• Users Online: 312
  • Home
  • Print this page
  • Email this page
Home About us Editorial board Ahead of print Current issue Search Archives Submit article Instructions Subscribe Contacts Login 


 
 Table of Contents  
ORIGINAL ARTICLE
Year : 2022  |  Volume : 12  |  Issue : 3  |  Page : 271-276

Trends and seasonal variations in human secondary sex ratio in Southwest Nigeria: A 10-year survey


Department of Cell Biology and Genetics, University of Lagos, Lagos, Nigeria

Date of Submission25-Nov-2021
Date of Acceptance04-Aug-2022
Date of Web Publication15-Sep-2022

Correspondence Address:
Mr. Samson Taiwo Fakorede
Department of Cell Biology and Genetics, University of Lagos, Lagos
Nigeria
Login to access the Email id

Source of Support: None, Conflict of Interest: None


DOI: 10.4103/aihb.aihb_165_21

Rights and Permissions
  Abstract 


Introduction: Secondary sex ratio is the number of live male births per 100 female births. In humans, the primary sex ratio, which is the sex ratio at conception, is expected to be 1:1 in natural populations based on the law of independent assortment of the X and Y chromosomes. This is not the case with the sex ratio at birth as it is largely affected by various social and cultural practices that it tends to bias towards one sex, with profound implications for population and demographic dynamics. Materials and Methods: We collected retrospective data of records of live births from three Southwestern Nigerian states, covering 10 years. The data were obtained from different Hospitals/Birth Centres from three states, i.e., Lagos, Ondo and Osun, between 2006 and 2017. Data analysis was performed to determine the monthly, quarterly, yearly and seasonal variation in sex ratios. Chi-square analysis was used to determine the significance of differences in sex ratios at P < 0.05. Results: Annual average sex ratios of 99.7, 105.6 and 106.0 were obtained for Lagos, Osun and Ondo, respectively, while the pooled data had a sex ratio of 104.2, indicating male preponderance. Sex ratios differ significantly according to season. Differences in sex ratio were significant during the dry season in Lagos and Osun states and in Ondo during the rainy season. Conclusions: Sex ratios from Southwest Nigeria are as diverse as other populations, with ratios ranging between 99.7 and 106.0.

Keywords: Demography, live births, seasonal variation, sex ratio, sex


How to cite this article:
Fakorede ST, Ojo SD, Shonde KM, Adekoya KO, Ogunkanmi LA, Oboh B. Trends and seasonal variations in human secondary sex ratio in Southwest Nigeria: A 10-year survey. Adv Hum Biol 2022;12:271-6

How to cite this URL:
Fakorede ST, Ojo SD, Shonde KM, Adekoya KO, Ogunkanmi LA, Oboh B. Trends and seasonal variations in human secondary sex ratio in Southwest Nigeria: A 10-year survey. Adv Hum Biol [serial online] 2022 [cited 2022 Dec 2];12:271-6. Available from: https://www.aihbonline.com/text.asp?2022/12/3/271/356110




  Introduction Top


The human secondary sex ratio (SSR) or sex ratio at birth (SRB) which conventionally means the number of live male births per 100 female births, continues to get attention from economic planners, demographers, cell biologists, biostatisticians and epidemiologists.[1],[2] This is because understanding the distribution of sexes in certain populations can help to proffer solutions to socio-economic problems, especially in the areas of planning and implementation of interventions as well as in the management and distribution of resources. In humans and some other sexually reproducing organisms, the SSR is expected to be similar to the primary ratio of 1:1 because of the segregation of the sex chromosomes; X and Y in humans. However, the SSR in humans is somewhat biased towards the male child as the natural SSR is believed to be 105–107, indicating that there are about 107 males for every 100 females.[3],[4],[5]

