Introduction

Globally, the sex ratio at birth and the total population remain relatively stable, with 105.6 boys born for every 100 girls and 101.016 males born for every 100 females, respectively1. This consistency occurs in the absence of deliberate manipulation2. Similarly, in Ethiopia, the average sex ratio at birth is 105.5 boys to 100 girls, while the population sex ratio stands at 101 males to 100 females3.

Fertility refers to the ability of individuals or couples to reproduce through regular sexual activity4,5. Fertility is commonly measured using indicators such as the Crude Birth Rate (CBR), Total Fertility Rate (TFR), and General Fertility Rate (GFR)6,7,8,9. Demographers use the TFR to estimate the average number of children a woman would bear over her lifetime if she experienced the current age-specific fertility rates throughout her reproductive years. Unlike generational measures, the TFR reflects fertility conditions in a single year. It is essential to distinguish between birth rate, defined as the number of live births per 1000 total population, and total fertility rates, which focus specifically on reproductive behavior. The GFR measures the number of live births per 1000 women of reproductive age in a given year5,9.

To maintain a stable population in a given area without factoring in immigration or emigration, a TFR of 2.1 children per woman is generally considered the replacement level5. While many regions worldwide still report fertility rates well above this replacement level, the global TFR has declined markedly since 1950. Between 1950 and 1955, the global average fertility rate was 4.97 children per woman, declining to 3.86 by 1975–1980, 2.5 by 2005–2010, and 2.32 by 20219. A pooled analysis of 33 sub-Saharan African countries estimated the regional TFR at five children per woman (95% CI 4.63–5.37) with higher rates in low-income countries (5.45) compared to lower-middle-income (4.70) and upper-middle-income countries (3.80). Between 2010 and 2018, regional variations within Africa were evident, with the following TFRs: East Africa at 4.74, Central Africa at 5.59, Southern Africa at 3.18, and West Africa at 5.3810.

Fertility plays a crucial role in shaping population size and age structure. In Ethiopia, as in many sub-Saharan African countries, high fertility has historically driven rapid population growth. Before the 1990 National Family and Fertility Survey (NFFS), detailed fertility data were scarce11. This gap has since been filled by a series of Ethiopian Health and Demographic Surveys (EDHS). These surveys reveal a steady decline in Ethiopia’s TFR, from 6.4 children per woman in 1990 to 4.6 in 2016. Specifically, the TFR trends reported by the NFFS and EDHS were as follows: 6.4 (1990), 5.9 (2000), 5.4 (2005), 4.8 (2011), 4.1 (2014), and 4.6 (2016). Between 2000 and 2016, the national TFR dropped by 1.3 children per woman. The decline was even more pronounced in the Amhara region, where the TFR fell from 5.9 to 3.7 in 2016, a reduction of 2.2 children per woman11,12,13,14,15,−16.

Both the GFR and CBR have shown downward trends over time. The GFR declined from 193 births per 1000 women of reproductive age in 2000 to 156 in 2016, reflecting a decline in fertility. Similarly, the CBR dropped from 41.3 per 1000 population in 2000 to 31.8 in 2016 and remained at that level through 2022. These trends represent significant shifts in reproductive behavior, potentially driven by increased access to family planning services, improved female education and employment opportunities, and broader socioeconomic changes12,13, −14,16,17.

The TFR typically determines long-term population growth. When each woman bears, on average, 2.1 children who survive to reproductive age, she and her partner are effectively replaced. If the TFR falls below this threshold, the population size will likely decline eventually, although the impact may be delayed due to factors such as age structure and migration5,18.

Ethiopia is a Major source of young female migrants to domestic labor markets in the Middle East19,20, with the Amhara region being one of the primary areas of origin21. This gendered selective migration contributes to unusually high adult-sex ratios (ASR) in affected areas10. Such imbalances can influence fertility by delaying marriage or causing prolonged spousal separation. Furthermore, female emigration may have broader demographic consequences, as communities with higher female proportions tend to exhibit higher birth rates in low-income settings22.

