Cohort and Period Birth Replacement in the European Republics of the
Former Soviet Union, 1950-2011
José Antonio Ortega
Dept of Economics and Economic History, University of Salamanca, Spain.
Proceedings of the XIV HSE April International Academic Conference on Economic and
Social Development, Higher School of Economics, Moscow, April 2-4, 2013.
All the European Republics in the Former Soviet Union with the exception of Azerbaijan
experienced lowest low levels of fertility since the 1990s below 1.3 children per woman. In most of
the republics there has been an increase in fertility since then, but strong migration currents since
independence mean that in some countries the observed number of births is still declining fast
compared to former generations. These developments are measured using a new indicator of
population replacement, the Birth Replacement Sum. Cohort Birth Replacement Sums put in
relation the number of children born to a generation of women to the number of women born in that
generation. Period Birth Replacement Sums are period equivalents that measure the extent to which
current births are enough to replace the mothers’ generation. Recent results for the Former Soviet
Union illustrate that many countries remain at lowest low birth replacement due to the combination
of a fertility level that remains low and emigration of women since independence. At present, the
Birth Replacement Sums remain depressed due to migration by more than 20 percent in Moldova,
Georgia and Armenia, and by more than 10 percent in the rest of republics with the exception of
Belarus, Ukraine and Russia. In Russia the recent fertility increase and a positive contribution of
migration lead to levels closer to birth replacement at 1.7.
In the 1990s lowest low fertility levels below 1.3 children per woman were observed for the first
time at the National level (Kohler, Billari and Ortega, 2002). All the European republics in the
former Soviet Union reported at some time such extremely low fertility with the exception of
Azerbaijan. In the year 2000, fertility was reported in Georgia and Armenia to be as low as 1.1.1
Such extremely low fertility, among the lowest in the world at the time, came as a surprise.
However, the 2001 Population Censuses showed that fertility had been underestimated since many
more people than expected had emigrated and the actual population was much smaller (Duthé et al.,
2010). Total fertility figures have been revised upwards to 1.31 in Armenia and 1.46 in Georgia.
But, does that revision make population dynamics less worrisome? The observed number of births
continued to be the same, a level less than half the size of some of the birth cohorts bearing children
at the time. This is what is relevant from a perspective of population replacement, independently of
whether such a low figure is due to low fertility or to emigration. In this article a new measure of
population replacement based on this idea is computed for the European republics that were part of
the Soviet Union. The Birth Replacement Sum is a very simple replacement measure based on
comparing births to the generation size of their mothers. It only depends on vital registration data so
that possible miscalculation of population size has no effect. The results show that births are not
being replaced in any of the Republics, reaching extremely low values in some of them. The
contribution of low fertility, migration trends and mortality to that result are heterogeneous among
A main motivation for the development of demographic methods has been the isolation of
demographic phenomena. Standard manuals of demographic analysis are full of chapters
documenting pure measures of mortality, fertility or migration. Those pure measures are then
typically put together into prospective synthetic cohort measures for the study of reproduction, the
joint result of the elementary demographic processes. This is exemplified by the Net Reproduction
Rate (NRR). Such synthetic measures are derived under some hypothesis such as constant rates of
fertility and mortality and zero migration.
In contrast to such practice, the methods proposed here for the study of population reproduction and
replacement2 show there is no need to separate the different demographic phenomena in order to
measure replacement, precisely because replacement is the net result of the different components of
growth. We only need to observe real populations, where all the components naturally mix. Only at
a later stage we can consider the separation of the contribution of each component to the observed
1 Computed from ASFRs in Statistical Yearbook of South Caucasus 2002
2 We follow Calot and Sardon’s (2001) distinction between reproduction and replacement by which replacement
measures include migration, whereas reproduction measures are only affected by fertility and mortality.
patterns of replacement. The newly proposed measures refer to replacement in real populations both
with a period and a cohort perspective. Cohort replacement takes a particularly simple form: a birth
cohort of women has replaced itself to the extent that the number of female children they bore
equals or exceeds their birth cohort size. Whereas a strict cohort measure would follow those
women over time wherever they lived, most often we are interested in replacement within a
geographical unit such as a country. In such a case births to foreign-born women from the same
birth cohort are counted as contribution to the cohort replacement, whereas births to native born
emigrants abroad are not. If we call B(c,x) the number of births born to women of birth cohort c of x
years of age, the cohort birth replacement sum (CBRS) is given by:
where B(c) is the total number of female births in the year of birth of the cohort. The BRD(c,x),
birth replacement densities, are the age-specific components of birth replacement playing an
analogous role to age-specific fertility rates (ASFRs) in the computation of the total fertility rate
(TFR). All these quantities are based solely on information on births, not requiring information
about population size. The extension to period birth replacement is straightforward: it is based on
the aggregation of birth replacement densities across period lines, instead of cohort lines:
where B(t-x) refers to the number of female births in period t-x, the mother’s birth cohort size for
each of the children born in period t. The period birth replacement sum is closely related to the birth
replacement ratio (Ortega, 2006; del Rey and Llorente, 2010; del Rey and Ortega, 2011), defined as
the ratio of the period number of births to a weighted average of the mothers’ cohort size. The
differences lie in the extra age-specific dimension provided by PBRS. Both indicators provide
retrospective measures of cohort and period birth replacement. The difference between the period or
cohort TFR and the Birth Replacement Sum is due to migration and mortality. Mortality reduces
replacement, irrespective of fertility, because women do not bear children after death. Migration can
go either way: women that migrate bear their children elsewhere, but immigrants bear their children
in the country. The following relationship between the ASFRs and the BRD is used in order to
transform one to the other:
Where E(t,x) is a measure of exposure to risk for women aged x at time t. In our case, the mid-year
population is preferably used to estimate exposure. When, depending on the Lexis region
considered, exposure corresponds to several cohorts, a weighted average of births’ cohort size is
obtained, with weights proportional to exposure time.