The imbalance or inequality in sex ratio, which is more prevalent in eastern Europe and Asia, has been attributed to several factors, chiefly of which are preferences for a male child. This trend is aided by several practices such as sex-selective abortions, infanticide and sex selection technology.[6],[7],[8],[9] On the other hand, unfavourable environmental conditions such as economic decline, earthquakes as well as excess male deaths as a result of war can lead to a low sex ratio.[10],[11],[12],[13] Other factors that are believed to affect the SRB include coital rate and parental age,[14] birth order,[15] maternal weight,[16] environmental toxins,[17],[18] hormonal imbalances,[19] chronic stress[20] and parental occupations.[21],[22],[23],[24]

SSR varies not only from one country to another but also among ethnic populations within a country. Mathews and Hamilton[25] reported sex ratios ranging from 103-105 for different populations groups in the United States. Among the Asian countries, the SRB was as high as 113.5 in China in 2015, largely due to government policy that significantly reduced fertility and the bias in the reporting of female births to state authorities. In South Korea, the sex ratio was 106.2 in 2014, and it was 111.3 in India between 2014 and 2016.[26],[27] In Nigeria, different values of sex ratios have been reported in different ethnic populations and geographical locations. Sex ratio values of 1.07 have been reported among the Igbo population in Eastern Nigeria[28] and 112 among the Hausas in the North.[29] Among the Yorubas in the Southwest, different values of male-to-female births were observed by several authors over a wide range of time, including 107.8[30] and 106[31] in Ibadan. Azeez et al.[3] and Oyeniyi[32] reported sex ratios of 102.7 and 114, respectively, for the region. Moreover, Eneni et al.[33] obtained a value of 120 in a South-Southern Nigerian population. The most recent study on the human SSR in the Southwest region was conducted by Azeez et al.[3] 2007. The study, which covered from 1995 to 2004, gave a combined ratio of 102.7 male births to 100 females. Our study was aimed at determining the trend as well as seasonal variation in sex ratios in Southwest Nigeria from 2007 to 2016.


  Materials and Methods Top


Population of study

Southwestern Nigeria [Figure 1] is occupied by the Yoruba ethnic individuals. Yoruba is one of the major ethnic populations in the country, in addition to the Igbos in the East and the Hausa-Fulani in the North. Other ethnic groups such as the Tiv, Efik, Ibibio, Ijaw and Kanuri constitute the minority. The Yorubas are found predominantly in 9 out of 36 states, including Ekiti, Lagos, Ogun, Ondo, Osun, Oyo and some parts of Kogi, Kwara and Edo states.
Figure 1: Map of Nigeria showing South-Western States (Modified from Fasasi and Alabi 2020.[34]

Click here to view


Sources of data and statistical analysis

The retrospective data used for the study were collected from the birth records of selected hospitals from three Southwestern Nigerian states as a representative of the Yoruba ethnic population. These are General Hospital, Badagry, Lagos state, (GH_BDG); Obafemi Awolowo University Teaching Hospital, Ile-Ife, Osun state, (OAUTH_IFE); and General Hospital, Iju-Itaogbolu, Akure, Ondo state (GH_AKR). The data consisted of the total number of live single births for both sexes from 1 January to 31 December of each year for 10 years, that is, 2007–2016. The number of live births from each hospital was analysed for monthly, quarterly, yearly and seasonal variation in sex ratios using the formula:



For seasonal variation, the months of the year were classified into two, i.e., rainy and dry seasons. The rainy season in Southern Nigeria includes the months of March to July (long rainy period) and September to October (short rainy period), while November to February (long dry period) and the month of August (short dry period) constitute the dry season. Chi-square analysis was performed to determine the significance of differences in sex ratios at 0.05 significant levels. All statistical analyses were performed using IBM SPSS Statistics for Windows, Version 26.0. Armonk, NY: IBM Corp. Data of multiple births were excluded from the study.


