Endometrial preparation for frozen embryo transfer is a relatively simple procedure for the woman. In classic preparation cycles, (with hormone treatment) the woman takes a course of estrogens orally or transdermally (for approximately two weeks) to achieve good endometrial thickness. This is followed by supplementation with progesterone vaginally and/or subcutaneously starting five days before injecting the embryo into the uterine cavity.

This pattern with hormone replacement therapy offers greater cycle planning and fewer follow-up controls, however, for some patients, medication is a risk factor and for this reason, increasingly, in older women with regular menstrual cycles, we opt for the preparation in natural cycle.

This natural cycle strategy consists of monitoring a woman's physiological cycle. Ultrasound controls and some hormonal analysis are required to see how the follicle selected in that month is growing and progressively producing its own level of estradiol that helps to thicken the endometrium in a natural way. Once we are sure that the follicular size is correct we perform the ovulatory discharge with HCG to ensure that there is ovulation that will help the formation of the corpus luteum and produce correct levels of intrinsic progesterone. 

Thanks to this own production of estradiol and progesterone the endometrium will be naturally prepared for the day of the transfer.

The preparation in natural cycle requires a closer and more meticulous follow-up of the patient, therefore more face-to-face controls are necessary than in the artificial cycle, but the medical advantages for the woman and the future pregnancy are more and more scientifically evident, in addition, the economic cost in medication is lower.

Cryotransfer in natural cycle is related to higher hormone levels and therefore favorable for pregnancy and implantation, as well as less placentation problems, i.e., less risk of hypertension and preeclampsia during pregnancy.