Cryopreservation
		After a sperm has penetrated deep inside the egg, an egg in the pronuclear stage (i.e., before the maternal and paternal chromosomes have fused) can be deep-frozen. The advantage is that such egg can be utilized in other cycles without the necessity of hormonal stimulation and surgical harvesting of eggs. The chances of success of each stimulated cycle can be considerably enhanced in this manner. Technique: Eggs in a special fluid (cryoprotective fluid) are dipped slowly (within two hours) into liquid nitrogen (-196 °C). They are stored in individual portions in so-called straws, which can be stored in liquid nitrogen for an unlimited time. Thawing For thawing, the temperatue of the deep-frozen straws is gradually increased to room temperature, or 37 °C. The eggs are rinsed from the straws and incubated for 24 to 48 hours. During this time, the pronuclei unite (conjugate) and cell cleveage begins. When they have reached the 8-cell stage, the embryos are transferred to the uterine cavity. In most cases, transfer of cryopreserved cells is performed during a natural cycle. The time of ovulation must be determined precisely by ultrasound and hormone assays; one day later, the pronuclei - oocytes are thawed, and transfer is performed three to five days after ovulation. As in IVF, the luteal phase is supported by Utrogest™ (progesterone).