Human birth sex ratios have been a subject of study since at least the 17th century, and during this time many hypotheses have been made trying to explain the higher number of male newborns [41]. Researchers from various fields, including evolutionary biology, cell biology, demography, gynecology, and pediatrics, have attempted to uncover the reasons behind the observed male predominance at birth [19]. However, despite extensive studies and analyses of various factors, the causes of male birth surpluses remain unclear.
The SRB in humans is an important demographic descriptor that serves as a measure of effective population size and prospective fertility [42]. The value of SRB is influenced by individual-level factors such as maternal or paternal age at conception, birth order, sex of the preceding child, maternal weight, family size, socio-economic conditions, and environmental conditions during pregnancy [43, 44, 45]. Initially, it is assumed that the sex ratio after conception should be 50:50 based on simple Mendelian segregation of X and Y chromosomes [41]. This assumption is confirmed by studies conducted by Orzack et al. [19], who karyotyped nearly 140,000 embryos (3–6 days old) created through assisted reproductive technologies and found that 50.2% were male. Among embryos with genetic defects, approximately 51% were male, indicating an increased risk of miscarriage for male embryos. These findings suggest a male-biased death rate in utero until week 2, followed by a female-biased death rate over the next 4 months, followed by a period of little bias before switching back to male-biased deaths from 36 weeks of pregnancy. Throughout pregnancy, including the last month, the utero sex ratio remains between 47 and 53% male. Therefore, the SRB is expected to fall within this range based on the findings of Orzack et al. [19].
However, the most commonly reported value of SRB in research results is between 105 and 107, indicating an excess of male births as a well-documented worldwide phenomenon. Moreover, depending on the factors being examined, the SRB values can significantly vary, emphasizing the male sex predominance. This discrepancy may be attributed to the fact that besides genetic abnormalities that increase the risk of miscarriage, the occurrence of miscarriage itself may be influenced by numerous other factors and their interactions [46, 47]. These factors may include physical and psychological health conditions. Sociodemographic factors such as age, occupation, and pre-existing health conditions like high or low body mass index, diabetes, high blood pressure, cancer, infertility, and other diseases or conditions can also contribute to pregnancy loss. Additionally, women may experience pregnancy-specific health conditions like gestational diabetes, preeclampsia, and eclampsia [48, 49, 50, 51]. So if spontaneous miscarriage is defined as the loss of a fetus before the completion of the 20th week of gestation, according to results presented by Orzack et al. [19], for most of this we should observe a higher probability of miscarriage of female fetuses. However, most studies on miscarriage incidence and risk factors do not record the sex of aborted fetuses, making it difficult to determine how these factors affect the sex ratio. Therefore, further research investigating the influence of these factors on the sex ratio at birth is warranted.
In the study sample, the sex ratio for twin pregnancies was 1:1, while for single pregnancies, the SRB ratio was 118.50. However, it is important to note that the percentage of twin pregnancies in the study sample was very low (0.42%). According to the literature, twin pregnancies typically account for about 1–2% of all pregnancies, and their prevalence has been increasing in recent years [52, 53]. This rise in twin pregnancies is believed to be a result of factors such as advanced maternal age at conception and the use of assisted reproductive techniques [54].
In our sample, the percentage of twins is notably lower, which can be attributed to the characteristics of the hospital where the newborns were delivered. As per Polish regulations, this hospital is classified as a second-degree referral center, which means that high-risk pregnancies, including multiple pregnancies, are typically transferred to a higher-level referral center, such as Rzeszów, located approximately 56 km away [55]. This is particularly relevant for preterm births, as twin pregnancies are at a higher risk of premature delivery, as well as other complications during pregnancy [56, 57, 58, 59].
Therefore, the observed sex ratio value of exactly 1:1 in the study sample may be distorted, considering the lower SRB values commonly observed in multiple pregnancies. Nonetheless, the obtained value is lower than in singleton pregnancies, which is consistent with previous research findings [27, 60].
In the current study, we observed that the likelihood of having a male child increased with the age of the mothers. The lowest sex ratio was found among mothers aged 25.0–29.9, with a value of 109.4. Furthermore, with subsequent births, the probability of having male offspring also increased. In terms of interpregnancy intervals, mothers who had birth after a gap of 4 years or more had a higher chance of having a son.
The season of birth significantly influenced the SRB. Newborns born in autumn (conceived in winter) had the highest SRB, while those born in spring (conceived at the end of summer) had the lowest SRB. Additionally, the heating season, associated with increased air pollution, also impacted the SRB. Newborns born during the October–April heating season had an SRB of 104.3, whereas those born outside the heating season had an SRB of 126.5.
Studies indicate a slightly higher number of male live births compared to females, and some scientists consider this to be a consistent pattern [4]. However, there is disagreement among scientists regarding the precise mechanisms operating during this period. It remains uncertain whether it is due to the higher fertilization capacity of sperm carrying the Y chromosome, disparities in the implantation of male and female embryos, or differences arising during embryogenesis. Furthermore, there is ongoing debate concerning the influence of maternal factors on environmental stress [61]. It is also believed that male fetuses are more susceptible to environmental and maternal factors, potentially leading to a higher incidence of prematurity, macrosomia, or congenital defects [62, 63]. Maternal factors encompass aspects such as the mother’s lifestyle and biological state, including age, previous pregnancies, deliveries, or cesarean sections, as well as the interval between births [64, 65].
In this study, we observed that mothers under the age of 19, the youngest group, had a lower proportion of male births compared to mothers in other age groups. This could be attributed to the biological immaturity of young women, as male fetuses may require greater metabolic investments from mothers to ensure their survival. The mother’s body may compete for resources with the developing fetus during this stage [66, 67].
