There are benefits as well as drawbacks of zona pellucida breaching. Narrow gaps in the zona may cause disintegration of the hatching trophectoderm, when embryos are cultured in vitro. An increase in the formation of monozygotic twins following micromanipulation may also occur, due to the forced separation of the inner cell mass during blastocyst expulsion in utero. Mouse studies indicate that one or several narrow zona openings (less than 5 microns) are detrimental. Such embryos may become trapped during hatching but may be rescued by drilling an additional larger gap elsewhere on the zona. The use of acidic Tyrode's solution for clinical assisted hatching of eight-cell embryos is currently under investigation. The findings suggest that large holes are efficient for promoting hatching (at least one-fourth of the embryos implanted thus far) and that embryos with unthinned zonae (those with normally the poorest prognosis) benefit mostly from assisted hatching. Results also indicate that embryos with the best zona morphology should be replaced without micromanipulation.