Serum oestradiol concentration at the time of frozen embryo transfer (FET) in artificial cycle are lower when using transdermal administration of oestrogen for endometrial preparation compared to the vaginal route. This difference could have consequences for placentation and establishment of maternal-foetal circulation. The aim of our study was to compare the birth weight of newborns and the perinatal issues after FET in an artificial cycle with regard to the route of administration of oestrogens.

Retrospective monocentric cohort study in the medically assisted reproduction department of the University Hospital of Angers, France, between January 2017 and October 2020. Inclusion criteria were age >18 years old and one live birth after FET in an artificial cycle. The main outcome was the birth weight of the newborns. The choice of oestrogens administration (transdermal or vaginal) was left to the patient.

804 FET in artificial cycle were included in our study. Oestrogens were administrated in 356/804(36.6%) patients using transdermal route and in 448/804(45.9%) patients using vaginal route. There were 68/345 (19.1%) live births in the transdermal group and 85/448 (19%) in the vaginal group. There was no difference in the birth weight of the newborns (3320[2100-4165] grams in the transdermal group vs 3327.5[915-4650] grams in the vaginal group, p=0.72). All the other perinatal issues were comparable between the two groups.

Birth weights and perinatal issues were comparable with regard to the route of administration of oestrogens (vaginal or transdermal) in the context of endometrial preparation before FET in an artificial cycle.