproductive period. Before this time the term incomplete fertility ^{4} is employed to show that the cohort’s cumulative fertility may be expected to increase.
 3. In computing cumulative birth rates and completed birth rates for birth or marriage cohorts of women, the denominator is usually the number of members of the cohort who are living at a specified age or date and the numerator is restricted to the births i,o these women before that age or dale, i. e., the members of the cohort who died previously and the birLhs to these women are excluded.
633
The fertility of marriage ^{1} or marital fertility ^{1} may be studied by means of durationspecific rates ^{2} which are the ratios of births within a particular segment of married life to the total number of personyears lived by the women (or men) within that segment. The index of current marriage fertility ^{3} and the average number of children per marriage ^{4} may be computed by summing the durationspecific rates for a given period. The results thus obtained refer to the current fertility of a hypothetical marriage cohort and not to the completed fertility (6323) of an actual marriage cohort.
634
A fertility table ^{1} (cf. 1531) or fertility schedule ^{1} commonly shows fertility rates classified by age or by duration of marriage (or by both) and often shows these rates by order of birth (6111). Such tables are sometimes said to be tables of fertility functions ^{2}. Some fertility tables show the actual birth experience of marriage cohorts (1162) as they proceed through life, and contain the rates mentioned in para. 632. Other fertility tables show the experience of different age groups (or cohorts) in, a given year or a short period of years. With the latter tables the sum of all agespecific fertility rates is called the total fertility rate ^{3} and represents the number of children, that would be born per 1,000 females (or males) if they experienced no mortality and were subject to the agespecific fertility rates of a specified fertility table. The female gross reproduction rate (7114) which is obtained by multiplying the total fertility rate by the proportion of births which are female, is used frequently. The proportion of female births is the complement of the masculinity proportion at birth ^{4}. The sex ratio at birth ^{5}, which is sometimes called the secondary sex ratio5 to distinguish it from the sex ratio at conception ^{6} or primary sex ratio ^{7} is generally expressed as the ratio of the number of live boys born to the number of live girls born. It would be desirable to know the masculinity proportion at conception ^{6} but as yet few data are available regarding sex in the case of foetal deaths (4106*) during the first week or month after conception (6021).
635
The distribution of women by the number of births they have had is called a parity distribution ^{1} or sometimes a prolificacy distribution ^{1} or family size distribution ^{1}. The last term is ambiguous because relatives other than children are included in some studies of family size ^{2} while other studies do not consider all children but only surviving children ^{3} or dependent children ^{4}. In the study of fertility of marriage, attention is often devoted to marriages of completed fertility ^{5} i.e., to marriages in which the wife has reached the end of her reproductive period.
636
The term birth order rate ^{1} is commonly used when the numerator of the birth rate (6301) is restricted to births of a given order (cf. 6111), e.g. general fertility rates (6317) are often subdivided by birth order. Parityspecific birthrates ^{2}, parityspecific fertility rates ^{2} or select issue rates ^{2} not only restrict the numerator to births of a given order but also restrict the denominator to the women of the parity (6116) at risk (1342), e.g. second order births to oneparity women. Such rates are usually agespecific (6318) or durationspecific (6332). Occasionally parityspecific birth rates are also specific for both age and duration. Parityspecific birth probabilities ^{3} are the same as parityspecific birth rates except that the denominator consists of the number of women at the beginning of the period who have had a birth of the next lower order to the one considered in the numerator. A related index of fertility is the parityprogression ratio ^{4} which is the fraction whose denominator is the number of women of parity in a population whose fertility is complete and whose numerator is the number of women of parity n + 1 in the same population. These ratios reflect all prior childbearing and may also be computed for women of a specified age or cohort (1162). The foregoing ratios could be, but in practice generally are not, computed for men.
637
Couples who do not practise contraception during the period studied are called noncontracepting couples ^{1}. The fertility of noncontracepting couples ^{2} is sometimes studied and the probability of conception (6021) in a menstrual cycle (6225) has been called their fecundability ^{3}. A woman’s pregnancy history ^{4} or pregnancy record ^{4} contains detailed information about her pregnancies, including the dates when each began and ended. If the record also provides data of other important periods of nonexposure to the risk of conception (e.g. prolonged absence on the part of the husband), a conception rate ^{5} or pregnancy rate ^{5} may be computed to show the number of conceptions per unit of time among women during periods of exposure to the risk of conception ^{6}. Pregnancy rates are often agespecific (6318) or durationspecific (6332) and may be computed separately for contraceptors and noncontraceptors or for periods with and without the use of contraceptives. They are generally stated per 100 months of exposure. The months of exposure per conception ^{7} obtained by reversing the previous procedure gives another index. It is obtained by dividing the months of exposure by the number of conceptions. A comparison between the pregnancy rates of noncontraceptors and contraceptors of equal fecundity will yield a measure of the efficacy of contraception ^{8}, efficiency of contraception ^{8} or contraceptive effectiveness ^{8}.
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