Department of Obstetrics Gynaecology and Paediatric Sciences, Gynaecology Unit, Policlinico of Modena, Via del Pozzo 71, 41100 Modena, Italy
1 To whom correspondence should be addressed. e-mail: cagnacci{at}unimore.it
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Abstract |
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Key words: body weight/conception/metabolism/sex ratio
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Introduction |
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Materials and methods |
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Pre-pregnancy weight, weight gain during pregnancy and weight at delivery were divided in quartiles, and sex ratio was stratified accordingly to the weight quartiles. In order to exclude the possibility that quartiles of weight gain during pregnancy are related to a different length of gestation or pre-pregnancy weight, length of gestation and pre-pregnancy weight was calculated for each quartile of weight gain during pregnancy. Contingency tables and the 2-test were used to perform the statistical comparison between the different quartiles. A P-value of 0.05 was used to test the null hypothesis. The results are reported as mean ± SEM.
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Results |
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Mean age ± SEM of the women was 31.12 ± 0.27 years, and did not stratify differently among the weight quartiles. Mean pre-pregnancy weight was 61.14 ± 0.05 kg, mean weight gain during pregnancy was 13.0 ± 0.02 kg and mean weight at time of delivery was 74.15 ± 0.05 kg. Weight limits defining the quartiles of pre-pregnancy weight, weight gain during pregnancy and weight at delivery are reported in Table I.
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Weight at delivery is the result of pre-pregnancy weight and weight gain during pregnancy. The sex ratio differed among quartiles of weight at delivery, with no clear trend (Table II).
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Discussion |
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Weight gain during pregnancy was also associated with modifications of secondary sex ratio. In particular, the greatest increase in weight was characterized by a decrease in the sex ratio. This was not observed in a recent study performed in a more limited number of pregnancies (Tamimi et al., 2003). A greater increase in weight may be the consequence of an increased duration of gestation. Because pregnancies terminating at different weeks of gestation show a different proportion of male offspring (Cooperstock and Campbell, 1996
; Bernstein 1998
; James, 2000
), a difference in length of gestation might explain the difference observed among the quartiles of weight gain. However, this possibility seems to be excluded by our analysis, which shows similar lengths of gestation in the four quartiles of weight gain. It is also possible that a greater increase in weight during pregnancy is the consequence of a low pre-pregnancy weight. Accordingly, women gaining more weight could be the same as the 1st quartile of pregnancy weight, i.e. those showing a reduced sex ratio in their offspring. This possibility also seems to be excluded by our analysis, which shows a similar pre-pregnancy weight among the four quartiles of weight gain. Weight gain during pregnancy may modify the secondary sex ratio only after conception and implantation, through an increase in the loss of male embryos. However, an alternative explanation is possible. The excessive increase of weight during pregnancy may indicate a metabolic tendency of the woman towards energy storage, rather than energy utilization, for reproductive processes. These mechanisms may be operative before pregnancy, and may influence sex ratio as early as at time of conception/implantation. Indeed, a reduction of sex ratio has been reported in women with diabetes (James, 1998
; Rjasanowski et al., 1998
), probably as the consequence of an altered availability of essential fatty acids for the embryo (Crawford et al., 1998
).
Although the mechanisms underlying sex selection are still unclear, the data seem to indicate that males are more fragile (Naeye et al., 1971), and that a greater attrition is exerted on them when reproductive/metabolic conditions are non-optimal (Crawford et al., 1998
). This view is supported by the documented decline of the sex ratio in the cases of unfavourable seasons (Cagnacci et al., 2003
), environmental pollution (James, 1996
; Astolfi and Zonta, 1999
; Mocarelli et al., 2000
), destructive earthquakes (Fukuda et al., 1998
), smoking parents (Fukuda et al., 2002
), and aged mothers (Juntunen et al., 1997
; Orvos et al., 2001
) or fathers (Jacobsen et al., 1999
; Nicolich et al., 2000
). In particular, the age of the parents becomes important in multiple pregnancies, when energy availability for the fetuses is more critical (Pollard, 1969
; Juntunen et al., 1997
; Jacobsen et al., 1999
; Nicolich et al., 2000
; Orvos et al., 2001
).
The present study has several limitations. Most of the data were based on anamnesis, and errors could have been made in the exact recall of pre-pregnancy weight. The height of the women was not recorded in the hospital charts, and it was not possible to calculate the body mass index, which would have allowed a better quantitation of fat stores. In addition, smoking habit data were missing from most of the records, and a different distribution of this parameter among the different quartiles cannot be excluded.
On the other hand, the data rather strongly indicate maternal weight, and probably maternal metabolism, as important determinants of secondary sex ratio in human offspring. The interaction of these parameters with other determinants of human sex ratio needs to be addressed in focused studies.
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References |
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Submitted on September 2, 2003; accepted on October 6, 2003.