Concern About the Association Between Sex and Birth Weight of Babies : A Cross-Sectional Randomized Finding From a Nigerian Hospital

Address for correspondence Dr. Baba Usman Ahmadu Department of Paediatrics, Federal Medical Centre Yola, Yola bye-pass Adamawa state, Nigeria. E-mail: ahmadu4u2003@yahoo.com How to cite this article ? Ahmadu BU, Abubakar IH, Halima A, Ruqayya A, Suleiman GM. Concern About the Association Between Sex and Birth Weight of Babies: A Cross-Sectional Randomized Finding From a Nigerian Hospital. J Nepal Paediatr Soc 2013;33(1):21-24. Concern About the Association Between Sex and Birth Weight of Babies: A Cross-Sectional Randomized Finding From a Nigerian Hospital


Introduction
"H ow much did he or she weigh?" is often the fi rst question proud parents are asked after they have announced the sex of their newly delivered baby.Sex chromosomes may have a disproportionate infl uence on birth weight of babies because both X and Y chromosomes are atypical in gene content and activity, as a result of their unique evolutionary trajectory 1 .Some investigators, however, believed that genetic factors and endocrine mechanisms related to sex could play signifi cant roles in determining birth weight of babies.Magnus et al 2 , in 1984 estimated that genetic infl uences account for about 80% of birth weight outcome in babies, and recently some of the specifi c genes and chromosomal loci involved have been identifi ed 3 .Hattersley and colleagues 4 in 1998 observed that mothers who had glucokinase gene defects were hyperglycemic, and the birth weights of their offspring were increased by a mean of 601 grams.Also, if a fetus had inherited a glucokinase mutation, the birth weight was decreased by 533 gram, equivalent to a fall from the 50th to the 25th birth weight percentile 4 .Insight into factors that may be involved in human fetal growth has been studied.In particular, insulin-like growth factor (IGF)-I, IGF-II, IGF receptor type 1, insulin and insulin receptor are all critical for normal fetal growth 3 .Furthermore, low birth weight (LBW) in babies has been linked to IGF-I gene defects in humans 3 .
Graves in 2010 argued that the placenta is the site where maternally and paternally derived alleles have infl uence on fetal growth 1 .This sexual antagonism is seen particularly in large numbers of genes that show imprinted expression on placenta more than any other tissue in humans 1 .This may have evolved because of the competing interests of the male parent, a larger offspring, and the female parent surviving to bear offspring from other partners 1 .Many genes on the male-specifi c region of Y chromosomes are dedicated to sex and reproduction, making the Y chromosome quite unique in its functional coherence 5,6 .This may possibly lead to higher birth weight in males compared to female babies.In this review, the association between sex and birth weight of babies was investigated at the University of Maiduguri Teaching Hospital (UMTH).To our knowledge, no such study was performed before in Maiduguri, Borno State, and the entire North-East sub-region of Nigeria.The knowledge obtained could bridge the gap on the paucity of published literature on the relationship between sex and birthweights of babies especially in the North-Eastern part of the country.