Vital event registration remains nearly nonexistent in Ethiopia, and fertility estimates were omitted from the 2019 EDHS23. As a result, key demographic indicators such as sex ratios and fertility trends have not been thoroughly studied in Ethiopia. To address this gap, the study presents novel data on key fertility indicators and population sex ratios for a rural community in Ethiopia. The data collection efforts are linked to the Birhan HDSS in the Amhara region, Ethiopia, in which the authors of this article were involved.

Objective

To evaluate the ASR and key fertility indicators within the Birhan HDSS in the Amhara region, Ethiopia, from December 1, 2021, to November 30, 2022.

Method

The Birhan Field site was established in June 2018 through a collaboration between Saint Paul’s Hospital Millennium Medical College (SPHMMC) and Harvard School of Public Health (HSPH) in the North Shewa Zone of the Amhara Region, Ethiopia. The study area includes Birhan HDSS, which contains fourteen rural and two urban kebeles (the smallest administrative units used in Ethiopia) (Fig. 1). The primary objectives of this HDSS are to ensure continuous and ongoing monitoring of the health and demographic conditions of the population in the area and to serve as a valuable resource as it provides a well-defined population sampling frame to conduct nested studies.

The primary activity of the Birhan HDSS consists of systematically conducting house-to-house visits within the surveillance area at regular three-month intervals. These visits are designed to actively monitor and update detailed information on both individual members and household-level dynamics, including births, deaths, migrations, marital status changes, and other demographic events.

If new households have been established within the village, either through immigration or household formation, these households are identified, and comprehensive baseline data are collected. Once baseline information is gathered, these households are formally enrolled in the HDSS and become part of the ongoing surveillance system. In addition to capturing routine demographic events, the HDSS data collectors play a critical role in the early identification of pregnancies within the community24.

The study team developed a Tangerine-based electronic data system for longitudinal data collection to support this process. This system operates in two ways: when syncing, the Tangerine Collect app allows data collectors to both upload (push) and download (pull) data from the Tangerine Aggregate central database. Data collected on tablets is regularly uploaded to the central database, and updates from the database are downloaded to the tablets whenever data collectors synchronize them, typically daily.

Fig. 1
figure 1

Map of the Birhan Health and Demographic Surveillance System. The map was generated using QGIS version 3.28 (QGIS Association; https://qgis.org.

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Within the Birhan HDSS population, there was a nested pregnancy and birth cohort study to estimate the causes of children’s and maternal mortality and morbidity. In this study, Women of reproductive age are regularly monitored for pregnancy screening by the cohort and HDSS data collectors, community key informants, and healthcare providers at household and health facilities within the catchment area. The woman is referred to the Birhan open cohort study team if a pregnancy is detected. Pregnant women who consented were enrolled and followed through antenatal, birth, and two years postpartum, with scheduled visits to assess the health and well-being of both women and newborns throughout pregnancy, delivery, development, and the postnatal period.

The central database served as a bridge for the HDSS and nested studies; updates made on each study participant were displayed for Birhan open cohort data collectors daily25.

We extracted one year of data (December 1, 2021, to November 30, 2022) from the HDSS and the Cohort. From the HDSS, we collected information on the average population of the study area during the study year and the number of women of reproductive age. Furthermore, we obtained the number of live births from the HDSS and the morbidity and mortality cohort datasets.

With this data, we calculated key indicators related to fertility rates and ASR. The CBR represents the number of live births per 1000 people. The GFR measures the number of live births per 1000 women of reproductive age. The TFR estimates the average number of children a woman is expected to have in her lifetime, based on current birth rates. In addition to fertility indicators, we also calculated the ASR. The adult sex ratio represents the proportion of males to females in the adult population (15–49 years old).

Our analytical strategies for measuring human fertility and ASR in the study area are based on descriptive statistics and involve the use of the following methods:

  • Crude Birth Rate (CBR): The CBR is calculated by dividing the total number of live births recorded over a year by the mid-year population and multiplying by 1000. CBR = (Total number of children born in a year) / (Mid-year Population) x 10008.

  • The GFR is calculated by dividing the total number of births per year by the female population in the reproductive age group and multiplying by 1000. GFR = (Total number of children born in a year) / (Female Population, ages 15–49) x 10009.

