In an earlier article (“When couples have fewer than two”, the Sunday Times, June 15) I discussed the impact of fertility trends in Singapore on the future size of our population.

Singapore women, as a whole, have been having too few babies to keep our population from declining in the future. But there are differences in birth patterns among women from the different ethnic and educational groups.

For instance, while Malay and Indian women have been having children at a rate of about two per family (the population replacement rate), Chinese women, on average, have been having fewer and fewer.

I will consider the implications of this trend later in the article. First, in order to understand better the population problem that faces Singapore, we need to look at some fertility figures.

Those who watch population trends find a number called the GRR (gross reproduction rate) to be a particularly useful measure of fertility.

It is the average number of daughters a woman will produce during her childbearing years — provided that she lives through the entire period.

Of course, not all women live until the end of their child-bearing period. So the actual average number of daughters born to women in the population can be calculated from the GRR by taking into account the death rate of women in this age group.

The resulting figure is the net reproduction rate (NRR).

If the NRR is exactly 1 (commonly referred to as the two-child family level, since each woman has one son and one daughter on the average), we can expect the population to replace itself in the future.

If it is less than 1, too few daughters are being born for the population to replace itself.

If it is more than 1, the size of the population will grow, since there will be more daughters than mothers.

When the NRR is exactly 1 in Singapore, the GRR will be about 1.025. This is the replacement fertility level.

Women in Singapore have been having fewer and fewer children since 1958. This decline continued into the early 1970s. In 1970, the average woman had 1.505 daughters over her lifetime.

In 1975, when the fertility rate first dropped below the replacement level of 1.025, the average woman had 1.006 daughters.