  Results Top


The total number of single births recorded in each of the study locations covered by this study were 11,680 for GH_BDG, 11,212 for OAUTH_IFE and 22,819 for GH_AKR. The monthly SSRs in these locations over 10 years are presented in [Table 1]. The highest monthly SSR (108.7) was recorded for GH_BDG in October, while the lowest ratio (83.3) was in July, with statistically significant deviations. For OAUTH_IFE, the month of October had the highest sex ratio of 120.8, while April recorded the lowest ratio of 93.3 over the 10 years covered by the study. The sex ratios for February and October were statistically significant in this location. The highest monthly SSR of 116.2 was recorded in January for GH_AKR, with the lowest being 90.1 in June. There were significant deviations of the sex ratios in January, February, May, June, October, November and December across the 10 years. Monthly analysis of data from all three studied locations showed that the highest (114.1) and lowest (95.3) sex ratios was recorded in October and June, respectively.
Table 1: Monthly secondary sex ratios in all study locations and combined, from 2007 to 2016

Click here to view


[Table 2] shows the annual sex ratio of live births from 2007 to 2016. It was observed that GH_BDG that year 2016 had the highest sex ratio of 115.4 males to 100 females, while the lowest ratio was in 2008 with 78.2 males to 100 females. There were statistical differences in the annual sex ratios of the years 2008, 2014 and 2016. The highest SSR of 117.4 was recorded for OAUTH_IFE in 2009, whereas the lowest sex ratio (96.2) was recorded in 2013, with values deviating significantly in 2009 and 2012. Annual analysis of the data from the GH_AKR showed that the sex ratio was highest (123.8) in 2009 and lowest (98.2) in 2012. Sex ratios were statistically significant in 2009, 2010 and 2013 for this location. Annual analysis of the combined data showed that year 2009 had the highest SRB of 116.3, while the lowest value (94.5) was in 2008.
Table 2: Annual secondary sex ratios in all study locations and combined, from 2007 to 2016

Click here to view


Results of the quarterly variation in sex ratios from the three locations for the 10 years covered by the study are presented in [Table 3]. For GH_BDG, the highest quarterly sex ratio of 107.3 was in the fourth quarter, while the lowest ratio of 94.6 was in the first quarter. OAUTH_IFE had the highest (111.5) and lowest (100.6) quarterly sex ratios in the first and second quarters, respectively. The highest and lowest sex ratios of 111.3 and 99.6 were recorded for General Hospital, Iju-Itaogbolu, Akure in the first and second quarters, respectively. Results further showed that there were no significant differences (P > 0.05) in quarterly sex ratios for all study locations.
Table 3: Quarterly sex ratio of live births in Southwest Nigeria from 2007 to 2016

Click here to view


[Table 4] presents the annual sex ratio for all three study locations. This includes an annual sex ratio of 99.7 for GH_BDG, 105.6 for OAUTH_IFE and 106.0 for General Hospital, Iju-Itaogbolu, Akure. The annual human SSR in Southwest Nigeria for the 10 years was 104.2 signifying more male births than females. There was no significant difference in sex ratios at a 0.05 significant level for the 10 years covered by this investigation.
Table 4: Sex ratio of live births in Southwest Nigeria over the 10 years

Click here to view


The results obtained of annual seasonal variation in sex ratios [Figure 2] shows that the highest sex ratio of 147.1 was recorded in 2008 for GH_AKR, and the lowest (77.7) was recorded in 2009 for GH_BDG during the dry season. The sex ratios differ significantly (P < 0.05) in GH_BDG and GH_AKR during the period under study. During the rainy season, the highest SSR at birth (120.4) was recorded in OAUTH_IFE in 2015, while the lowest ratio (78.5) was recorded in GH_BDG in 2008. A significant difference in sex ratios was only observed in GH_AKR during the rainy season (P < 0.001).
Figure 2: Seasonal variation in the sex ratio of live births in Southwest Nigeria from 2007 to 2016 during rainy (a), and dry (b) seasons. GH_BDG: General Hospital, Badagry, OAUTH_IFE: Obafemi Awolowo University Teaching Hospital, Ile-Ife, GH_AKR: General Hospital, Akure.

