Chemical removal of the outside of the zona pellucida of day 3 human embryos has no impact on implantation rate.

Two hundred eighteen consenting patients entered a randomized study of the application of chemical zona pellucida thinning on their day 3 embryos, prior to uterine transfer. Of those control patients (n = 108), whose embryos remained unmanipulated, 40 (37.0%) have ongoing/delivered pregnancies, while in the experimental group (n = 110), whose embryos had their zonae pellucidae chemically thinned, there are 49 patients (44.6%) who have ongoing/delivered pregnancies. Although this difference is not significant, clearly the application of this micromanipulative intervention has not been detrimental, and this bodes well for routine application of embryonic micromanipulation procedures in general. Certain patient subgroups were studied including older women, those with elevated basal follicle stimulating hormone levels, patients with embryos of differing zona thickness, and patients with embryos of differing uniformity of zona thickness. No significant influence of chemical removal of the outside of the zona on the implantation rate of embryos in any of these subgroups was observed other than a marginally significant (P = 0.095) improvement of implantation of embryos with less than 4.0 microns variation in zona thickness when chemical zona thinning was applied. Failure of chemical zona thinning to enhance human embryo implantation significantly, compared to assisted hatching by complete zona drilling, strongly suggests that the bilayered human zona pellucida needs to be fully breached, unlike that of the mouse.