The computation of Birth Replacement Densities and Sums requires only data on births classified by
period and age/cohort of the mother. Such data has been estimated for all the European republics of
the Former Soviet Union since 1960, until 2010-2011. Figure 1 shows the number of births since
1960 for all the republics based on data from Demoscope(2013) supplemented by data for 2011
from National Statistical Offices (NSOs). Births have been standardized to 100 in 1985. In this way,
the level in 2011 represents the extent to which births have decreased since the late soviet period,
and it is also connected to replacement since the 26 years since 1985 roughly correspond to mother’s
age at birth. Azerbaijan stands out with levels close to 100. Russia has increased recently to levels
close to 80. Most of the rest of republics cluster at levels 40 to 60 percent of 1985 levels.
Cohort birth replacement sums require births classified by period and cohort throughout a cohort
reproductive experience. Reconstructed births for Lexis triangles from Human Fertility Database
since 1959 are available for Estonia, Lithuania, Russia, Belarus and Ukraine (Max Planck Institute
for Demographic Research and Vienna Institute of Demography, 2013). Cohort birth replacement
sums can then be compared to cohort total fertility computed from the same source.
Period birth replacement sums are estimated from period births classified by cohort (ideally) or age.
Eurostat(2013a) provides cohort-period births by single years of age until 2011 for Estonia, Latvia,
Lithuania, Belarus, Russia and Ukraine in the demo_fagec database, and Eurostat(2013b) age-period
birth until 2010 for Moldova, Armenia, Azerbaijan and Georgia in the demo_fager database. Female
birth counts since 1960 are used as denominators. The resulting BRS can be compared to TFR levels
from Demoscope(2013) supplemented/replaced by data from NSOs for Azerbaijan, Armenia,
Belarus and Russia.
COHORT BIRTH REPLACEMENT
Cohort Birth Replacement Sums are particularly simple to interpret. Take the 1960 birth cohort of
women from the Russian Federation as an example. There were over their reproductive lifetime 2.4
million children born in Russia to women born in 1960. This 2.4 million divided by the 1.36 million
women born in 1960 gives a Cohort Birth Replacement Sum of 1.76. In contrast, the cohort TFR for
that generation is 1.85. The small difference is basically the effect of mortality. In any case, the
generation born in 1960, a generation that had already experienced most of its childbearing during
the Soviet period, did not reproduce itself.
Table 1 provides cohort BRS and TFR for two cohorts of women that have already (basically)
completed their lifetime childbearing, the 1950 and 1965 cohorts, for the countries covered by the
Human Fertility Database. The only generations that were reproducing themselves (cohort BRS
bigger than 2.05, for a sex ratio at birth of 1.05), were those born in Estonia, despite the fact that
their cohort TFR was lower than those in Russia in Lithuania for 1950. The reason is the high
immigration received in Estonia of women born in other Soviet republics. This was so despite
relatively high mortality levels, particularly for the 1950 cohort. This meant BRS to TFR ratios in
the 80 percent range for most countries for the 1950 cohort, in contrast with levels close to 100
percent for the 1965 experiencing lower mortality. As a result, while fertility levels had declined for
the generations born in 1965 experiencing a large part of their reproductive life in the post-
independence period, their birth replacement was higher in all cases except Estonia due to the
reductions in mortality, particularly infant mortality. The comparison between Estonia and Belarus
for the 1950 cohort serves to highlight the large impact of differential mortality and migration on
birth replacement: cohort TFR is exactly the same at 1.96, but BRS are widely different at 2.10 and
1.45 respectively, a 45 percent difference.
