The egg – as well as the embryo – is surrounded and protected by a shell, the zona pellucida. Shortly before the embryo nests in the uterine mucosa the zona pellucida opens – with the help of special enzymes and with the pressure of the embryo’s growth – and the embryo “hatches”.

Some studies indicate that the in-vitro culture of the embryo – and the cryo-preservation – can lead to hardening of the zona pellucida and so complicate the “hatching” or prevent its occurrence.

• to patients with multiple unsuccessful IVF or ICSI attempts, in spite of them having “good embryos";
• to patients over 38 years of age;
• if embryos have a thickened zona pellucida or if embryos that developed from cryo-preserved fertilized egg cells are transferred.