We thank Abengochea et al. for their interest in our article and their comments.1 We agree that there are individual variations in fetal weight and accessory structures, such as the placenta and amniotic fluid, which may affect the degree of compression of the inferior vena cava (IVC) by the gravid uterus in the supine position and changes in the IVC volume in the 15° left-tilt position. Certainly, these variations may account for the differences between the four parturients in our study in which IVC volume was somewhat increased in the 15° left-tilt position and the other parturients in which IVC volume was not increased compared with those in the supine position. It should be noted, however, that the principal aim of our study was to challenge an “ancient” practice, based on an assumption that had never been morphologically validated.1,2 We did not aim to perform a multivariate analysis of the characteristics of pregnant women. The sample size in our study was determined to evaluate whether there was a significant difference in the IVC volume between pregnant and nonpregnant women. Although we do not know how the left-lateral position is applied in the hospital of Abengochea et al., we believe that, in general, the left-lateral position is routinely used, not only in those limited parturients with supine hypotensive syndrome or obesity but also in healthy pregnant women without consideration of the fetal weight and accessory structures, such as the placenta and amniotic fluid. In addition, in our opinion, the study design should be simple. Therefore, we limited our study to healthy pregnant women and excluded parturients with supine hypotensive syndrome or obesity.
As we stated in our article, and Palmer also pointed out in the editorial view accompanying our article, our study has several drawbacks. One drawback of our study is the small number of subjects (only 10 subjects per group).1,2 Accordingly, the statistical power of our study was not sufficient to perform a multivariate analysis. However, we understand that Abengochea et al. are curious about individual variations in fetal weight and accessory structures. The estimated fetal weight based on ultrasound images and abdominal characteristics of the pregnant women when magnetic resonance images were obtained are listed in table 1. The pregnant women numbers was the same as in table 2 in our article.1 The relation between estimated fetal weight and IVC volume in the supine position was not significant. The estimated fetal weight of the four parturients (patient numbers 1, 3, 5, and 9) in which the IVC volume was somewhat increased in the 15° left-tilt position was significantly different from that of the other parturients in which the IVC volume was not increased. The same results were obtained when comparing abdominal characteristics.
The authors declare no competing interests.