What Explains the Discrepancy between Changes of Fertility Rates and Local Birth Figures?

Occasionally, we receive questions from the public asking why the numbers published by the Census and Statistics Department sometimes seem to differ from what they intuitively perceive; similar statistical figures also appear to have discrepancies. Take fertility figures as an example, Hong Kong's total fertility rate (TFR) has risen for two consecutive years since the society returned to normalcy, climbing from a low of 0.701 in 2022 to 0.841 in 2024 (TFRs cited in this article are calculated on a “per woman” basis for easy understanding). Meanwhile, though the number of births has increased, the growth rate is noticeably lower than that of the TFR. So, could the fertility rate be slightly overestimated?

Calculating TFR: The mother's resident status is key

When discussing the fertility situation of Hong Kong residents, many people refer to the number of local births. Another useful indicator in demographic analysis is the “fertility rate”. However, unlike the former, the fertility rate focuses on “babies born to Hong Kong women” rather than “local births”. This subtle conceptual difference can lead to discrepancies between the two indicators.

Hong Kong is a highly mobile society, where people frequently move in and out conveniently. Every year, a significant number of Hong Kong women choose to give birth in the Mainland or overseas. Some of them leave Hong Kong shortly before their due date and return with their babies to reside in Hong Kong soon after giving birth. Depending on how long they stay in Hong Kong, these women are counted as part of the territory’s resident population.

Based on the aforementioned concept, we calculate Hong Kong's TFR by taking the number of local births as a base and adding in those babies born abroad to Hong Kong women (i.e. babies born outside Hong Kong and brought back to Hong Kong before their first birthday). Conversely, we exclude babies born in Hong Kong whose parents are not Hong Kong residents for various reasons. As the number of included babies often exceeds the number excluded in recent years, the total number of babies counted in the fertility rate calculation is often higher than the number of local births. This is the main reason why the growth rate of TFR has outpaced that of local births in recent years.

Even without considering the differences in coverage between “local births” and “births to Hong Kong women”, the changes in fertility rates and birth numbers would not align with each other. Why is that? Let’s look into how TFR is calculated and what it aims to measure.

Women are main focus when it comes to fertility

TFR is an indicator that reflects the fertility level of a place. It measures the number of children a woman is expected to have over her lifetime, based on the fertility rates of different age groups (usually 15 to 49 years old, grouped in five-year intervals) in a given year.

Calculating the TFR for a specific year is straightforward. First, calculate the fertility rate for each age group of women in that year (the “fertility rate” equals the “number of babies” divided by the “number of women”). Then, sum the fertility rates of different age groups to obtain the TFR for that year.

Fertility rates can be understood as the “probability” of women giving birth, reflecting their willingness to have children. However, the actual number of babies born also depends on the number of women of childbearing age in the population, and their age distribution, as the probability of giving birth typically decreases with age in the later stages of childbearing period.

Due to the intensifying trend of ageing in Hong Kong’s population, the number of women of childbearing age has significantly declined since 2019, with an increasing proportion of relatively older women. Therefore, even though the total fertility rate indicates that Hong Kong residents’ willingness to have children has significantly rebounded since 2022, the actual increase in the number of births will still be limited.

This might be the reason why, in occasion, people may feel that fertility rates and local birth figures show inconsistent changes.

SUN Siu-kei, Johnson
Statistician
25 June 2025