Click here to view



  Discussion Top


The human SSR varies from population to population and from one geographical location to another. This variation can be attributed to several factors, including cultural practices such as preference for a male child and patriarchal family system, government policy of small family size in some countries, sex-selection technology, environmental pollution, parental age, stress, war, natural disasters and genetics. Analysis of the SRB data published over several decades shows that regardless of race, ethnicity, nationality and geographic location, the SRB tends to go through periods of highs and lows regardless of genetic or environmental interplay. In the present study, we evaluated monthly, annually, quarterly and seasonal trends in human SRB in selected Yoruba populations of the Southwestern states of Lagos, Osun and Ondo, Nigeria. The study covers 10 years, spanning 1 January, 2007, to 31 December, 2016. Our findings revealed that the sex ratios of the selected locations differ monthly, quarterly and annually, with more than half of the months in each year having a high sex ratio, i.e., more male live births than females. On average, annual sex ratios in the three towns covered by this investigation are 99.7 for GH_BDG, Lagos state, 105.6 for OAUTH_IFE, Osun state and 106.0 for General Hospital, Iju-Itaogbolu, Akure, Ondo state. There was no statistical difference in the average sex ratios of studied locations (χ2 = 0.231; P = 0.891). Our finding is in agreement with several studies that have established that there are more male live births than females except for Lagos, with a slight female preponderance. The incidence of more female births than males has also been previously reported by Mosuro.[35] The low sex ratio recorded in Lagos, as revealed by this study and that of Mosuro,[35] which are over two decades apart, signifies that the sex ratio of Lagos is generally low, and this may be attributed to environmental factors such as pollution and increased stress among the adult males, Lagos being a highly industrialised and densely populated metropolitan area with poor transportation systems, unlike the Ile-Ife and Akure that are less industrialised. There are established relationships between environmental pollution and sexual differentiation.[36],[37],[38] The timing and/or extent of exposure to toxicants significantly affects the rate of cell division. Rapidly dividing cells, like the foetal gonadal tissue (FGT), are more susceptible to replication error than the slowly dividing ones. Hence, exposure of the FGT to environmental assault may lead to abnormalities that could compromise sex determination. Furthermore, somatic mutations in the sex-determining gene on the Y chromosome (SRY gene, a gene which initiates the determination of male sex in mammals) resulting from environmental pollutants as well as damage to the Y chromosome might lead to the predominance of female offspring.[39],[40] stress, on the other hand, has been implicated in increased levels of an adrenocorticotrophic hormone known as corticotrophin. Corticotrophin affects men and women differently. In men, elevated levels of corticotrophin bring about a reduced amount of testosterone and increased oestrogen leading to more female offspring. In women, however, high levels of corticotrophin cause the adrenal cortex to synthesise more testosterone, thereby creating hormonal balance for male offspring.[41] Variation in the sex ratios of the study locations is also ascribable to differences in living standards, but more evidence is needed for a valid argument.

The reported overall sex ratio of 104:100 for the entire study period showed that Southwestern Nigeria had more male birth to a female and falls in the 1.03–1.07 range for humans, especially the African populations.[42] There are several data from Nigeria and other parts of the world that agree with this finding. While Ayeni[30] observed a sex ratio of 107.85:100 for Ibadan, Southwestern Nigeria, Effiong et al.[31] reported a sex ratio of 1.06 among Yoruba children in Ibadan. Furthermore, sex ratio values of 107:100[43] and 1.12[29] have been reported among the Northern Hausas. Furthermore, for the sex ratio of Igbo births, Egwuatu[28] reported a value of 107. A 2007 study by Azeez et al.[3] revealed a sex ratio value of 102.7:100 in South West, Nigeria, while Oyeniyi[32] reported a value of 1.14 another Southwestern Nigerian population and, more recently, Eneni et al.[33] gave a high ratio of 120:100 in their study of a South-South Nigerian Teaching Hospital. According to a 2001 survey, Belgium's SRB was 1.04, while Italy, Switzerland and Ireland had a joint sex ratio of 1.07. A 1970–2002 report of sex ratios in the United States gave ratios 1.03, 1.04 and 1.05 for the African Americans, Mexican Americans and the white non-Hispanic populations, respectively. In China, there was an upsurge in the observed SSR ranging from 106 in 1979, 111 in 1990-117 in 2001 and as high as 130 in some rural communities.[7],[44] The last population census in Nigeria conducted in 2006 put the sex ratio of the entire population at 1.05, while the CIA World Factbook[44],[45] estimated the country's SRB to be 1.06. It is interesting to note that these figures align with the results obtained in the present study.

