Re: Are there more male babies born or are there more females born?

The first thing that has to be said is that I dont think that one single figure can be given for the ratio of female to male births around the world because this ratio is affected by a number of environmental factors and practices in different cultures and societies. I will try and illustrate this by considering 4 or 5 different examples.

A study has been carried out by Jakobovits and Zubek(1991) in the Toldy Ferenc hospital in Hungary on the sex ratios in adolescent pregnancies where it was found that there was a slight reduction in the percentage of male births compared to that observed in non-adolescent mothers.

Tarrin et al(1995) have discussed whether in fact in vitro selection programmes carried out around the world may in fact be inadvertently affecting the ratio of female to male babies. In this study, three groups of IVF patients were described based on the number of cells present in the embryo transferred rerlative to the mean number of cells in all embryos that were ready for transfer. It was found that if you transfer an embryo that has a total number of cells less than the mean in the total number of embryos, then there is a very large increase in the number of female births (88.8% of births being female). If in fact you transfer an embryo that has a number of cells greater than that mean in the total number of embryos available for transfer , then the situation is reversed and you observe 90% of offspring being male.

So by selecting for fast cleaving embryos, those that have high numbers of cell divisions, and slow cleaving embryos, those that have a low number of cell divisions, it is possible to affect the sex ratios in the subsequent offspring (Tarrin et al, 1996). A second finding is that in fact maternal age has an effect on the ratio of male to female offspring. It was found that in patients below 35 years of age, 62.7% of births were males and in patients above 35 years of age 71.4 were female. So clearly these results would suggest that by selecting slow cleaving(slow dividing) and rapidly cleaving( fast dividing) embryos and by maternal age, it is actually possible to affect the ratio of female to male babies born through in vitro fertilization. A similar study carried out by Ng et al (1995) would seem to oppose these conclusions.

Another study has been carried out by Mordel et al(1995) to study the sex ratios of babies born as triplets following ovulation treatment in the mother. Thye results obtained from this were extremely interesting. Firstly such ovulation treatments seemed to increase the so called spontaneous incidence of triplet births from 1 in 10000 to 1 in 1000. In their study Mordel et al(1995) examined 36 triplet pregnancies that had occured in their hospital and 2717 triplet births that had occured elsewhere. It was found that of these, 58% of babies born were male whilst 42% were female. Triplets of the same gender were found to comprise 33% of all triplet births.

Finally Krishnamurthy and Joshi(1993) have assessed the potential involvement of tobacco on the ratio of male to female newborns. Here it was found that the ratio of males to females was 80.6 to 100 in mothers that 'took' tobacco compared to 105.5:100 in non-users of tobacco.

So whilst a general figure can be given for the overall ratio of females to males in a given society, the above discussion shows firstly that this ratio is not determined by a single factor and that this ratio may change over a given period of time dependent on the habits, attitudes and medical practices of people within that population. I hope that this answers your question. sorry for the delay in providing the answer.

Robert

Jakobovits AA, Zubek L Neonatal sex ratio in pregnancies of adolescent mothers. Acta Med Hung 48 (1-2): 45-49 (1991)

Krishnamurthy S, Joshi S Gender differences and low birth weight with maternal smokeless tobacco use in pregnancy. Trop Pediatr 39 (4): 253-254 (Aug 1993)

Mordel N, Mor-Yosef S, Simon A, Zajicek G, Schenker JG, Sadovsky E Sex ratio in triplets. J Perinat Med 23 (4): 265-271 (1995)

Ng E, Claman P, Leveille MC, Tanphaichitr N, Compitak K, Suwajanakorn S, Wells G Sex ratio of babies is unchanged after transfer of fast- versus slow-cleaving embryos. J Assist Reprod Genet 12 (9): 566-568 (Oct 1995)

Tarin JJ, Bernabeu R, Baviera A, Bonada M, Cano A Sex selection may be inadvertently performed in in-vitro fertilization-embryo transfer programmes. Hum Reprod 10 (11): 2992-2998 (Nov 1995)