Some interest has been generated in studying the rationale for assisted hatching . some reports suggest that embryos with lower zona pellucida thickness ( zpt ) and higher zona pellucida thickness variation ( zptv ) had a better chance of successful hatching and so the patient become pregnant ; another reports suggest that in vivo aging ; in vitro culture and exposure to ultralow temperature may induce zona hardening and then impair the embryo ability of hatching 有作者提出胚胎透明带厚度( zonapellucidathickness ， zpt )小的胚胎孵出时阻力减小，而透明带厚度变化率( zonapellucidathicknessvariation ， zptv )高的胚胎由于透明带上有一个明显薄的区域而使胚胎易于孵出；还有作者推测病人年龄、体外培养天数的增加及冷冻处理会增加透明带的硬度，使胚胎孵出困难。