There were incidences of geographical variations in SRB in our sampled locations. The monthly ratio in GH_BDG was highest in October and lowest in July, whereas the sex ratio at General Hospital, Iju-Itaogbolu, Akure, was highest in January and lowest in June. The highest and lowest monthly sex ratios for OAUTH_IFE were recorded in October and April, respectively. Overall, the SRB for combined data from all locations during the period covered by the study was highest in October and lowest in June. The quarterly analysis showed that the incidence of sex ratio in OAUTH_IFE and General Hospital, Iju-Itaogbolu, Akure followed a similar pattern and differently from GH_BDG. There were more male births, i.e., high sex ratios recorded in either the first (GH_BDG) or third quarters (OAUTH_IFE and GH_AKR), while the lowest sex ratios were seen in the first quarter for GH_BDG and fourth quarter for OAUTH_IFE and GH_AKR. The highest and lowest annual sex ratios were obtained in GH_BDG in 2016 and 2008, respectively. OAUTH_IFE and GH_AKR both had their highest sex ratio in 2009. However, while the lowest sex ratio was observed in OAUTH_IFE in the year 2013, it was 2012 for GH_AKR. Geographical variations in SRB have been documented by previous studies,[46],[47],[48] and it is suggested that ambient temperature might skew the sex ratio in favour of a particular gender.[49],[50] For instance, studies have revealed that warmer temperatures tend to favour the birth of the male child.[50],[51],[52],[53] The underlying mechanism for this phenomenon remains unclear. This may, however, be attributed to the events occurring within the testis. A study by van Zelst et al.[54] showed that increased temperatures affect the disjunction of meiotic X and Y chromosomes in mice. In addition, Lerchl[49] demonstrated that the sex ratio of humans is affected by temperatures, with higher proportions of boys resulting from increased environmental temperatures. On the other hand, Kelsey et al.[55] showed that low ambient temperature affects the SRB by triggering stress responses in individuals that are protected against it. Furthermore, low temperatures can lead to increased stress in populations by disrupting resources, for example, economy, that sustain them.[56] Nevertheless, temperatures may not be the sole factor responsible for the differences observed in our findings because the study locations fall in the same geographical altitude and share relatively the same climatic conditions. Hence, other socio-economic and environmental factors might play significant roles.

The seasonal variation in SRB revealed significant differences (P < 0.05) in sex ratios during the rainy season for GH_AKR only, and GH_BDG and GH_AKR in the dry season, but there was no specific pattern in the monthly distribution of sex ratios in across the studied locations. Results of Azeez et al.[3] and Eneni et al.[33] from South-West and South-South, Nigeria, also agrees with our finding that sex ratios do not follow a general trend month-wise. Our findings further agree with several other reports indicating that sex ratios vary from one geographical location to another.[46],[48] Furthermore, in ex-Yugoslav nations, a study showed that sex ratios vary with little change in latitude.[57] Apart from geography, several factors such as socio-economic development as well as levels of fertility, industrialisation and health-care systems may together affect SRB within specific countries or regions.


