Frequency of twinning in southwest Nigeria

BACKGROUND: In the human species, twin is a type of multiple birth in which the mother gives birth to two offspring from the same pregnancy. The occurrence and frequency of twinning, however, varies across human populations. The maternal age, socio-environmental factors, increase in the use of contraceptives, the race of human population, increase in the spontaneous abortion rate, and seasonal variations are among the factors that could influence twinning rate. Information on twinning rates in southwest Nigeria is limited. AIMS: This study presents information on the frequency of twinning, as well as its analysis by maternal age, in four urban settings in southwest Nigeria. This is with the aim of extending current knowledge on the frequency of twinning in southwest Nigeria and contributing to the demographic studies in the country. MATERIALS AND METHODS: Data on single births and twin births from January 1995 to December 2004 were collected from the Oyo State General Hospital (OSGH), Wesley Guild Hospital (WGH), Obafemi Awolowo University Teaching Hospital (OAUTH), and Ekiti State Specialist Hospital (ESSH) in Ogbomoso, Ilesa, Ile-Ife, and Ado-Ekiti respectively. These were analyzed by year and maternal age groups of 15-19, 20-24, 25-29, 30-34, 35-39, 40-44, and 45-49 years according to the standard method. RESULTS: A frequency of twin births of 46.5 per 1000 deliveries and 46.2 per 1000 deliveries was recorded for Ilesa and Ile-Ife respectively. The frequency recorded for Ogbomoso and Ado-Ekiti was 38.5 and 22.1 per 1000 deliveries respectively. The overall average frequency of 40.2 per 1000 deliveries for the four hospitals ranks among the highest recorded rates of twin births in the world. The maternal age group of 25-29 years had the highest occurrence of twin births, while the lowest was recorded in the 45-49 years age group. CONCLUSION: This analysis reveals high incidence of twinning in the studied areas and supports previous assertion that the southwestern part of Nigeria has the highest twinning rate in the country and in the whole world. It is our opinion that diet, maternal history of twinning, and some socio-environmental factors may have inß uenced the results.


Introduction
Twins in animal biology is a form of multiple births in which the mother gives birth to two offspring from the same pregnancy -some of the same gender, others of opposite.Giving birth to twins is a relatively rare event in humans, where occurrences vary considerably across populations. [1,2]The human female usually has a single baby in each pregnancy; but one in 90 pregnancies is a twin pregnancy; one in 8,100 pregnancies is a triplet pregnancy; and one in 729,000 pregnancies is a quadruplet pregnancy. [3]Alfonso et al. [4] reported that the rates of multiple births were the highest during the last three decades of the 18 th century, when the twinning rate was more than 17‰, the triplets rate was more than 3‰ and the quadruplet rate was almost 7 per 1 million births.
In the whole world, analyses and estimation of multiple births have revealed approximately 125 million twins and triplets, out of which 10 million were just identical twins. [5]ins have been reported to be more frequent in African and Asian countries, and this is due to the higher proportion of fraternal twins; as identical twins occur equally often, whatever the race or age of the mother.
Among Caucasians, fraternal twins occur more frequently in families with a history of twins, in older women, among women who have had several children previously, and after fertility treatment to induce ovulation. [3]Among the factors that could inß uence twinning are the maternal age and the race of human population, [1,6] socio-environmental factors, increase in the use of contraceptives, increase in the spontaneous abortion rate, [6][7] and seasonal variations. [8]requency of twin birth varies considerably across human populations.Among the earliest reports of the incidence of twinning in Nigeria is that of Bulmer, [9] who reported an incidence of 44.9 twin births per 1000 births in Ibadan.Between 1967 and1969, Nylander [10] reported twinning incidence of 48.3‰ and 57.2‰ for Ibadan and Igbo-ora respectively.Mosuro [11] obtained a higher incidence of twin births: 68.1‰ births for Igbo-ora, 37.2‰ births for Ibadan, and 26.9‰ births for Lagos.In Ilorin [12] and Calabar, [13] the incidence of twinning was 35.1‰ and 26.5‰ births respectively.
In the northern part of Nigeria, the twinning rate was reported to be 39.7‰ [14] and 28‰ [15] births.

Most reports on incidence of twinning in southwest
]16] These are just two out of the six states that constitute the southwestern part of Nigeria.In this study, we present information on the incidence of twinning in Ogbomoso, the second

Materials and Methods
The data used for this study were collected from the birth records of four different hospitals in three states of southwestern Nigeria [Figure 1]., viz., 15-19, 20-24, 25-29, 30-34, 35-39, 40-44, and 45-49 years, were considered for the possible inß uence of mother's age on twinning rate.All sets of data were not analyzed for type of twins delivered, i.e., whether monozygotic (identical) or dizygotic (fraternal), because of inadequate records.This is not expected to affect the results obtained in this study.The number of single births (x), the number of twin births (y), and the number of single births for every twin delivery (x/y) were determined.The number of twin births in every 1000 deliveries was computed as Twin deliveries (y) × 1000 Total deliveries (z)

Results
Table 1 shows the annual incidence of twin births recorded in General Hospital, Ogbomoso, Oyo state,      3].