Regarding optimal maternal ages, previous studies have reported that younger women tend to give birth to more sons than older women, although not all findings are conclusive [68, 69, 70, 71]. Rapaport et al. [72] observed that for maternal age ≤ 22, girls were born more often, while the likelihood of having a son increased for middle-aged mothers, with the highest probability occurring at around 31.3 years of age. However, for the oldest women, approaching the perimenopausal period, the probability of having a son decreased again. The authors note that the study population was not affected by urbanization-related factors such as access to contraception, processed food, or air/water pollution. It is worth noting that the women from Krosno County, the population under study, also belong to a traditional population strongly influenced by the doctrine of the Catholic Church. In this group, lower SRB values were observed among younger women, while higher values were noticed among older women above the age of 30. These relationships may also be associated with ongoing social transformations. Currently, it is observed that women are delaying motherhood decisions and placing great importance on completing higher levels of education and pursuing professional careers. Numerous studies also highlight that advanced maternal age trends are linked to poorer newborn outcomes and a higher risk of conditions such as Down syndrome and autism [73, 74]. Additionally, it is worth mentioning that research by Sánchez-Barricarte [3] indicates that the age of mothers, rather than fathers, may have a significant impact on SRB differences.
In this study, a decrease in the SRB value was observed for the mother’s second delivery, while an increase was observed for the third and subsequent pregnancies. This finding differs from the results reported by Jacobsen et al. [27], where changes in the coefficient value were found to be insignificant. This discrepancy suggests that this factor may not have a significant influence on the secondary sex ratio. Some researchers believe that newborns from earlier pregnancies are more likely to be male, while those from subsequent pregnancies are more likely to be female. However, not all data support this pattern. Furthermore, some researchers have not found a significant association between the number of previous births and the sex ratio [75].
The lack of a noticeable effect of the number of pregnancies on the SRB may be partly explained by the influence of birth intervals between pregnancies. In this study sample, birth interval data were available for only a portion of the surveyed women (5,330), but the results indicate a relationship between birth intervals and SRB. Table 9 provides a summary of the descriptive statistics regarding the time between successive births based on their order. Based on these findings, it can be concluded that the shortest interval occurs between the first and second pregnancy, while longer intervals exceeding 4 years are observed between subsequent pregnancies, with the maximum interval occurring between the third and fourth pregnancy. The impact of birth intervals on the SRB is not well understood. However, in the case of women with higher parity and shorter interdelivery intervals, a higher frequency of daughters being born has been reported. Some scientists attribute this trend to maternal depletion [76]. In certain countries, for social or cultural reasons, women who already have a son may choose to have longer interdelivery intervals compared to women who do not have a male child, which could indirectly affect the SRB [77, 78].
Table 9
Time between pregnancies in years.
Time between pregnancies [years] | Mean (+/−SD) | Range |
First–second (244) | 3.1 (4.95) | 0.6–20.8 |
Second–third (3,230) | 4.6 (3.12) | 0.7–22.1 |
Third–fourth (1,381) | 5.8 (3.87) | 0.7–21.1 |
Above the fourth (475) | 4.3 (3.19) | 0.6–18.9 |
Total (5,330) | 4.8 (3.50) | 0.6–22.1 |
The work also discusses the influence of environmental factors. In the current study, we found that the highest SRB value was observed for newborns conceived in winter, while the lowest value was recorded for children conceived at the end of summer. The season of conception is believed to play a role in determining the sex of the fetus, with more male fetuses conceived under more favorable environmental conditions. Some researchers have noted that mothers who are pregnant during summer and winter are more likely to give birth to female babies compared to those pregnant in the spring [79].
The seasonality of SRB is a topic that is still widely debated. It is suggested that the seasonality of human reproduction may be influenced by biological factors related to seasons, insolation, and food availability [80, 81]. Research conducted in Germany in the late 1990s indicated that the highest SRB values were observed in May and December, while the lowest values were observed in March and October. This suggests that pregnancies conceived in late summer and spring tend to result in more male births, while those conceived in early summer and winter tend to result in more female births. One possible explanation for this phenomenon is the better reproductive success of men born in the spring compared to those born in the autumn [82].
When considering the seasonality of births, it is also worth mentioning the availability of vitamin D and insolation. Some authors propose a positive relationship between SRB values and maternal vitamin D levels before conception. Optimal levels of vitamin D are believed to reduce inflammation, which could be detrimental to male embryos [83]. Reports from the USA also indicate that higher solar radiation is associated with a higher SRB compared to regions with lower solar radiation. Researchers compared SRB data from southern states with higher insolation to northern states, and significant differences were observed [3].
In Poland, the SRB may also be influenced by pollution levels during the heating season. In this study, we observed that newborns born between October and April (conceived during the winter season and early spring) had a lower SRB compared to those born outside the heating season (conceived in autumn and early winter). The impact of pollution on the SRB is not fully understood, and research results are varied. Some studies exclude the influence of pollution on the SRB [3], while others suggest that pollution, particularly water pollution, can decrease the SRB value [39]. Other reports indicate that maternal exposure to dioxins, pesticides, and lead may result in lower SRB values, and inhalation of particulate matter may also affect the SRB [84, 85, 86]. It is worth noting that in many developed countries, the SRB has decreased over the past five decades, and experts attribute this decline to maternal exposure to pollutants [87].
The SRB is a fascinating topic that is being studied worldwide. However, research results are inconsistent, although some studies suggest the influence of maternal factors and environmental stress on the SRB. Many scientists believe that under better environmental and maternal conditions, male births are more likely, while under worse conditions, female births are more prevalent. The sex ratio at birth varies by region and country, with more boys than girls reported globally [88].