During a natural cycle, no drugs are used for the stimulation of the ovaries. The development of the single leading follicle and the maturation of the endometrium are monitored with a series of ultrasounds and hormone tests. Due to a high rate of premature ovulation and loss of the oocyte, the natural cycle has been replaced by the modified natural cycle.

During a modified natural cycle (MNC), antagonist and minimal doses of gonadotrophins are administered in the last days of the follicular phase. In this way premature ovulation is avoided. The advantages of natural cycles are no/minimal drugs, no side effects or complications, short duration, low cost etc.

The disadvantage is that from a single follicle we must retrieve a single oocyte, which must be mature, get fertilized, divide into a good quality, chromosomaly normal embryo with high developmental competence, capable of implanting in the uterus and result in pregnancy.

Indications for natural/modified natural cycle include poor ovarian response, several previous IVF attempts, the desire to avoid drug administration, and some rare contraindications of drug stimulation.