Materials and Methods
The study was carried out at the Department of Paediatrics, and Obstetrics unit of the UMTH, Nigeria.Apart from being the largest health facility in the area, UMTH serves as a referral centre for the six North-Eastern states and neighboring countries of Chad, Cameroon and Niger Republics.
The study protocol was reviewed and authorised by the Medical Research and Ethics Committee of the UMTH.All work was performed according to the international guidelines for human experimentation in clinical research 7 .Informed consent from parents was also obtained.Parents had unlimited liberty to deny consent without any consequences.
The minimum sample size was determined using a statistical formula appropriate for detecting differences between two means when using paired sampling units: the effect size was set at 0.2, alpha level at 0.05 and power at 95% 8,9 .However, 50% of the calculated minimum sample was added to maximize power.Therefore, the sample size for this study was one hundred babies.Babies were eligible for participation in this study if they were delivered at the labour ward of UMTH and met the following study inclusion criteria: (i) singleton birth at term based on fundal height estimate for gestational age or Naegale's rule 10 .(ii) Mothers had no known underlying pregnancy complications, chronic illness and not on drugs other than those used for routine antenatal care.Babies whose mothers smoked cigarettes or drank alcoholic beverages or coffee were excluded from this study.Babies were enrolled in this study using the systematic random sampling method where the fi rst of every four baby was selected at the labour ward.Where the fi rst baby did not fulfi l the inclusion criteria above, the immediate next baby that qualifi ed was selected.
On enrolment of babies, information was obtained on delivery outcome which included the sex of the babies.The birth weight of babies in grammes was determined using the bassinet weighing scale which has a sensitivity of 50 grams.
Mean and standard deviation (SD) were calculated for birth weight of babies.The 95% confi dence intervals of the means were calculated as described by Hanley et al 11 .Likelihood ratio chi-square test of association based on William's criterion was used to investigate the effect of sex on birth weights of the babies 12 .Statistical analysis was performed using statistical package for social science (SPSS) statistical software version 16, Illinois, Chicago USA.Statistical signifi cance was defi ned as a p value <0.05.

Results
One hundred babies participated in the study.There were 53 (53.0 %) males and 47 (47.0 %) females giving a male to female ratio of 1.1:1.Majority of the babies 82 (82.0 %) had normal birth weight (Table 1).Association between birth weight and sex of the babies was signifi cant (χ 2 = 9.317, p = 0.025).

Discussion
Sex contributed to birth weights of babies in the present study with males having greater birth weights than females.The reason for this fi nding may possibly be found in the heterogenic nature of male chromosomes unlike the female chromosomes that are homogenous.A past study has documented that the male chromosome may confer some advantages to male over female babies, and this may include males having increased birth weights than females 13 .Some authors have advanced the concept of sexual selection to explain the increase in birth weight of male babies compare to their female counterpart 1 .This was based on the relatively bigger nature of some male organs particularly the brain and testis, believed to be due to accumulation of genes producing multifunctional proteins.One of these genes is SRY, the sex determining gene, which secretes androgens that is responsible for masculinizing male babies 6 .Male babies could have more muscle mass than female babies, thus male babies would have elevated birth weight than female babies.However, other workers have argued against sexual selection; they believed that gene expression in brain and testis that codes for multifunctional proteins in male babies was the result of evolution and not of function 14 .
Other investigators in Scandinavia have reported a contrasting fi nding to the one made in this study; they reported that parental birth weights correlated with birth weights of their babies 3 .Maternal and paternal birth weights were signifi cantly lower in families who had LBW babies compared with families with no LBW babies.To add, a LBW black mother stand a tripple chance of delivering a LBW baby, compared to double risk seen in their white counterparts 3 .Thus there is evidence of familial trends in determining birth weight of babies possibly due to autosomal genes rather than sex mediated genes.The placenta is critically involved in transporting nutrients and act as a barrier to infection, and fetuses have been observed with confi ned placental mosaicism 15 .Based on this, other researchers believed that cytogenetic abnormality in the placenta and not the sex of the fetus could be responsible for LBW 15 .How confi ned placental mosaicism affects fetal growth could presumably be related to alteration in placental functions.
Our inability to assess other possible factors such as information on parental birth weight, placental weight and structure that could contribute to LBW in babies was a drawback to this investigation and remains a limitation of this study.These limitations may have affected the adequate assessment of the association between sex and birth weight outcome of our babies.Future research incorporating these aspects is hereby advocated.

Conclusion
Sex was found to be a signifi cant determinant of birth weight of babies; with males having higher means birth weight than their female counterparts.

Table 2 :
Sex and birth weight profi le of the study population

Table 2
shows sex and birth weight profi le of the subjects.Overall mean (SD) birth weight of the babies was 3.02 (0.59), 95 CI (2.91 -3.14).