Assisted Hatching

The natural growth phases of embryos, from conception in the fallopian tubes to implantation into the uterus, are well defined. During fertilization, sperm must penetrate a hard shell surrounding the egg, called the zona pellucida. Once such penetration occurs, the zona hardens to prevent additional sperm from fertilizing the egg. Following initial interaction between sperm and eggs, zygotes (fertilized eggs) are formed. In the following three days, each embryo undergoes cleavage within the zona pellucida, whereupon slow cell division takes place. On the fourth day of development, cleavage stage embryos begin a rapid cell division, becoming morulas (typically between 16 to 64 cells per embryos). Finally, on the fifth day of development, embryos become blastocysts as they begin to form cavities called blastocoeles, still within the zona pellucida. Embryo development up to the blastocyst stage occurs in the fallopian tubes. When embryos reach the blastocyst stage they leave the fallopian tube and enter the uterine cavity.

Prior to implantation into the uterine lining, embryos must first "hatch" out of their shell, the zona pellucida. If no hatching occurs, the embryo cannot implant and no pregnancy occurs. In the natural setting, as the blastocoele cavity expands, the embryo shell weakens and breaks, allowing the embryo to evacuate its zona and begin implanting. In the laboratory however, embryos may behave differently. Certain clinical situations have been associated with hardening of the zona pellucida, which can impede the hatching process, leading to decreased implantation and pregnancy rates. Advanced reproductive age, elevated FSH levels/decreased ovarian reserve, prior implantation failure, thickened zona, and frozen-thawed embryos have all been shown to benefit from assisted hatching prior to embryo transfer. Assisted hatching in such clinical scenarios may result in improved implantation and pregnancy rates. In assisted hatching a small hole is introduced in the zona pellucida in order to weaken it just prior to embryo transfer.