  Conclusions Top


Sex ratios from Southwest Nigeria are as diverse as other populations from the literature, with ratios ranging between 99.7 and 106.0. These figures may not represent the accurate SRB from this region; however, since records of births occurring at home and abandoned infants were not captured by this study.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

1.
Orzack SH, Stubblefield JW, Akmaev VR, Colls P, Munné S, Scholl T, et al. The human sex ratio from conception to birth. Proc Natl Acad Sci U S A 2015;112:E2102-11.  Back to cited text no. 1
    
2.
Upreti P, Singh RK. Sex ratio at birth and its determinants: A cross sectional hospital based study. Hindu 2017;299:74-5.  Back to cited text no. 2
    
3.
Azeez MA, Akinboro A, Bakare AA. Human sex ratio at birth in South West Nigeria. Indian J Hum Genet 2007;13:59-64.  Back to cited text no. 3
[PUBMED]  [Full text]  
4.
Ross L, Blackmon H. Sex determination. Encycl Evol Biol 2016;4:81-8.  Back to cited text no. 4
    
5.
Chao F, Gerland P, Cook AR, Alkema L. Systematic assessment of the sex ratio at birth for all countries and estimation of national imbalances and regional reference levels. Proc Natl Acad Sci U S A 2019;116:9303-11.  Back to cited text no. 5
    
6.
Das Gupta M, Zhenghua J, Bohua L, Zhenming X, Chung W, Hwa-Ok B. Why is son preference so persistent in East and South Asia? A cross-country study of China, India and the Republic of Korea. J Dev Stud 2003;40:153-87.  Back to cited text no. 6
    
7.
Hesketh T, Xing ZW. Abnormal sex ratios in human populations: Causes and consequences. Proc Natl Acad Sci U S A 2006;103:13271-5.  Back to cited text no. 7
    
8.
Hesketh T, Lu L, Xing ZW. The consequences of son preference and sex-selective abortion in China and other Asian countries. CMAJ 2011;183:1374-7.  Back to cited text no. 8
    
9.
Hesketh T, Min JM. The effects of artificial gender imbalance. Science & Society Series on Sex and Science. EMBO Rep 2012;13:487-92.  Back to cited text no. 9
    
10.
Fukuda M, Fukuda K, Shimizu T, Møller H. Decline in sex ratio at birth after Kobe earthquake. Hum Reprod 1998;13:2321-2.  Back to cited text no. 10
    
11.
Graffelman J, Hoekstra RF. A statistical analysis of the effect of warfare on the human secondary sex ratio. Hum Biol 2000;72:433-45.  Back to cited text no. 11
    
12.
Catalano RA. Sex ratios in the two Germanies: A test of the economic stress hypothesis. Hum Reprod 2003;18:1972-5.  Back to cited text no. 12
    
13.
Saadat M, Ansari-Lari M. Sex ratio of birth during wartime and psychological tensions. Hum Reprod 2004;19:465.  Back to cited text no. 13
    
14.
James WH. Coital rate, sex ration, and parental age. Lancet 1971;1:1294.  Back to cited text no. 14
    
15.
Jacobsen R, Møller H, Engholm G. Fertility rates in denmark in relation to the sexes of preceding children in the family. Hum Reprod 1999;14:1127-30.  Back to cited text no. 15
    
16.
Cagnacci A, Renzi A, Arangino S, Alessandrini C, Volpe A. Influences of maternal weight on the secondary sex ratio of human offspring. Hum Reprod 2004;19:442-4.  Back to cited text no. 16
    
17.
Pillarisetti JR. Skewed sex ratio, environmental toxins and human wellbeing: The need for policies. Int J Environ Stud 2016;73:692-701.  Back to cited text no. 17
    
18.
Pavic D. A review of environmental and occupational toxins in relation to sex ratio at birth. Early Hum Dev 2020;141:104873.  Back to cited text no. 18
    
