Assisted hatching (AH) is a specialized laboratory procedure performed during In Vitro Fertilization (IVF) treatment. The technique uses micromanipulation to create a tiny opening in the embryo’s protective outer layer, known as the zona pellucida. This artificial breach assists the developing embryo in escaping its shell, a necessary step for implantation. The goal is to improve the chances of successful implantation into the uterine lining, increasing the likelihood of pregnancy.
The Biological Rationale for Needing Assistance
Every embryo is initially encased in the zona pellucida, a dense, non-cellular structure that serves as a protective barrier. For pregnancy to establish, the embryo (now a blastocyst) must naturally shed this shell in a process called “hatching.” This event allows the embryo’s cells to make direct contact with the endometrium, enabling implantation.
Factors associated with infertility or the IVF process can cause this outer layer to become abnormally hard or thick, preventing the embryo from breaking free. For example, advanced maternal age (women over 37) is correlated with a more rigid zona pellucida. Additionally, cryopreservation (freezing and thawing embryos) can chemically alter the shell, making it more difficult to penetrate. When the zona pellucida is compromised, the embryo’s natural efforts to hatch may fail, leading to non-implantation.
The Procedure Explained
Assisted Hatching is a precise, high-magnification procedure performed by an embryologist in the laboratory. It is typically done one to two days before the embryo is transferred into the uterus. Historically, methods used to breach the zona pellucida included mechanical, chemical, and laser-assisted hatching. The technique utilized by most clinics today is Laser-Assisted Hatching (LAH), due to its precision and safety profile.
LAH uses a focused infrared laser to create a small, localized opening in the zona pellucida without physical contact or chemicals. The laser delivers rapid, short pulses of energy that ablate a small section of the shell, creating a controlled aperture for the embryo to escape. This method minimizes stress on the embryo and reduces the risk of damage to the internal cells (blastomeres).
Older techniques are less common now. Mechanical hatching, or partial zona dissection, uses a fine glass microneedle to physically tear a slit in the shell. Chemical hatching involves applying a localized stream of an acidic solution, such as acidified Tyrode’s solution, to dissolve a small area of the zona pellucida. These older methods carry a slightly higher risk of inadvertently damaging the embryo or exposing it to harmful substances.
Patient Selection Criteria
Assisted hatching is recommended for specific patient groups where the risk of hatching failure is elevated. A common indication is advanced maternal age, generally defined as women aged 37 years or older, whose eggs often have a naturally thicker or more hardened zona. Another criterion is the presence of a visibly thick zona pellucida, which is measured during microscopic evaluation of the embryo.
Patients who have experienced previous unsuccessful IVF cycles, especially those with repeated implantation failure despite the transfer of high-quality embryos, are strong candidates. AH is also frequently recommended for embryos that have been cryopreserved, as the freezing and thawing process can increase the rigidity of the outer shell. Embryos displaying signs of poor quality, such as significant fragmentation or slow development rates, may also benefit from the procedure.
Potential Outcomes and Considerations
The intended outcome of assisted hatching is to increase the rate at which an embryo successfully attaches to the uterine wall. For patients with specific risk factors, such as a history of failed implantation or the use of frozen-thawed embryos, the procedure has been shown to improve clinical pregnancy rates. However, results vary, and the procedure does not improve outcomes for all IVF patients.
Although the procedure is highly controlled, a small risk of physically damaging the embryo during manipulation exists, particularly when using older mechanical or chemical methods. A specific consideration is a slightly increased risk of identical (monozygotic) twinning. This phenomenon is thought to occur because the artificial opening in the zona pellucida may cause the inner cell mass of the blastocyst to inadvertently split, leading to the formation of two fetuses from a single embryo.