  • The TFR measures the average number of children each woman would have over her reproductive age if the age-specific birth rates remain constant. It is calculated by summing the age-specific birth rates (ASFR) for each age group (e.g., 15–19, 20–24, 25–29, … 45–49) and multiplying by 5. TFR = 5 x ∑(ASFR)6.

  • The ASR is calculated by dividing the number of adult males (15–49 years old) by the number of adult females (15–49 years old) and multiplying by 100.

We obtained protocol approvals for the HDSS and open cohort studies from the Institutional Review Boards (IRB) of SPHMMC and HSPH. All methods were carried out in accordance with these approved protocols.

Result.

The results suggest that a higher male-to-female ratio characterizes the reproductive-age population of the region. The mid-year population was 72,776, with a higher number of males (38,454) than females (34,322). The population pyramid indicates a predominantly young population, characterized by a broad base that represents a large number of children and adolescents. Population numbers decline with increasing age, a pattern typical in developing regions.

Notably, the population pyramid highlights a skewed sex ratio favoring males in the 15–25-year age group (Fig. 2). Specifically, women comprised 37.4% of the 15–24-year age group, resulting in a sex ratio of 167.4 males for every 100 females. Across the broader reproductive age group (15–50 years), the sex ratio was 125 males per 100 females. Overall, the study area exhibited a male-dominant population, with 112 males for every 100 females (Table 1).

Table 1 Sex ratio in Birhan HDSS study population in Amhara Region, Ethiopia, 2022.
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Fig. 2
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Population Pyramid of Birhan HDSS Study Population in Amhara Region, Ethiopia, 2022 midyear population.

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The population of women of reproductive age (WRA) in the study area totaled 14,086, among whom 1476 live births occurred, comprising 748 boys and 728 girls. This results in a sex ratio at birth of 102.7 boys for every 100 girls, consistent with natural sex ratio patterns observed globally.

Fertility rates were highest among women aged 20 to 34 years, reflecting both biological factors and prevailing social norms regarding childbearing26,27,28. Specifically, women in the 25–29 age group had the highest number of live births (424), followed closely by those aged 30–34 years (399) and 20–24 years (322). Fertility declined substantially after age 35, with markedly lower rates among women aged 40–49.

Compared to national trends, the fertility rate in the Birhan HDSS population has declined. The TFR for 2022 was 3.44 children per woman, lower than Ethiopia’s national TFR of 4.6 in 2016 and significantly lower than the 6.4 recorded in 1990. The decline in fertility is also evident across all ASFRs, with adolescent fertility (ages 15–19) notably lower in Birhan HDSS (22 per 1000 women) compared to the national rates reported in previous EDHSs (Table 2 and Fig. 3).

Table 2 Trends in ASFRs and TFR in Ethiopia from 2000–2016 and in Birhan HDSS in Amhara Region, Ethiopia, 2022.
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Fig. 3
figure 3

Trends in ASFRs in Ethiopia from 2000–2016 and in Birhan HDSS in Amhara Region, Ethiopia, 2022.

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The 2022 fertility analysis from the Birhan HDSS indicates a notable decline in key fertility indicators compared to previous national averages reported by EDHSs. The TFR in Birhan HDSS was 3.44 children per woman, a 25.22% decrease from the national TFR of 4.6.

Similarly, the CBR in Birhan HDSS was 19.98 births per 1000 population, which is 37.24% lower than the national average of 31.84 per 1000 reported in 2016. The GFR, which measures the number of live births per 1000 WRA, was also lower in Birhan, at 104.78, compared to the national figure of 156 in 2016. This represents a decrease of 32.83%.

Together, these reductions in TFR, CBR, and GFR underscore a significant decline in fertility in the Birhan HDSS area, diverging from national patterns observed in previous decades Table 3 and Fig. 4).

Table 3 CBR, GFR, and TFR in Ethiopia from 2000–2016 and in the Birhan HDSS study population in Amhara Region, Ethiopia, 2022.
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Fig. 4
figure 4

Trend of CBR, GFR, and TFR in Ethiopia from 2000 to 2016 and in the Birhan HDSS Study Population in 2022.

