Department of Genetics and Microbiology, University of Pavia,via Abbiategrasso 207, 27100 Pavia, Italy
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Abstract |
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Key words: birth order/fetal gender/maternal age/preterm delivery
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Introduction |
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In order to investigate the importance of fetal sex in comparison with other factors, we examined the risk of preterm delivery in a recent sample of Italian births. In addition to fetal gender we included in the analysis three other characteristics well known to influence not only neonatal survival and health, but also pregnancy duration: maternal age at delivery, maternal educational level, and the baby birth order.
Maternal age, which has been widely reported to influence pregnancy outcome (Fretts et al., 1995; Lansac, 1995
; Breart, 1997
; Zonta et al., 1997
; Astolfi et al., 1999
) and which therefore might also affect pregnancy duration, is increasing in importance since the frequency of mothers bearing children at advanced age has been rising over the last 20 years. Maternal educational level, which is generally accepted as a reliable indicator of the socio-economic status of the family, might at least partially account for variation in gestational age, because of the differences in availability of antenatal care.
Babies of different birth orders, who have been shown to run different stillbirth risks (Astolfi et al., 2000), might also be differently exposed to the risk of a shortened pregnancy. At present, however, because of the reduced size of Italian families, a comparative analysis is suitable only between first- and second-borns. The analysis of only the first two orders has therefore avoided the problem of possible interactions between fetal gender and maternal age discussed by several authors in relation to deliveries of very high order (Almagor et al., 1998
; James, 1998
).
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Materials and methods |
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For each newborn, information was available on gender (SX), birth order (BO), gestational age (GA), maternal age (MA), and educational level (ED). Gestational age, estimated from last menstrual date, ranged from 20 to 44 weeks, and deliveries occurring before the 37th week were considered preterm, in accordance with the World Health Organization (WHO) definition (http://www.who.int/p11/ter/). The risk of preterm delivery was evaluated in mothers older than 30 years, grouped into three age classes (3034, 3539, 40), which covered the upper 50% in the maternal age distribution. As for maternal education, the best available indicator of the family socio-economic level, two levels (>8 and
8 years of schooling) were considered.
Data analysis
Univariate analyses were carried out to evaluate the effects of single factors on pregnancy duration and the risk of preterm delivery.
The relative risk of preterm delivery was estimated by logistic regression analyses, with fetal gender and birth order, the maternal age and education, and interactions of all factors included as explanatory variables.
Statistical analyses were performed by means of SPSS/PC 8.0 and SAS V6.10 procedures.
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Results |
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Birth order
In agreement with previous and current observations that second children are generally less prone to the risk of stillbirth (Ulizzi et al., 1998; Astolfi et al., 2000
), a significantly lower percentage of preterm babies was found among second- compared with first-borns (3.894% versus 4.426%; odds ratio (OR) = 0.875, 95% confidence interval (CI) 0.8630.887).
Maternal educational level
Antenatal care, which may be differently available according to family socio-economic and cultural levels, may have influenced not only the outcome but also the duration of the pregnancy. We therefore evaluated the risk of preterm delivery with respect to maternal education. As expected, among the less educated mothers a significantly higher quota of preterm deliveries occurred (4.4 versus 4.0%, OR 1.09 95% CI 1.0761.105), and the risk was not significantly different between first and second deliveries.
Maternal age
Mean pregnancy duration behaved as a second-degree function as maternal age increased: it reached the maximum length (39.5 weeks) in 22-year-old mothers and the absolute minimum (about 38.75 weeks) in mothers older than 41 years (Figure 1). However, the difference between the extreme values, which amounted to less than 1 week, seemed negligible from the biological point of view.
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Multifactorial analyses
The risk of preterm birth was evaluated by logistic regression as a function of the gender and birth order of the baby, the maternal age (from 30 years onwards), the number of years of schooling, and their interactions. Since no order interactions were found to be significant, the simplest model with only the main factors was considered. According to this model, first-born children had a higher risk than second-borns, and maternal age seemed to be the most important factor contributing to an increased risk of premature babies (Table I). The relative weights of low educational level and male gender were lower but still significant. From the total sample equation, the predicted minimum and maximum probabilities of a preterm birth, computed considering all favourable and all unfavourable conditions respectively, turned out to be Pmin = 0.033 and Pmax = 0.118. This was interpreted as meaning that a first-born male child delivered to an uneducated mother older than 39 years had a risk of being born preterm of more than 3.5 times higher than a second-born female child delivered to a well-educated mother aged 3034 years.
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Discussion |
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The high quota of preterm births in very young (<20 years) mothers (Figure 2) may be indicative of unfavourable socio-economic situations: the current average maternal age at delivery is approaching 30 years (Ulizzi et al., 1998
) and only 3.2% of mothers face childbirth at a very young age. In addition, it cannot be excluded that the weeks of gestation may be underestimated more frequently in poorly educated and/or in very young mothers (James, 1997b
).
Baby birth order, maternal education, and fetal gender were of descending importance though always significant. Second-born children, previously reported to be the least prone to stillbirth risk (Ulizzi et al., 1998), seem to be less vulnerable even to preterm risk; and in spite of the improvement in health care, less educated mothers still run a higher risk of preterm deliveries, probably due to the relative lack of prenatal care in unfavourable living conditions. Finally, the factor with the lowest weight in assessing a preterm delivery risk was found to be fetal gender.
In conclusion, in our sample we found a clear indication that advanced age of the mother, more than male gender of the baby, contributes an increased risk of preterm delivery. Biological, most probably hormonal, factors related to maternal ageing seem more important than the sex of the fetus in determining the premature onset of labour.
On the other hand, the excess of males among preterm babies is in agreement with the general finding of biological and even genetic weakness of male fetuses, witnessed by the high sex ratio observed in unfavourable pregnancy outcome. In fact a higher male proportion holds not only for stillborns, but also for spontaneously aborted fetuses (Cann and Cavalli-Sforza, 1968; Hassold et al., 1983
; Jakobovits, 1991
; Zonta et al., 1996
). It should be noted, however, that appropriate health care might increase the number of preterm babies born alive even after a very short gestation, i.e. those babies who otherwise would have been either aborted or stillborn.
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Acknowledgments |
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Notes |
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References |
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Submitted on May 6, 1999; accepted on July 14, 1999.