Department of Obstetrics Gynecology and Pediatric Sciences, Gynecology 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: conception/delivery/rhythms/seasons/sex ratio
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Introduction |
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Materials and methods |
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For the institute database, sex ratio was stratified according to both the month of conception and the month of birth. The length of gestation of each single pregnancy, and the sex ratio of pregnancies terminated at different weeks of gestation was also calculated. For the County database, sex ratio was stratified according to the month of birth. Circa-annual rhythmic distribution of sex ratio and number of conceptions of vital pregnancies was evaluated by the periodogram method using the RHYTHM program (Van Cauter, 1979). The periodogram was adapted to analyse the 12 months rhythm. As originally described (Van Cauter, 1979
), the method initially tests the significance of the observed time fluctuations against the hypothesis of their purely random occurrence via two different tests. The alternative to pure randomness is, for the first test, the existence of local correlations, implying that values at given times depend on values at other times, and for the second test, the existence of periodic fluctuations. When the hypothesis of random occurrence of the data is rejected, the periodogram method is applied to detect and estimate the possible significant components. A sum of sinusoidal components with periods equal to integer divisors of the observation span (i.e. 12 months/1; 12 months/2; 12 months/3; etc.) is fitted on the series. A decision procedure devised by Fisher (Van Cauter, 1979
) allows the selection of the significant periodicity underlying the process at a given probability of P < 0.05. Up to the first three significant periodical components are included in the theoretical description of the profile. A theoretical pattern is computed according to the minimum of the residual sum of squares. The amplitude of the theoretical pattern is defined as half the difference between the maximum and minimum of the theoretical pattern, while the acrophase is the time corresponding to the occurrence of the first global maximum of the theoretical curve. Both the amplitude and the acrophase are furnished with a confidence interval. Periodograms are considered significantly different when described by different periodical components or alternatively when the confidence intervals of either the amplitude or acrophase do not overlap. Accordingly, the periodogram analysis estimations of the relative contributions of low and high components in the time dependence of the profile and indications regarding the frequency range and the periodicity or non-periodicity of any components are obtained. Contingency tables and the
2-test were used to perform the statistical comparison among sex ratio at different times of gestation and the sex ratio observed in the different seasons of the year.
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Results |
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The analysis of the institutional medical records and the historical registry of the Modena county showed a slight circa-annual fluctuation of sex ratio evaluated at time of delivery. Fluctuations were similar for both analyses with peaks of sex ratio in March and September. In both analyses, these fluctuations were not significant at the periodogram evaluation.
From the institutes medical record we calculated that the sex ratio of fetuses born between 3037 weeks (n = 2237) gestation was significantly higher than that observed in fetuses born between 4042 weeks (n = 6013) gestation (0.525 versus 0.498; P < 0.02, 2). Intermediate sex ratio was observed in fetuses born between 3839 weeks gestation (n = 5867; sex ratio = 0.519). Accordingly, at conception the rhythm of sex ratio was different from that at birth.
Indeed, sex ratio calculated at the time of conception showed a significant seasonal rhythm (period = 12 months) with an amplitude of 2.4% and a theoretical maximum in October (±43 days) (Figure 1). The sex ratio was significantly higher in the 3 months of the peak (September, October and November) than in the 3 months of nadir (March, April and May) (0.530 versus 0.504; P < 0.05).
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Discussion |
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References |
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Submitted on November 21, 2002; accepted on January 8, 2003.