Table 6 presents live births by age of mother and live-birth order for as many years as possible between 1990 and 1998.
Description of variables: Age is defined as age at last birthday, that is, the difference between the date of birth and the date of the occurrence of the event, expressed in completed solar years.
Live-birth order refers to the numerical order of the live birth in relation to all previous live-born issue of the mother, irrespective of whether pregnancies were nuptial or extra-nuptial.
Except where otherwise indicated, eleven categories are used to classify live birth order: 1 through 9, 10 or more and number of live births unknown.
Reliability of data: Data from civil registers of live births which are reported as incomplete (less than 90 per cent completeness) or of unknown completeness are considered unreliable and are set in italics rather than in roman type. Table 1 and the technical notes for that table provide more detailed information on the completeness of live-birth registration. For more information about the quality of vital statistics data in general, and the information available on the basis of the completeness estimates in particular, see section 4.2 of the Technical Notes.
Limitations: Statistics on live births by age of mother and live birth order are subject to the same qualifications as have been set forth for vital statistics in general and birth statistics in particular as discussed in section 4 of the Technical Notes.
The reliability of data, an indication of which is described above, is an important factor in considering the limitations. In addition, some live births are tabulated by date of registration and not by date of occurrence; these have been indicated by a (+). Whenever the lag between the date of occurrence and date of registration is prolonged and, therefore, a large proportion of the live-birth registrations are delayed, birth statistics for any given year may be seriously affected.
Another factor which limits international comparability is the practice of some countries not to include in live-birth statistics infants who were born alive but died before the registration of the birth or within the first 24 hours of life, thus underestimating the total number of live births. Statistics of this type are footnoted.
The principal limitation of this table is that in some cases it shows data for total births while in others it presents data for legitimate births only. This is because many countries or areas compile birth order data only for legitimate births. Thus data found within this table tend to be less comparable than data on live births by age of mother and age of father presented in other tables as the latter two have been restricted to total births only.
Furthermore, some of the methods used to estimate birth order deviate from the definition stated above. Some countries and areas base birth order on the sum of previous live births and late foetal deaths, on legitimate live births, on live births during existing marriage and on a variety of other combinations. These differences in definition impair international comparability of the data. Within the time series for any one country or area, however, data may be compared, provided the definition has not changed radically in the interim bearing in mind the limitations noted in the next paragraph.
At one time the vast majority of births occurred after marriage. Therefore, if information on birth order was not available for women having an illegitimate birth, there was little bias in the data due to this omission. A similar point concerns the level of divorce and remarriage. Omission of births in previous marriages, when birth order applied only to current marriages, did little to bias birth order distributions. However, when births occur outside of marriage and divorce with remarriage is more common, these factors contribute greater limitations to the use of statistics on live births by birth order.
As in other age distributions, lack of frequencies in the unknown-age category is not necessarily an indication of accurate age reporting. In some cases, births for which age of mother was unknown were distributed among known age frequencies before tabulation; in other cases, marked decreases in the frequencies in the unknown age class indicate more emphasis on completeness and accuracy of age reporting. It will also be noted that large frequencies in unknown order of birth are usually correlated with small frequencies in first birth order. This may be due to failure to record a reply to the question when the response is “none”. This same problem appears in data on women by number of children ever born.
Reporting errors are also a possibility to be borne in mind in using these data. For example, very large numbers of births reported at high birth orders and young ages of mother are not credible; they are probably due to age-reporting errors.
Coverage: Live births by age of mother and live-birth order are shown for 96 countries or areas.
Earlier data: Live births by age of mother and live-birth order have been shown previously in issues of the Demographic Yearbook featuring natality. For information on specific years covered, readers should consult the Index.
 Principles and Recommendations for a Vital Statistics System Revision 2, Para.57, Sales No. E.01.XVII10, United Nations, 2001.