Table
12
Table 12 presents
live births by gestational age for as many years as possible between 1990 and
1998.
Description
of variables: Gestational age of a live-born infant is defined as the interval,
in completed weeks, between the first day of the last menstrual period of the
mother and the day, month and year of delivery[1].
Three new definitions associated with gestational age were adopted at the
Twenty-Ninth World Health Assembly following this recommendations of the Ninth
Revision Conference, 1975, as follows: pre-time is less than 37 completed weeks
(less than 259 days); term is 37 through 41 completed weeks (259 to 293 days);
post-term is 42 or more completed weeks (294 days or more)[2].
The
gestational age classification used in this table is as follows: under 27 weeks,
28-31 weeks, 32-35 weeks, 36-39 weeks, 40 weeks and over and an unknown
category. Where age has been
reported in months - either calendar or lunar - it has been converted to
gestational weeks according to the following scale:
0-6
months= under 28 weeks
7
months = 28-31
weeks
8
months = 32-35
weeks
9
months = 36-39
weeks
10
months = 40 weeks and over
Reliability
of data: Data from civil registers of live birth 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 gestational age are subject to the same
qualifications which 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
the data, an indication of which is described in the above paragraph, 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 or areas not to include in live-birth statistics infants who were born
alive but died before registration or within the first 24 hours of life, thus
under estimating the total number of live births. Statistics of this type are
footnoted.
The use
of the onset of the last menstrual period as the basis for computing gestational
age provides, in effect, only an estimate of the length of pregnancy, which is
actually the time from conception to birth. Since the exact time of conception
cannot be determined, it is recommended that gestational age, computed from the
onset of the last menstrual period, be used to approximate the true period of
gestation, although it probably overestimates the true period of gestation by
about two weeks.
The
actual estimation and recording of gestational age itself are subject to
considerable uncertainty. In many
cases, gestational age is estimated from the woman’s recollection of the date of
onset of the last menstrual period or medical records of this date, while in
other cases gestational age is inferred from the birth weight or other
characteristics of the infant or from other information about the probable time
of conception.
Moreover,
gestational age can only be determined by inference for those conceptions that
occur during post-partum amenorrhoea, and there is considerable variation
between countries in the proportion of conceptions occurring in this period.
However, even where it is possible to use the date of onset of last menstrual
period to estimate gestational age, memory errors and errors arising in the
conversion of this date to the gestational age of the infant may introduce
biases into tabulations by gestational age.
It is
important to note that gestational age should not be estimated on the basis of
birth weight because it is often desirable to study the correlation of these two
variables, which is not possible if gestational age is estimated on the basis of
birth weight. Data on both
gestational age and on birth weight, the latter presented in table 11 are useful
in the study of infant mortality because infant of low birth weight and those of
curtailed gestation are subject to higher risks of mortality and
morbidity.
The
relatively limited coverage of this table is due to the fact that many countries
or areas do not collect and tabulate data on gestational age.
Coverage: Live
births by gestational age are shown for 42 countries or areas.
Earlier data: Live
births by gestational age have been shown in previous issues of the Demographic
Yearbook featuring natality. For
information on the years covered, readers should consult the Index.
[1]
Principles
and Recommendations for a Vital Statistics System Revision 2, United
Nations Publication, Sales No. E.01.XVII.10, Para.142., New York,
2001.
[2] Manual of the International
Statistical Classification of Disease, Injuries, and Causes of Death Based on
the Ninth Revision Conference, 1975, and Adopted by the
Twenty-ninth World Health Assembly, 1975 Revision, vol. 1, p.764.,
World Health Organization, Geneva,
1977.