Author contributions

Arisa Ijuin and Kenichi Masui contributed to all aspects of this manuscript, including study conception and design; acquisition, analysis, and interpretation of data; and drafting of the manuscript. Masaki Sato, Nagayoshi Umehara, Shoichiro Amari, and Yasuyuki Suzuki contributed to the conception and design of the manuscript. Jumpei Saito contributed to the analysis of ropivacaine concentration and drafting of the article. Mayuko Abe, Wataru Matsunaga, and Yoko Yamashita contributed to the acquisition of data. All authors revised the manuscript and approved the final manuscript.

Acknowledgments

We thank all obstetric anesthesiologists, obstetricians, neonatologists, and midwives for caring deliveries at the National Center for Child Health and Development, Tokyo, Japan. We thank Reiko Ohara MD, PhD (Division of Obstetric Anesthesiology, National Center for Child Health and Development, Tokyo, Japan) for recruitment support. We thank Professor Takahiro Mihara (Department of Health Data Science, Yokohama City University Graduate School of Data Science, Yokohama, Japan) for his advice on statistical analysis. We also thank LSI Medience Corporation, Tokyo, Japan for measuring the fentanyl concentrations and Cactus Communications K.K. for English editing service.

Disclosures

Kenichi Masui received payments for delivering domestic lectures from Aspen Japan K.K. (Tokyo, Japan), Sandoz K.K. (Tokyo, Japan), Terumo Corporation (Tokyo, Japan), Maruishi Pharmaceutical Co., Ltd. (Osaka, Japan), Mundipharma K.K. (Tokyo, Japan), Masimo Japan Corporation (Tokyo, Japan), Covidien Japan Inc. (Tokyo, Japan), Pfizer Japan Inc. (Tokyo, Japan), and MSD K.K. (Tokyo, Japan) and for consultancy from Mundipharma K.K. (Tokyo, Japan), Terumo Corporation (Tokyo, Japan), and Nihon Kohden Corporation (Tokyo, Japan). Yasuyuki Suzuki received payments for delivering domestic lectures from Maruishi Pharmaceutical Co., Ltd. (Osaka, Japan), Mundipharma K.K. (Tokyo, Japan), Masimo Japan Corporation (Tokyo, Japan), Covidien Japan Inc. (Tokyo, Japan), Pfizer Japan Inc. (Tokyo, Japan), and Sanofi K.K. (Tokyo, Japan) and for consultancy from Masimo Japan Corporation (Tokyo, Japan). Nagayoshi Umehara received payments for delivering domestic lectures from Kaken Pharmaceutical Corporation (Tokyo, Japan) and for consultancy from Kaken Pharmaceutical Corporation (Tokyo, Japan) and Gyne Mom Corporation. (Saitama, Japan). The other authors declare no conflicts of interest.

Funding statement

National Center for Child Health and Development, and Yokohama City University

Prior conference presentations

This manuscript was presented in part at the 2nd Annual Meeting of the Japanese Association for Perinatal Anesthesiology (25–26 March 2023, Fukuoka, Japan) and was presented in part at the 55th Annual Meeting of the Society for Obstetric Anesthesia and Perinatology (3–7 May 2023, New Orleans, LA, USA).

Editorial responsibility

This submission was handled by Dr. Ronald B. George, Associate Editor, Canadian Journal of Anesthesia/Journal canadien d’anesthésie.