To assist or not to assist embryo hatching

Abstract

At early stages of development, the human embryo is protected by a surrounding two-layer coating—the zona pellucida. As the embryo develops further, this coating becomes thinner and, at the blastocyst stage, the thin coating is broken off and the embryo ‘hatches’ such that it can implant into the uterine wall. Following in vitro fertilization and in vitro growth, many embryos do not implant after embryo transfer. Cultured embryos are known to hatch and implant at a lower rate than naturally developed embryos in vivo. This lower hatching rate could be attributed to a thicker zona pellucida. A thicker zona has, for instance, been seen in older women and women with high basal FSH levels. It was therefore suggested that making a hole in or thinning of this outer layer may help embryos to ‘hatch’, thus increasing the implantation rates. In 1990 such an assisted hatching procedure was developed (Cohen, 1991).