It is not always possible to do a transfer of frozen embryos in a natural cycle. First of all, it is necessary for the woman to have regular menstrual cycles and to be normo-ovulatory, that is, to ovulate every month normally.
In addition, the natural cycle has other disadvantages. For example, there is less flexibility in the dates of the ultrasound examinations and in scheduling the transfer, since the window of implantation of the endometrium is very limited. On the other hand, the probability of cycle cancellation due to early ovulation is greater.