In this study, a GWG of ≤ 5.7 kg at 30 weeks of gestation significantly increased the incidence of preterm births. Additionally, a GWG of ≤ 7.2 kg significantly increased the rate of delivery of SGA infants among Japanese UPW. The level of neonatal intensive care was also higher when the GWG at 30 weeks of gestation was ≤ 5.7 kg. Adverse outcomes that are known to increase with excessive GWG, such as CS, HDP, and macrosomia, revealed no statistically significant increases with weight gain. Contrary to the findings of previous studies, the proportion of GDM decreased with more GWG. These results suggested that firm weight restrictions for UPW may have led to an increase in the incidence of adverse pregnancy outcomes without reducing the risk of CS, HDP, GDM, and macrosomia.
This study demonstrated that adequate weight gain in UPW can reduce the rate of preterm births and delivery of SGA infants. A low pre-pregnancy BMI is, in itself, a risk factor for preterm births.11 However, in this study, the preterm birth rate was the highest in group A, while the rates in groups B, C, and D were similar to the Japanese average preterm birth rate (approximately 5.7% since 2000).7 This suggests that even if pre-pregnancy BMI is underweight, appropriate GWG can prevent preterm births. In the present study, the higher the maternal weight gain, the lower the proportion of SGA. According to the Developmental Origins of Health and Disease theory, LBW infants have higher risks of future academic deficits, type 2 DM, and cardiovascular disease.12–14 Therefore, the firm weight restriction for Japanese UPW can increase not only the incidence of birth rate of SGA neonates but also those of future diseases.
To the best of our knowledge, this is the first study to evaluate the association between antenatal maternal weight (not estimated weight at 40 weeks) and perinatal adverse outcomes among Japanese UPW. Based on our analysis, a GWG of > 5.7 kg at 30 weeks is recommended to prevent preterm births, and a weight gain of > 7.2 kg is required to reduce the rate of SGA infants. Pregnant women are advised to manage their own weight during pregnancy, and clinicians should monitor pregnant women’s weight at every prenatal visit.15 In most previous studies, the optimal GWG refers to the predicted estimated weight gain at 40 weeks or the weight gain per week.16 These studies were epidemiological studies using perinatal databases that included only the maternal weight data at the time of delivery, not the maternal weight data during pregnancy. The antenatal weight targets from our results are more useful for pregnant women to manage their own weight and prevent preterm births and reduce the rate of delivery of SGA neonates.
This study has some limitations. First, although all weights during pregnancy were measured using the same weighing scale, pre-pregnancy weight was based on self-reported weights. Second, as this was an observational study, we did not evaluate the methods of intervention. Therefore, it remains unclear if the mode of gaining weight (e.g., through diet) reduces perinatal adverse outcomes, such as preterm births and delivery of SGA neonates. Regardless of stern weight gain restrictions, underweight women may be less likely to gain weight during pregnancy. Furthermore, the proportion of underweight women in the younger population has increased in Japan. In 2017, 21.7% of Japanese women of reproductive age were underweight compared with 13.4% in 1981.17 Thus, preconception counseling is required, as well as education and awareness before pregnancy, as this would help pregnant women achieve an appropriate BMI.
In conclusion, a GWG of > 5.7 kg at 30 weeks is recommended to prevent preterm births and a weight gain of > 7.2 kg is required to reduce the rate of SGA neonates. Firm weight restrictions for UPW may have led to an increase in the incidence of adverse pregnancy outcomes without reducing the risk of CS, HDP, GDM, and macrosomia. Therefore, it is critical to revise the optimal antenatal weight gain range based on this study.