After the publication of the original article (Kikuchi et al., 2021), the authors noticed that the article contained some errors in the manuscript. The type of specimen to be analyzed in the study was serum, not plasma. Progesterone, estradiol, and testosterone can be measured in either serum or plasma (World Health Organization, 2002). The context of the findings, which is that the delivery of a placenta/fetus with high gonadal steroid production may cause a wider range of fluctuations in maternal gonadal steroid concentrations in the circulation, is not affected after this correction. The authors apologize for any inconvenience this has caused.