Assisted Hatching (AH) is a micromanipulation technique performed by embryologists as an adjunct to In Vitro Fertilization (IVF). The procedure mechanically alters the protective outer layer of the early embryo. By making a small opening, AH supports the embryo’s ability to successfully attach to the uterine wall, improving the chances of achieving a pregnancy.
The embryo is encased in the zona pellucida, which serves as a protective barrier during fertilization and early development. This layer prevents more than one sperm from fertilizing the egg and holds the developing cells together. Before implantation in the uterine lining, the embryo must escape from this shell, a process known as natural hatching.
The embryo, now a blastocyst, expands, creating internal pressure that causes the zona pellucida to thin and rupture. This typically occurs around the fifth to seventh day after fertilization, allowing the cells to emerge and interact with the endometrium. If the zona pellucida is too tough or fails to break open, the blastocyst cannot escape and consequently cannot implant, leading to a failed cycle.
Technical Methods Used in Assisted Hatching
Creating an opening in the zona pellucida requires precise micromanipulation tools under a microscope. Historically, three distinct methods have been developed. The chemical method involves using a small stream of acidified Tyrode’s solution to carefully dissolve a small area of the zona pellucida. This technique requires meticulous control to ensure the acid does not contact the sensitive inner cells of the embryo.
Another approach is mechanical hatching, which involves using a fine glass needle or a specialized pipette to physically tear or puncture the zona pellucida. While effective at creating an opening, mechanical methods carry a slightly higher risk of inadvertently damaging the embryo’s internal structure.
The most commonly used method today is laser-assisted hatching, which utilizes a focused infrared laser beam to create a precise, microscopic opening in the zona pellucida. The laser offers superior control, allowing for the rapid creation of a hole or thinning of the shell with minimal exposure time outside of the incubator. This efficiency and accuracy have made the laser technique the preferred standard in most embryology laboratories.
Patient Selection Criteria
Assisted hatching is reserved for specific patient and embryo characteristics associated with a lower chance of successful implantation. One of the most common indicators is advanced maternal age, typically women 38 years old or older. The zona pellucida is theorized to be thicker and more resistant to natural hatching in embryos derived from older oocytes.
Embryo-specific characteristics also guide the decision, particularly the finding of an unusually thick or dark zona pellucida. A zona thickness exceeding a certain threshold, often around 18-20 micrometers, suggests the embryo may struggle to hatch on its own. Furthermore, embryos that have undergone cryopreservation are often candidates for AH.
The process of freezing and thawing can cause the zona pellucida to harden, making it more difficult for the blastocyst to escape. Patients who have experienced repeated implantation failure in previous IVF cycles, even with good-quality embryos, represent another significant group considered for the procedure. AH is employed in an attempt to overcome a presumed hatching deficit and facilitate successful uterine attachment.
Clinical Outcomes and Safety Profile
The effectiveness of assisted hatching is evaluated by its impact on clinical pregnancy and live birth rates. In certain populations, particularly those with poor prognosis factors like advanced age or a history of multiple failed IVF cycles, AH has been shown to increase clinical pregnancy rates. However, for patients with a favorable prognosis, the benefit of AH on live birth rates is less clear.
The procedure is generally considered safe, but two primary risks are associated with the micromanipulation. The most immediate concern is the potential for direct damage to the embryo’s cells during the manipulation process, although this risk is minimal when performed by a skilled embryologist using modern laser technology.
A second documented risk is an increase in the incidence of monozygotic, or identical, twinning. The small breach in the zona pellucida may contribute to the splitting of the inner cell mass, leading to the formation of twins from a single embryo. AH is typically reserved for specific groups where the potential benefit is thought to outweigh the small risks.