Discussion
In this study, the incidence of twinning in Ogbomoso,   or abandoned infants go unrecorded.Only 37.3% of births in Nigeria take place within health facilities. [17]ta collected for this study are assumed to be the births reported in Calabar; [13] 28 per 1000 in Jos; [15] 35.1 per 1000 in Ilorin; [12] and 39.7 per 1000 births for the Hausa population in northern Nigeria. [14]They are also higher than 11‰ twin births recorded in Kenya, [18] 20‰ twin births observed in Sub-Saharan Africa, [2] 16.1‰ twinning rate recorded in Nepal, [19] and 33.4/1000 and 26.6/1000 recorded for Accra and Kumasi in Ghana respectively. [20]The values in this study are higher than the African rate of 20‰ twin births [21] and rank among the highest twinning rates in the world.
Apart from genetic predisposition, a factor that may have also been inß uencing high twinning rate in southwest Nigeria is diet. [22]There is a general belief that the Yoruba's predisposition to high twinning rate is due to consumption of yam (Discorea sp.), which is believed to contain a natural hormone phytoestrogen, which may stimulate multiple ovulation. [23]Indigene of the studied areas are known to have preference for food prepared in different forms from yam.Maternal history of twinning (though not considered in this study) might also have contributed to the high incidence of twinning observed in this study and previous studies from southwest Nigeria.This is because the probability of a subsequent twin pregnancy is increased fourfold in mothers of twins, and the risk of having dizygotic (DZ) twins is roughly double for a woman whose mother or sister has DZ twins. [24]e incidence of twinning among the races of the world has been extensively studied.Worldwide changes have occurred in the pattern of twinning rates in recent decades.The highest twinning rate is observed to occur among the Negroids, while the lowest occurred among the Mongoloid population. [1]In Nigeria, the incidence of twinning was estimated to be 45‰. [7]Bulmer [9] recorded 44.9‰ incidence of twinning in Ibadan, while 48.3‰ and 57.2‰ incidences of twin births were respectively obtained for Igbo-Ora and Ibadan between 1967 and 1969. [10]On an average, the highest record of twinning is among the Yorubas of southwest Nigeria, with approximately 50-53 twin births per 1000 deliveries. [25]ese rates were suggested then as the highest in the world.Between 1985 and 1989, 68.1‰ twin births were recorded for Igbo-Ora, while 37.2‰ and 26.9‰ twin births were recorded for Ibadan and Lagos respectively. [11]e values obtained in this study, especially those of Ogbomoso and Ado-Ekiti, are lower than the overall estimated average for the Yorubas of southwest Nigeria. [10,25]This may mean that there is a decline in the rate of twinning in southwest Nigeria, an observation that is in concordance with the observations of Mosuro. [11]The studied locations are urban towns inhabited by the Yorubas.But with migration, other ethnic groups are also found living in these areas.This might have led to interbreeding resulting in admixture of genes and hence reduction in twinning rates.In general, twinning rates appear to be modiÞ ed by both migration and inter-ethnic mixing. [26]other factor that may have inß uenced the twinning rate in the present study is social class.Nylander [6] reported that the twinning rate in the lowest social class was over twice that in the middle and highest social class in Nigeria.This was particularly true of Igbo-Ora, Nigeria, [10] which was and still is a rural settlement that is believed to be homogenous culturally, socially, ethnically, and may even be consanguineous; and it is having the highest incidence of twin births in Nigeria, as well as in the world.Ogbomoso, Ilesa, Ile-Ife, and Ado-Ekiti are towns with middle and high social classes in Nigeria, hence the decline in the twinning rate.Changing societal values such as reduction in polygamy, family planning policy on reduction in the number of children produced per family, and more women pursuing career goals have had a resultant effect of decrease in twinning rate.
In some studies in other countries, the decline in twinning rate has been related to an increase in industrialization and urbanization [27] and population density, [28] resulting in "psycho-social stress." [29]The high population growth rate, immigration from the rural areas into the towns, the gradual breakdown of family structure, and the continuing urbanization of areas that were traditionally agricultural may have all produced an environment in which psycho-social stress is a signiÞ cant factor.It seems possible, therefore, that psycho-social stress factors may be responsible, at least in part, for the decline in twinning rate in the studied locations.It is not possible to substantiate this theory with the data available from the present study.However, the fact that previous studies from southwest Nigeria [11,16] showed a possible continuing decline in incidence of twinning in urban settings in southwest Nigeria, coupled with the belief that psycho-social stress factors in the studied areas might have increased over this period in terms of a continuing increase in population density and political strife, does not contradict the theory.
In addition, a history of oral contraceptive use has been suggested to contribute to the decline in twinning by acting directly to reduce the probability of double-ovulation. [30]he decline may also be inß uenced by a decrease in the probability of conception.It seems, therefore, that no single factor can explain the trends observed.It is increasingly evident that the explanation of the variability of twinning rates is very complex and is not just of interest per se but may disclose new insights into the role of genetic versus environmental factors in the reproduction of man.This seemingly declining rate of the incidence of twinning is not peculiar to Nigeria.Several studies of populations elsewhere in the world have shown a decline in twinning rate during the 20 th century. [7,29,31,32]e maternal age group that had the highest twin birth rate in Ilesa, Ile-Ife, and Ogbomoso was the 25-29 years age group; while at Ado-Ekiti, it was the 30-34 years age group.The 25-29 years age group of mothers with highest twin births obtained for Ilesa, Ile-Ife, and Ogbomoso is in concordance with the records obtained in Igbo-Ora, Ibadan, and Lagos; [11] Jos [15] and Calabar. [13]This appears to be the age group with the highest record of twin births in southwest Nigeria.
According to Mosuro, [11] the relative youthfulness of the age group with the highest twinning rate in the present study may be related to modern trend in Nigerian women toward earlier marriage and having fewer children, particularly in urban areas.As a result of such social changes, there are fewer older mothers now having children.This translates to a lowering of birth rate generally, and consequently, a downward trend in the twinning rate in the studied locations.The data of Ado-Ekiti for the 30-34 years age group is in accord with data obtained from Europe and White United States populations, [1] and is still within the general belief that the rate of twinning appears to increase with maternal age reaching a peak at about 37 years. [1]This may be the reason for the lowest twinning rates in the 15-19 years age group since women in this age group are most likely to have their Þ rst delivery.Also, women within the age group 46-49 years in most cases are perimenopausal or may have completed their families, hence the lower incidence of twin birth in this age group.
This study has presented data on twin births in four major towns of southwestern Nigeria.Analysis of data reveals high incidence of twinning in these towns and supports the previous assertion that the southwestern part of Nigeria has the highest twinning rate in the country and in the whole world.Further studies are needed on the possible inß uence of diet, environment, and societal changes on rates of twinning in Nigeria.
largest town after Ibadan in Oyo state [Figure 1]; Ile-Ife and Ilesa towns in Osun state [Figure 1]; and Ado-Ekiti, the capital of Ekiti state [Figure 1].It is hoped that data obtained will extend current knowledge on the frequency of twinning in southwest Nigeria and contribute to the demographic studies in the country.