MOST RECENT PERIOD BIRTH REPLACEMENT
While the cohort BRS illustrate how important was mortality in the past, and the potential role of
migration in birth replacement, the large population movements in the post-soviet space after
independence largely affected generations that have not completed yet their childbearing. Large
proportions of National populations left the country in the Baltics, Moldova and the Caucasus. The
main recipient within the former Soviet Union has been Russia, with a Net Migration peak of
845000 people in 1994 (Sievert et al., 2011). These population movements are as large a factor in
accounting for the evolution in the number of births depicted in figure 1 as fertility levels. Period
BRS and TFR are shown for the most recent date for all the European former Soviet republics in
table 2, together with the relative number of births for the same year. There are only two countries
that remain at fertility levels of 1.3: Moldova and Latvia. But both countries have lost a good part of
their female population of childbearing age due to emigration. As a result, BRS is 20 percent lower
than TFR at extremely low levels close to 1 children per woman. They are associated with current
births less than half the number in 1985. Georgia, another important sending country, shows the
importance of migration on birth replacement: while TFR at 1.8 is moderately high, countries like
Belarus with a TFR of 1.5 have BRS levels as high (or rather, as low) as Georgia, at 1.45. The
rankings in births correspond to those in BRS and not to those of TFR. The three countries with a
lowest BRS/TFR ratio are Moldova, Georgia and Armenia in that order, with more than 20 percent
less births than expected according to TFR. The Baltics and Azerbaijan have lost a more moderate
amount of births, in the 10-19 percent range. Belarus and Ukraine are almost on balance, while
Russia, the main recipient country, benefitting from natality levels 7 percent higher than indicated
by the TFR. The comparison between Armenia and Russia is telling: in both countries TFR is very
similar, slightly below 1.6. In Armenia, a large sender, BRS is as low as 1.2 in contrast with 1.7 in
Russia: a 37 percent difference, similar to the difference in relative births. The only country with
fertility levels above replacement, Azerbaijan with a TFR of 2.3, is not able to reproduce itself when
we take migration into account with a BRS slightly below 2.
The main advantage of BRS over Birth Replacement Ratios (BRR), is the possibility of an age-
specific analysis of birth replacement densities. Figure 2 displays the most recent birth replacement
densities for the years shown in table 2. Besides the differences in level corresponding to the
differences in BRS, the most salient characteristic lies in the contrast between the Baltic republics
and the rest regarding the timing of fertility. In the three Baltic republics the mean age at reduced
birth is relatively late, at around 29, despite the low fertility levels. This is in contrast with mean
ages around 26 years of age in the Caucasus. Russia and Belarus occupy an intermediate position of
27 years, closer to the Caucasus. The late fertility of the Baltic states is connected with a relatively
intense postponement transition by which union formation and first births occur at a later age
(Kohler et al., 2002). This process is still at its initial stages in the Caucasus. Postponement has
strong implications for both fertility and birth replacement. While postponement is taking place, the
relative dearth of women currently having children depresses natality in what has been named
tempo-effect. Bongaarts and Sobotka (2012) have shown how the recent fertility rise in Europe is
mainly connected with the decline in the pace of fertility postponement, in the same way that lowest
low levels were connected with the acceleration of postponement. The implications for the future of
birth replacement are therefore very different in the Baltics and in the rest of republics. In the Baltics
postponement has basically already taken place, so that tempo effects are not expected to be as
important as in the Caucasus, where fertility is still very young and there is a potential for
postponement that could be associated with further reductions of fertility and birth replacement. In
the case of Russia, there is still controversy regarding the extent to which the current increase in
fertility is connected to a slower pace of postponement or to the impact of recent pronatalist policies
(Frejka and Zakharov, 2012). The still relatively young fertility in Russia, Belarus and Ukraine
indicates the potential for further postponement in the future with a depressing effect on fertility
In order to understand birth patterns in the post-soviet Republics, migration trends have been and
remain as important a factor as fertility. The large migration flows after independence is the reason
behind moderate increases in the number of births in many republics despite recent fertility
increases. Cohort and Period Birth Replacement Sums highlight the contribution of changes in
mortality and migration to the number of births, with birth replacement levels 10-25 percent lower
than TFR levels due to the missing women of childbearing age that have emigrated. Russia, the
main recipient in the region, shows the opposite phenomenon with the presence of immigrant
women adding births on top of a relatively higher fertility level.
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Figure 1: Number of births (1985=100) in the European Republics of the Former Soviet Union
Figure 2: Most recent Birth replacement densities in the European Former Soviet Union
Table 1: Cohort Birth Replacement Sum and Total Fertility for selected Former Soviet Union
republics. Female cohorts born in 1950 and 1965.
Source: Own elaboration based on data from Human Fertility Database, Max Planck Institute for
Demographic Research and Vienna Institute of Demography (2013).
Table 2: Most recent period Birth Replacement Sum, Total Fertility and relative births
(1985=100) for the European Former Soviet Union republics.
Source: Own elaboration based on data from Eurostat (2013a, 2013b), Demoscope(2013) and
National Statistical Offices (TFR).