In a normal cycle, after ovulation, progesterone begins to rise so that the endometrial tissue can be modified to allow implantation and the development of the eventual embryo. When we perform an embryo transfer, what we need is to reproduce this situation so that the embryo can implant.
Normally, the progesterone treatment starts a few days before the embryo transfer. In case of transfer in the same cycle as the follicular puncture, it starts the same day of the puncture, otherwise, it varies according to the stage of embryo development.
In case of transfer in a natural cycle (when no hormonal treatment is given to prepare the endometrium), progesterone is started on the same day of the transfer, but it is very unlikely that progesterone does not have to be taken after the embryo transfer because it is always preferred to support the luteal phase with exogenous progesterone.