The findings of this study suggest that advanced and young maternal ages were respectively associated with adverse pregnancy outcomes after adjustment for potential confounders. Both older and younger pregnant women were at increased risk of preterm birth, low birth weight, an Apgar score < 7 at 5 min, and fetal death. However, the risk patterns of some outcomes may differ for older and younger women. Specifically, younger women had a higher risk of SGA while older women had negligible difference in risk for this outcome compared to 20–34 years old. In addition, maternal age was positively correlated with the risk for GDM, gestational hypertension, preeclampsia, PROM, postpartum hemorrhage, cesarean delivery, and fetal distress.
AMA is associated with increased odds of adverse maternal outcomes, whereas YMA is just the contrary. The incidence of GDM generally increases with maternal age. Several population-based studies showed that when compared to pregnant women with appropriate age, those at an AMA were more likely to have a higher risk of GDM[10, 11], while those with YMA were less likely to develop GDM[6, 22], which was in accordance with our study. This increasing trend may be attributed to the progressive decline of pancreatic β-cell function with aging, leading to a reduced insulin affinity[23]. The risk of gestational hypertension or preeclampsia also increases continuously with increasing maternal age. Consistent with other studies[6, 8], we noted that the risk of gestational hypertension and preeclampsia increased in women with AMA and decreased in those with YMA compared to controls. The opposite phenomenon between older and younger women may be explained by biological ageing that was associated with decreased nitric oxide availability or elevated production of oxidative stress, which may result in impaired uterine and endothelial vascular function[24]. We also observed a linear relationship between maternal age and PROM, postpartum hemorrhage, and cesarean delivery. Pregnant women with AMA had a higher risk of PROM, postpartum hemorrhage, and cesarean delivery while those with YMA had a lower risk for these outcomes compared to 20-34-year-old women, which were in line with those of studies conducted in Iran[25] and other areas of China[6, 10]. The increasing prevalence of GDM, gestational hypertension, preeclampsia and prior cesarean with aging may contribute to these trends[26, 27]. In addition, AMA was associated with an elevated risk of placental abruption[11], placenta previa[28], abnormal fetal position[8, 29], and polyhydramnios[10], which may be linked to uteroplacental ischemia[30] and higher proportions of infertility treatment, prior abortion, and pregnancy-induced diabetes or hypertension.
In addition, both advanced and young maternal ages are associated with adverse infant outcomes. In our study, pregnant women aged 35 years or older were more likely to have fetal distress, while those less than 20 years old had an inverse trend after adjusting for potential confounders. This was illustrated in several studies, which found a significant increase in the risk of fetal distress with advancing maternal age[31–33]. AMA and YMA were both correlated with an elevated risk of preterm birth and low birth weight. Yet, only YMA was associated with an increased risk of SGA, implying that both premature delivery and intrauterine growth restriction may contribute to the underweight state of newborns whose mothers were younger than 20 years. In comparison, preterm birth instead of growth restriction was the primary cause of low birth weight among those aged 35 years or older. Consistent with our findings, the population-based data from the Taiwan Birth Notification System showed a U-shaped association between maternal age and the risk of preterm birth and low birth weight. In contrast, there was an L-shaped association between maternal age and the odds of SGA[34]. A multicenter retrospective study conducted in mainland China indicated that pregnant women at an AMA or YMA appeared to have a higher risk for preterm birth and low birth weight, and the prevalence of SGA seemed to be decreased with advancing age[35]. However, the relationship between maternal age and the risk of SGA is still controversial. Several retrospective studies conducted in the United States[33], Sweden, and Norway[36] found that AMA was also associated with a significantly increased risk of SGA, which was contrary to the studies performed in China[34, 35]. This discrepancy may be explained by the difference in race between whites and yellows[13]. We also found a bidirectional trend that the risk of macrosomia was upward to downward with increasing maternal age, with a peak age of 35–39 years, which was consistent with the study of Zhang et al.[37]. Besides, elevated risks of fetal death and low Apgar score at both younger and older maternal ages were found in our research. The findings were in line with those of studies conducted in the United States[38], Thailand[39], and China[34, 37]. Advancing maternal age is accompanied by an increased risk of sclerotic lesions in the myometrial arteries that could cause reduced uteroplacental perfusion and consequently results in placental dysfunction, which is associated with a wide range of adverse pregnancy outcomes[40].
The strengths of our study are the quality of data sets and the assessment of both advanced and young maternal age affecting pregnancy outcomes. However, there are two limitations to note in this study. First, as a multi-institution-database-based study, it is difficult to standardize the measuring instruments used by different hospitals and may thus lead to detection bias; however, due to the comparable proportion of these groups in each hospital, the impact of this bias may be minor. Second, some potential confounders, such as maternal educational level and body mass index, are necessary to consider when evaluating the risks for maternal and infant outcomes but are unavailable in the dataset.
In summary, this study found a close association between maternal age and the risk of adverse pregnancy outcomes. AMA is an independent risk factor for all adverse pregnancy outcomes except SGA. However, YMA has a bidirectional effect on pregnancy outcomes. YMA was associated with increased risk of preterm birth, low birth weight, SGA, low Apgar score, and fetal death, but decreases the odds of obstetric complications, macrosomia, and fetal distress. Given the health hazards of younger or older pregnancies, extreme young pregnancies should be avoided, and perinatal care for older pregnant women should be strengthened to prevent these complications or outcomes.