IVF involves fertilizing an egg with sperm in a laboratory setting. The resulting embryo is then transferred into the woman’s uterus. Successful pregnancy requires implantation, where the embryo attaches to the uterine lining. Assisted Hatching (AH) is a specialized micromanipulation technique performed by embryologists to potentially aid this attachment process during an IVF cycle.
Understanding Natural Embryo Hatching and Implantation
Before implantation, the embryo must naturally escape its outer protective layer, a process called hatching. This layer is a thick, non-cellular shell known as the Zona Pellucida (ZP), which protects the embryo during its initial development.
The embryo typically reaches the blastocyst stage by day five or six after fertilization, consisting of hundreds of cells and forming a fluid-filled cavity. To attach to the receptive uterine wall (endometrium), the blastocyst must physically break through the ZP. It expands and contracts, generating pressure until the ZP tears open, allowing the embryo to emerge and begin attachment.
Failure to shed this shell (hatching failure) is a potential reason why a healthy embryo may not implant successfully. Once emerged, the blastocyst interacts directly with the endometrial cells. Following a day five blastocyst transfer, implantation usually begins within one to five days.
The Procedure: Defining Assisted Hatching Techniques
Assisted hatching is a laboratory procedure performed before embryo transfer to create a small opening or thin the ZP. This intervention bypasses the natural physical barrier, facilitating the embryo’s ability to hatch. The procedure uses specialized micromanipulation tools under a high-powered microscope.
Three primary methods have been developed for performing assisted hatching.
Mechanical Hatching
One of the earliest techniques was Mechanical Hatching, which involved using a fine glass needle to manually tear the ZP. This method is now rarely used due to the risk of damage.
Chemical Hatching
Another method is Chemical Hatching, where a small, localized amount of an acidic solution, such as acidified Tyrode’s solution, is applied to the ZP to dissolve a tiny section.
Laser Hatching
The most common and precise technique currently used is Laser Hatching. This method employs a focused infrared laser beam to create a microscopic opening or to thin the ZP with high accuracy. The laser pulse is brief and highly controlled, minimizing the exposure of the embryonic cells to manipulation. Laser-assisted hatching allows the embryologist to precisely control the size and location of the breach, significantly reducing the risk of harm to the embryo.
Clinical Evidence: Does Assisted Hatching Speed Up Implantation?
Assisted hatching is designed to overcome a physical barrier preventing implantation, not to accelerate the biological timeline. Implantation timing (one to five days after transfer) is governed by complex biological factors, such as uterine receptivity and the embryo’s developmental pace. AH ensures the embryo is not physically delayed by a thick shell, but it does not change the fundamental window required for attachment.
Studies suggest AH benefits specific patient groups facing a higher risk of hatching failure. These candidates include women of advanced maternal age (typically over 37) whose eggs may produce a thicker ZP, patients with multiple failed IVF cycles, and cryopreserved embryos whose ZP may have hardened during freezing.
In these high-risk populations, AH improves the probability of successful implantation and clinical pregnancy rates. However, for the general IVF population, evidence does not indicate a significant increase in overall live birth rates. AH focuses on removing a physical obstacle to ensure implantation, not on speeding up the process.