376 EPIDEMIOLOGY
much of the last century (but attaining much higher fi gures
in some years) down to below 10 in many countries today.
It has been very dependent on general social conditions: low
wages, poor housing, and bad nutrition, all having shown
close correlation with high IMRs. When infections were
rife, and brought into the home by older children, the rate
was higher. But with the improvement of infection preven-
tion and treatment, much related to sanitation, vaccination,
and antibiotics, infant mortality has occurred close to the
time of birth. For this reason, the national neonatal mortality
rate (NMR) has been used, a neonate being defi ned as up
to the age of 28 days. The same denominator is used as for
the IMR, and the difference between them is known as the
postneonatal mortality rate. Defi ned in this way, as it is, it
contravenes the proper defi nition of a rate, which should
refer to the ratio of the number to whom some event has
happened (e.g., death) to all those who were at risk for that
event. The denominator of the postneonatal mortality rate
is the number of live births, just as it is for the IMR and
the NMR. But all those who succumbed as neonates are no
longer at risk in the postneonatal period, and thus should be
excluded from the denominator. The difference, however,
is usually small, and it is more convenient to use two rates,
which add to the overall IMR.
Further reductions in the deaths at this period of life
have focused attention nearer to the time of birth. Deaths
in the fi rst week of life (up to the age of 7 days) have been
recorded for many years now, as well as separately for each
of those 7 days, and even for the fi rst half hour of life. Clearly
many of the causes of those very early deaths will have orig-
inated in the antenatal and intrauterine period. They will
share causes with those born dead (stillbirths), and indeed
they are combined together in the prenatal mortality rate.
This includes both stillbirths taken together. The stillbirth
rate (SBR) alone must of course use the same denomina-
tor, since all births were at risk of death in the process of
birth, to which the stillbirths fall victim. All of these rates
have been devised to highlight specifi c areas of importance,
especially in pediatrics. Closely related is the measurement
of the material morbidity rate (MMR). Here the numerator
is the deaths of women from maternal or puerperal causes,
and the denominator, interestingly, is the total number of
births, live and still. A moment’s refl ection will show that
it is the occasion of birth (whether live or still) that puts
a woman at risk of this cause of death, and that if she has
twins—or higher orders of multiple births—she is at risk
at the birth of each, so that the correct denominator must
include all births.FERTILITY RATESThe information collected on the birth certifi cate usually
permits the tabulation of fertility rates by age and number of
previous children. Age-specifi c fertility rates are defi ned as
the number of live births (in a calendar year) to a thousand
women of a given age. If they are expressed for single years
of age, and they are separated into male and female births,then we add together all the rates for female births to give
what is known as the gross reproduction rates (GRRs). If this
quantity is close to unity, then it implies that the number of
girl children is the same as the number of women of repro-
ductive age, and the population should thus remain stable in
number. But no allowance has been made for the number of
women who die before the end of their reproductive life, and
thus will fail to contribute fully to the next generation. When
this allowance is made (using the female mortality rates
for the appropriate ages) we obtain the net reproductive rates
(NNRs). Note, however, that there remains an assumption
that may not be fulfi lled—that the age-specifi c rates remain
unchanged throughout the reproductive age range (usually
taken as 15 to 45), that is, for a period of 30 calendar years.
Indices such as the NRR were devised as attempts to pre-
dict or forecast the likely future trends of populations. The
crude birth rates (CBRs), defi ned as the ratio of the number
of births to the total of the population, is like the crude death
rate in being very sensitive to the age structure of the pop-
ulation. Nonetheless, their difference is called the rates of
natural increase (RNI) and provides the simplest measure of
population change:CBR CDR RNIThe measure excludes the net effect of migration in changing
the population numbers: in some countries it is very rigidly
controlled, and in others it may be estimated by a sampling
process at airports, seaports, and frontier towns.POPULATION TRENDSPreviously it has been noted that both the GRR and NRR
make the assumption of projecting the rates observed in
1 calendar year to cover a 30-year period (15 to 45). It would
of course be possible to follow a group of women, all of the
same age, from when they were 15 up to the age of 45 in
the latest year for which fi gures are available. Such a group
would be called a “cohort”—the term used in epidemiology
for a group defi ned in a special way. To cover this cohort
would necessitate obtaining fertility rates for up to 30 years
back, and in any case that cohort would of course have com-
pleted its reproductive life. The highest fertility rates are
commonly found at younger ages: it is possible to show
graphically a set of “cohort fertility rates” by age labeled
by their year of birth (often a central year of birth, since
the cohort may be more usefully defi ned as a quinquennial
group). If they are expressed in cumulative form (i.e., added
together) and refer only to female birth, it will become clear
how nearly they approach unity, from below or above, if the
population is increasing. No adjustment for female mortality
in the period is required, since the rates are, for each year (or
quinquennium), calculated for those women of that cohort
alive at that time. The method therefore represents the most
useful prediction of future population trends, which can be
projected further forward by assumptions that can be made
explicit in their graphical depiction.C005_011_r03.indd 376C005_011_r03.indd 376 11/18/2005 10:25:41 AM11/18/2005 10:25:41 AM