We examined the independent effects of maternal age, delivery period, and maternal birth cohort on the trends in LBW, PTB, and SGA births in Shanghai. The “U-shaped” relationship between maternal age and LBW/PTB was examined in this study. Mothers born before the 1980s had a lower incidence of PTB than those born recently. Meanwhile, the risk of SGA declined with advancing age and in cohorts since 1960. However, there were no obvious fluctuant trends in the three birth outcomes by period, suggesting that the observed temporal changes were mostly influenced by the maternal birth cohort.
Our findings on the association between maternal age, birth cohort, and LBW/PTB are consistent with those of previous studies [18, 19]. Extremes of maternal age increased the incidence of LBW/PTB, suggesting that natural ageing or social environments, or an interaction of both, should account for the association. Generally, older women are believed to have more obstetric complications, which in turn is a high-risk factor for ABOs. Young mothers, whose reproductive system is not biologically developed, are more likely to have a lower degree of education and socioeconomic strata [20].
We noted that the maternal birth cohort remarkably affected LBW/PTB. This pattern reflects that unique individual characteristics, including maternal nutrition and socioeconomic status prior to pregnancy, could profoundly influence pregnancy outcomes [21]. One potential explanation is that social and economic changes, such as experiencing remarkable lifestyle changes, increasing environmental pollution, and higher stress, may affect the female reproductive system [22, 23]. Maternal smoking, secondhand smoke exposure, drinking, and sedentary behaviour are the main unhealthy lifestyle factors leading to a higher prevalence of ABOs [24, 25]. An epidemiological study has also found that exposure to traffic-derived pollutants increases the risk of LBW, PTB, and growth retardation [26].
Interestingly, younger maternal age was associated with SGA in this study, while some studies have elucidated an opposite association [27–31]. The debatable association could be attributed to several factors: (1) various fetal growth curves or birth weight percentiles used to define SGA [32]; (2) ethnic or genetic differences across populations [33, 34]; and (3) the possibility of some small fetuses to achieve their normal growth potential. Although it is difficult to explain the decreased trend of SGA, we hypothesised that mothers could have benefited from accessible prenatal interventions and nutritional improvements. Due to rapid urbanisation since the 1980s, the nutritional and health status of Shanghai residents has greatly improved, and more fertility policies have been promoted [9], meaning that women born in more recent cohorts were unlikely to have SGA births. However, the opposite influence was found for mothers with higher education levels and primiparous mothers.
To the best of our knowledge, only a few studies have identified maternal APC effect on PTB/SGA [18, 30]. Although our study aimed to analyse the temporal influence of LBW, PTB, and SGA births, several important limitations should be considered. First, other determinants of ABOs, including maternal smoking, gestational weight gain, pregnancy complications, and paternal factors, were not collected in the database, which could illuminate the internal mechanisms of maternal age and cohort effects on ABOs [24, 35]. Second, the estimated gestational age, based on the first date of a woman's last menstrual period and not on ultrasound-based methods, may not accurately classify PTB/SGA infants. Although misclassification might influence the results mentioned above, it is unlikely to contribute to the temporal trends entirely. Third, the data was collected from a single birth registry database, which does not represent the nationwide population. However, Shanghai is a megacity with a large population (almost 25 million), which could be representative of the other developed cities in China and some Asian developed countries.
Our study suggests independent effects of maternal age, delivery period, and maternal birth cohort on trends in ABOs. Within the context of the universal 2-child policy, more women of advanced age prefer to raise a second child in China [36]. Although older women obtained better education and higher socioeconomic strata through social selection, they were more likely to suffer from obstetric complications. Both young and advanced mothers are more likely to have LBW/PTB; accordingly, more prenatal care and public education should be provided to younger and older pregnant women.