Figure 1 :
Figure 1: Map of Nigeria showing the states of the Federation.Data were collected from 3 states (arrowed) in Southwestern Nigeria, (a) Oyo state, (b) Ekiti state and (c) Osun state.Igbo-Ora in Oyo state (a), has the highest rate of twinning in the country and worldwide

Figure 2 :
Figure 2: Annual incidence of twin births recorded in 4 hospitals in Southwest Nigeria between January 1995 and December 2004 rate at which twin births occurred in the places where the hospitals are located.There was approximately the same number of twin births (2‰ deliveries) at Ile-Ife and Ilesa town in Osun state; and their values of 46.5‰ and 46.2‰ were the highest in this study.These values were the double of what obtained for Ado-Ekiti and about 10‰ above the value recorded in Ogbomoso.With the exception of Ado-Ekiti, the incidences of twinning in the other three locations are higher than the 26.5 per 1000 These hospitals are Oyo

Table 3
presents the annual incidence of twin births recorded in Wesley Guild Hospital, Ilesa, Osun state.The variation in the incidence of twin births per 1000 deliveries over the years covered followed no speciÞ c pattern.The highest twin birth rate of 59.7‰ was recorded in 1998, while the lowest value of 32.7‰ was obtained in 1997.The average number of twin births per 1000 deliveries pooled was 46.5‰, indicating that 20.5 single births were recorded for each twin delivery.The age group 25-29 years had the highest record of 167 twin births (16.7 twin births per year), while the least number (0.3 twin birth per year) was recorded in the 15-19 years age group [Table3].Table 4 shows the annual incidence of twin births recorded in State Specialist Hospital, Ado-Ekiti.The highest incidence of twin births per 1000 deliveries of 30.0‰ was recorded in 1998, while the lowest value of 11.8‰ was recorded in 1997.The pooled average number of twin births per 1000 deliveries was 22.1‰, indicating that 44.2 single births were recorded for each twin delivery.The highest number of twin births was recorded in the 30-34 years age group, while the lowest number of twin births was recorded in the age group 15-