*2.2. Study Design and Procedures*

This was a prospective, interventional, non-pharmacological, and randomized controlled study, with a repeated-measure design.

Participants were randomly distributed in the experimental or in the control group according to a randomization with a 1:1 ratio obtained through a web-based computer system (randomization.com). The pregnant women in the control group received only routine prenatal care, which included emotional support from a midwife, the ability to take free positions during labor, massage and/or the application of hot packs in the lumbosacral area. The pregnant women in the intervention group received routine prenatal care plus the aromatherapy with vapor diffusion.

The essential neroli oil was diffused continuously through an aroma diffuser and using standard concentration at four drops of aroma oil per 300 mL of diffused water. The aromatherapy lasted the whole time of labor. Anxiety and pain intensity perception were assessed during 3 stages of labor: the latent phase (cervical dilatation of 3–4 cm), early active phase (cervical dilatation of 5–7 cm), and late active phase (cervical dilatation of 8–10 cm).

To better promote the spread of the active ingredients of neroli oil, a water (50%) and alcohol (32%) based formulation was made in which a percentage of 5% of the essential neroli oil was added. The formulation was completed by a phenolic antioxidant agent, BHT or butylhydroxytoluene, used at 0.1% and finished with the addition of PEG-40 hydrogenated castor oil, which has emulsifying functions, and propylene glycol, a carrier that makes the fragrance more lasting.

All participants provided written informed consent. The study was approved by the Ethical Committee of Psychological Research of the University of Naples Federico II (protocol number 2/2021; date of approval: 9 February 2021), designed with respect of the principles of the Declaration of Helsinki, and conducted following the EU General Data Protection Regulation. The clinical trial was retrospectively registered on the Deutsches Register Klinischer Studien (n◦ DRKS00027563).