19.
James WH. The sex ratios of offspring of diabetic parents. Diabet Med 2006;23:1043-4.  Back to cited text no. 19
    
20.
Kemkes A. Secondary sex ratio variation during stressful times: The impact of the French revolutionary wars on a German parish (1787-1802). Am J Hum Biol 2006;18:806-21.  Back to cited text no. 20
    
21.
Dickinson HO, Parker L. Sex ratio in relation to fathers' occupations. Occup Environ Med 1997;54:868-72.  Back to cited text no. 21
    
22.
Magnuson A, Bodin L, Montgomery SM. Father's occupation and sex ratio of offspring. Scand J Public Health 2007;35:454-9.  Back to cited text no. 22
    
23.
Grant VJ, Metcalf LE. Paternal occupation and offspring sex ratio. Sexualities Evol Gend 2003;5:191-209.  Back to cited text no. 23
    
24.
Ruckstuhl KE, Colijn GP, Amiot V, Vinish E. Mother's occupation and sex ratio at birth. BMC Public Health 2010;10:269.  Back to cited text no. 24
    
25.
Mathews TJ, Hamilton BE. Trend analysis of the sex ratio at birth in the United States. Natl Vital Stat Rep 2005;53:1-17.  Back to cited text no. 25
    
26.
Festini F, de Martino M. Twenty five years of the one child family policy in China. J Epidemiol Community Health 2004;58:358-60.  Back to cited text no. 26
    
27.
Tafuro S, Guilmoto CZ. Skewed sex ratios at birth: A review of global trends. Early Hum Dev 2020;141:104868.  Back to cited text no. 27
    
28.
Egwuatu VE. The sex ratio of Igbo births. Int J Gynaecol Obstet 1984;22:399-402.  Back to cited text no. 28
    
29.
Sule ST, Madugu HN. Sex ratio at birth in Zaria, Nigeria. Ann Hum Biol 2004;31:258-62.  Back to cited text no. 29
    
30.
Ayeni O. Sex ratio of live births in South-Western Nigeria. Ann Hum Biol 1975;2:137-41.  Back to cited text no. 30
    
31.
Effiong CE, Laditan AA, Aimakhu VE, Ayeni O. Birthweights of Nigerian children. Niger Med J 1976;6:63-8.  Back to cited text no. 31
    
32.
Oyeniyi T. Secondary sex ratio in a South-Western Nigerian town. Int J Genet 2012;4:92-4.  Back to cited text no. 32
    
33.
Eneni NO, Mukoro GD, Tabowei B. Trend of male-female birth ratio in a South-South Nigerian Teaching Hospital. Internet J Health 2013;14:1-4.  Back to cited text no. 33
    
34.
Fasasi MI, Alabi MA. Personality Type and Experience of Domestic Violence among Married Women in Southwest Nigeria. Curr Res J Soc Sci 2020;3:255-61.  Back to cited text no. 34
    
35.
Mosuro AA. Sex ratio of live births in South West Nigeria. J Sci Res 1997;3:113-7.  Back to cited text no. 35
    
36.
Williams FL, Lawson AB, Lloyd OL. Low sex ratios of births in areas at risk from air pollution from incinerators, as shown by geographical analysis and 3-dimensional mapping. Int J Epidemiol 1992;21:311-9.  Back to cited text no. 36
    
37.
James WH. Offspring sex ratio as an indicator of reproductive hazards associated with pesticides. Occup Environ Med 1995;52:429.  Back to cited text no. 37
    
38.
Davis DL, Gottlieb MB, Stampnitzky JR. Reduced ratio of male to female births in several industrial countries: A sentinel health indicator? JAMA 1998;279:1018-23.  Back to cited text no. 38
    
39.
Davis DL, Friedler G, Mattison D, Morris R. Male-mediated teratogenesis and other reproductive effects: Biologic and epidemiologic findings and a plea for clinical research. Reprod Toxicol 1992;6:289-92.  Back to cited text no. 39
    
