STUDY QUESTION
Are there any differences in physical and psychosocial well-being among women undergoing modified natural cycle frozen embryo transfer (mNC-FET) with or without vaginal progesterone as luteal phase support (LPS)?

SUMMARY ANSWER
Women undergoing mNC-FET with vaginal progesterone supplementation were more likely to experience physical discomfort but there was no difference in psychosocial well-being between the two groups.

WHAT IS KNOWN ALREADY
mNC-FET can be carried out with or without vaginal progesterone as LPS, which has several side-effects. It is commonly known that fertility treatment can cause stress and psychosocial strain, however, most studies on this subject are conducted in fresh cycle regimes, which differ from NC-FET and results may not be comparable.

STUDY DESIGN, SIZE, DURATION
This is a sub-study of an ongoing RCT investigating whether progesterone supplementation has a positive effect on live birth rate in mNC-FET. The RCT is conducted at eight fertility clinics in Denmark from 2019 and is planned to end primo 2024. The sub-study is based on two questionnaires on physical and psychosocial well-being added to the RCT in August 2019. On the time of data extraction 286 women had answered both questionnaires.

PARTICIPANTS/MATERIALS, SETTING, METHODS
Women who had answered both questionnaires were included in the sub-study. Participants were equally distributed, with 143 in each of the two groups. Participants in both groups received the same questionnaires at two time-points: on cycle day 2–5 (baseline) and after blastocyst transfer. Participants in the progesterone group had administered progesterone for 7 days upon answering the second questionnaire. All items in the questionnaires were validated. Items on psychosocial well-being originate from the Copenhagen Multi-Centre Psychosocial Infertility—Fertility Problem Stress Scale (COMPI-FPSS) and from the Mental Health Inventory-5.

MAIN RESULTS AND THE ROLE OF CHANCE
Women receiving progesterone experienced more vaginal itching and/or burning than women in the non-progesterone group (P < 0.001). Women in the progesterone group also experienced more self-reported vaginal yeast infection, this was, however, not significant after adjustment for multiple testing (P/adjusted P = 0.049/0.881). No differences regarding psychosocial well-being were found between the two groups. Within the progesterone group, a shift toward feeling less ‘downhearted and blue’ was found when comparing response distribution at baseline and after blastocyst transfer (P < 0.001).

LIMITATIONS, REASONS FOR CAUTION
All items on physical symptoms were self-reported. The item on vaginal yeast infection was therefore not diagnosed by a doctor. Inclusion in the study required a few extra visits to the clinic, participants who felt more burdened by fertility treatment might have been more likely to decline participation. Women who experienced a lot of side-effects to progesterone prior to this FET cycle, might be less likely to participate.

WIDER IMPLICATIONS OF THE FINDINGS
Our results are in line with previous known side-effects to progesterone. Physical side-effects of progesterone should be considered before administration.

STUDY FUNDING/COMPETING INTEREST(S)
The RCT is fully supported by Rigshospitalet’s Research Foundation and a grant from Gedeon Richter. Gedeon Richter were not involved in the design of protocol nor in the conduction of the study or analysis of results. A.P., L.P., and N.I.-C.F. report grants from Gedeon Richter, Ferring and Merck with no relations to this study. N.I.-C.F. has received travel support from Ferring, Merck A/S, & Gideon Richter, and is the head of the steering committee for the Danish Fertility Guidelines made by the members of from the Danish Fertility Society. A.P. reports consulting fees from Preglem, Novo Nordisk, Ferring, Gedeon Richter, Cryos, & Merck A/S, honoraria from Gedeon Richter, Ferring, Merck A/S, Theramex, and Organon, has received travel support from Gedeon Richter (payment to institution), participated on an advisory board for Preglem and was loaned an embryoscope from Gedeon Richter to their institution. A.L.S. has stock options for Novo Nordisk B A/S. B.A. have received unrestricted grant from Gedeon Richter Nordic and Merck and honoraria for lectures from Gedeon Richter, Merck, IBSA, and Marckyrl Pharma.

TRIAL REGISTRATION NUMBER
The RCT is registered on ClinicalTrials. gov (NCT03795220) and in EudraCT (2018-002207-34).