Association of grand multiparity with adverse birth outcomes and sociodemographic characteristics: an analysis of nationwide birth data in Japan

In Japan, grand multiparity’s associations with representative adverse birth outcomes such as preterm birth and low birth weight and the sociodemographic features of grand multiparity have not been investigated yet. We investigated these using nationwide live birth data. Live birth data of the Vital Statistics: Occupational and Industrial Aspects in Japan in 2010, 2015, and 2020 were used. Parity was categorized into primiparous (1 delivery), non-grand multiparous (2–4 deliveries), and grand multiparous (≥ 5 deliveries). Preterm birth (< 37 gestational weeks), low birth weight (< 2,500 g), macrosomia (≥ 4,000 g), small-for-gestational-age infant (weighing less than the 10th percentile for gestational age), and large-for-gestational-age infant (weighing more than the 90th percentile for gestational age) were considered adverse birth outcomes, while the parental age groups, parental nationalities, parental occupations, and household occupation were considered sociodemographic characteristics. A log-binomial regression model was used to investigate the association between grand multiparity and the chosen adverse birth outcomes, as well as its sociodemographic features. In total, 2,623,696 singleton live births within marriage were used for the analysis. The proportions of preterm birth, low birth weight, macrosomia, and large-for-gestational-age infant for grand multiparous women were higher than those of primiparous and non-grand multiparous women, and grand multiparity was significantly associated with a higher risk of those adverse birth outcomes. In addition, a non-Japanese mother was significantly associated with a higher prevalence of grand multiparity, while households with a full-time worker at a larger company and upper non-manual workers had a significantly lower prevalence of grand multiparity than the other household occupations and parental occupations, respectively. Grand multiparity was significantly associated with a higher risk of adverse birth outcomes. In addition, sociodemographic characteristics such as maternal nationality and parental occupations were shown to be associated with grand multiparity.