40.
Gustafson ML, Donahoe PK. Reproductive embryology and sexual differentiation. In: Wallach EE, Zacur HA, editors. Reproductive Medicine and Surgery. St Louis, Mo: Mosby-Year Book; 1995. p. 39-59.  Back to cited text no. 40
    
41.
James WH. Hormonal control of sex ratio. J Theor Biol 1986;118:427-41.  Back to cited text no. 41
    
42.
Garenne M. Sex ratios at birth in African populations: A review of survey data. Hum Biol 2002;74:889-900.  Back to cited text no. 42
    
43.
Rehan NE. Sex ratio of live-born Hausa infants. Br J Obstet Gynaecol 1982;89:136-41.  Back to cited text no. 43
    
44.
Hesketh T, Zhang J, Qiang DJ. HIV knowledge and risk behaviour of female sex workers in Yunnan Province, China: Potential as bridging groups to the general population. AIDS Care 2005;17:958-66.  Back to cited text no. 44
    
45.
CIA World Factbook Nigeria; 2020. Available from: https://www.cia.gov/the-world-facbook/countries/nigeria. [Last accessed on 2021 Feb 09].  Back to cited text no. 45
    
46.
Masoudi M, Saadat M. Altitude, latitude and sex ratio at birth in Iran. J Epidemiol Community Health 2007;61:172.  Back to cited text no. 46
    
47.
Grech V. Secular trends and latitude gradients in sex ratio at birth in Asia during the past 60 years. Pediatr Int 2013;55:219-22.  Back to cited text no. 47
    
48.
Pavic D. Secular trends and geographical variations in sex ratio at birth. Early Hum Dev 2015;91:811-5.  Back to cited text no. 48
    
49.
Lerchl A. Sex ratios at birth and environmental temperatures. Naturwissenschaften 1999;86:340-2.  Back to cited text no. 49
    
50.
Catalano R, Bruckner T, Smith KR. Ambient temperature predicts sex ratios and male longevity. Proc Natl Acad Sci U S A 2008;105:2244-7.  Back to cited text no. 50
    
51.
Trivers RL, Willard DE. Natural selection of parental ability to vary the sex ratio of offspring. Science 1973;179:90-2.  Back to cited text no. 51
    
52.
Helle S, Helama S, Jokela J. Temperature-related birth sex ratio bias in historical Sami: Warm years bring more sons. Biol Lett 2008;4:60-2.  Back to cited text no. 52
    
53.
Navara KJ. Humans at tropical latitudes produce more females. Biol Lett 2009;5:524-7.  Back to cited text no. 53
    
54.
van Zelst SJ, Zupp JL, Hayman DL, Setchell BP. X-Y chromosome dissociation in mice and rats exposed to increased testicular or environmental temperatures. Reprod Fertil Dev 1995;7:1117-21.  Back to cited text no. 54
    
55.
Kelsey RM, Alpert BS, Patterson SM, Barnard M. Racial differences in hemodynamic responses to environmental thermal stress among adolescents. Circulation 2000;101:2284-9.  Back to cited text no. 55
    
56.
de Vries J. Measuring the impact of climate on history: The search for appropriate methodologies. J Interdisc History 1980;10:599-630.  Back to cited text no. 56
    
57.
Grech V. Secular trends in newborn sex ratios. Early Hum Dev 2014;90:755-60.  Back to cited text no. 57
    


    Figures

  [Figure 1], [Figure 2]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4]



 

Top
 
 
  Search
 
Similar in PUBMED
   Search Pubmed for
   Search in Google Scholar for
 Related articles
Access Statistics
Email Alert *
Add to My List *
* Registration required (free)

 
  In this article
Abstract
Introduction
Materials and Me...
Results
Discussion
Conclusions
References
Article Figures
Article Tables

 Article Access Statistics
    Viewed331    
    Printed30    
    Emailed0    
    PDF Downloaded33    
    Comments [Add]    

Recommend this journal


[TAG2]
[TAG3]
[TAG4]