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Discussions

This study, conducted in the Birhan HDSS site in the Amhara region of Ethiopia, examined adult sex ratios and fertility trends. Among individuals aged 15–24, only 37.4% were women, corresponding to a sex ratio of 167.4 males per 100 females. Among women of reproductive age (15–50 years), the sex ratio was 125 males per 100 females. In 2022, the TFR in Birhan was 3.44 children per woman, marking a 25.22% decline compared to Ethiopia’s national TFR of 4.6 in 2016. Other fertility indicators also declined: the CBR fell to 19.98 per 1000 population, representing a 37.24% decrease, and the GFR decreased to 104.78 per 1000 women of reproductive age, a 32.83% drop from the 2016 national level16.

Globally, fertility rates have shown a persistent decline over the past few decades. The average TFR dropped from 4.97 in 1950–55 to 3.86 in 1975–80 and further to 2.5 in 2005-10. By 2021, the global TFR had declined to 2.329. This reduction has led to a slowdown in population growth, from a global peak of around 2% in the 1970s to 1.1% in 201729. In East Africa, the pooled regional TFR remains higher at 4.74, and the region recorded its highest growth rates of 2.7% in 201710,29.

In the Amhara region, the TFR declined steadily from 5.9 in 2000 to 3.7 in 2016, with intermediate values of 5.1 in 2005 and 4.2 in 201112,13,14,16. Fertility in the Birhan HDSS followed a similar declining trend. In 2022, the Birhan TFR was 25.2% lower than the national average in 2016 and closely aligned with the regional average, differing by only about 7%. This suggests that fertility patterns in Birhan reflect broader regional and national demographic shifts.

Although the EDHSs do not report CBR and GFR at the regional level, the observed declines in Birhan, 37.24% for CBR and 32.83% for GFR since 2016, are substantial16. These reductions underscore the utility of TFR as a robust fertility indicator, as the absolute number or proportion of women in the population has less influence on it30,31.

One important contextual factor influencing those trends is the international migration of young Ethiopian women, particularly to Middle Eastern countries for domestic work. This migration is shaped by a complex interplay of economic, social, cultural, and environmental factors. Women, especially those from rural areas, often face high unemployment and limited local opportunities, making international domestic work a more viable alternative to internal migration. Structured recruitment pathways and high demand for female domestic workers in the Middle East reinforce this trend32.

Cultural expectations further drive this gendered migration, as women are often expected to provide financial support for their families during times of economic hardship33. Migration is also frequently viewed as a means of achieving greater autonomy and financial empowerment34. Furthermore, formal migration channels facilitated by recruitment agencies tend to be more accessible for women, thereby increasing female international migration35.

This outmigration of young women likely contributes to the skewed sex ratios and declining fertility observed in the Birhan HDSS. Migration reduces the number of women of reproductive age, particularly those most likely to give birth. This aligns with the broader demographic transition in Ethiopia, where rural women increasingly migrate abroad to finance their eventual urban relocation19,20,36. The impact is particularly pronounced among young women, who typically begin childbearing earlier, with a national median age at first birth of 18.7 years22,23,37.

Beyond demographic changes, migration also affects representative health and fertility behaviors. Studies show that 29.2% of female returnees from the Middle East experience common mental disorders38. Migrants also tend to have lower conception rates during and after migration, due to both temporary separation from partners and lifestyle changes associated with improved living conditions post-migration39. Even after returning or settling in urban areas, many maintain lower fertility levels than non-migrants. These fertility reductions likely stem from both material improvements and psychological or relational challenges encountered abroad39,40,41.

The quality is a major strength of this study. The Birhan HDSS conducted pregnancy screening surveillance four times a year, using both community health workers and a dedicated team to register births at both the household and facility levels. This prospective population-based approach yields more reliable data than standard surveys.

Conclusion

The study reveals a young population with markedly skewed sex ratios, especially among younger age groups, and fertility rates significantly below national averages. The findings suggest that gendered migration, especially the emigration of young women, contributes to both the high male-to-female ratios and declining fertility. These changes are likely amplified by reduced conception rates among migrants due to separation from partners and shifts in socioeconomic conditions. The study presents novel data on key fertility indicators and population sex ratios for a rural community in Ethiopia, providing robust evidence to inform demographic planning and policy.