Assisted hatching

Last updated date: 06-Jan-2023

Originally Written in English

Assisted Hatching

Assisted Hatching


Assisted hatching is a laboratory process that is occasionally performed in conjunction with IVF therapy. IVF involves combining eggs and sperm in a laboratory (rather than within a woman's body as in spontaneous conception). When a sperm successfully penetrates an egg, it is deemed fertilized. The fertilized eggs are watched for 3 to 6 days as they divide and mature into embryos during IVF.

The best embryo is then transferred into the woman's uterus in the hopes of assisting her in becoming pregnant, or it is frozen for future use. The embryo is enveloped by cells that form a protective shell as it grows (zona pellucida).

As the embryo grows, it spontaneously breaks free from this shell. Occasionally, the doctor may instruct the laboratory to develop a small "crack" in the embryo's outer shell just before it is implanted into the woman's body (assisted hatching). The goal is that aided hatching will allow the embryo to grow, implant into the uterine wall, and eventually result in a pregnancy.


How Embryos Hatch?

Embryos Hatch

Understanding the natural hatching of an embryo is helpful in understanding assisted hatching. You start with an oocyte or egg before you obtain an embryo. The oocyte is surrounded by a protein shell known as the zona pellucida. The zona pellucida has several roles in embryo development, and these responsibilities shift as the embryo matures.

The zona pellucida combines with sperm cells before the egg develops into an embryo. This union marks the beginning of the fertilization process. The zona pellucida hardens when a single sperm cell enters the shell and fertilizes the egg. This hardening precludes further sperm cell entry into the now-fertilized embryo.

The tough shell also prevents the embryo from implanting early in the fallopian tubes, resulting in an ectopic pregnancy. It also helps to keep the blastocyst cells together.

The zona pellucida swells and begins to weaken and degrade as the zygote travels down the fallopian tube and develops into the blastocyst stage. The zona pellucida splits open about day four of development, allowing the blastocyst/embryo to emerge, leaving behind the thin protein shell. The embryo hatching process is represented by this stage.

The blastocyst attaches itself into the endometrium within a few days after hatching. A blastocyst cannot embed itself into the uterine wall unless it hatches, and so a pregnancy cannot occur.


What is Assisted Hatching?

Assisted Hatching

Fertilization occurs in the lab during IVF therapy. However, as every couple who has had IVF therapy knows, having a fertilized embryo does not ensure pregnancy. For pregnancy to occur, the transplanted embryo must implant itself into the endometrium and "stick."

The percentage of embryo transfers that "stick" (implantation rate) for women under the age of 35 is around 50%. However, it is less than 10% for women over the age of 42.

There are various ideas as to why this happens, one of which is because the embryo does not hatch correctly. This lack of hatching might be due to the embryo having an abnormally hard shell, or it could be due to anything in the lab environment, such as the cultures used to keep the embryo alive or the cryopreservation (freezing) chemicals, interfering with the hatching process.

Assisted hatching is intended to overcome any obstacles that are impeding hatching. It is also believed that it may increase the chances of successful implantation and, eventually, pregnancy.


Who Benefits From AZH?


Assisted hatching is most useful for the following types of patients:

  • Patients who are older than 38 years of age.
  • Patients with elevated levels of follicle stimulating hormone (FSH).
  • Patients whose embryos have a thicker than normal zona pellucida, as measured by trained embryologists.
  • Patients who have had failed natural fertilization during a previous IVF cycle. This may occur even though the male partner appears to have normal sperm counts, motility, and morphology.
  • Patients using frozen sperm that may be limited in number and/or quality.
  • Diagnosis of antisperm antibodies bound to spermatozoa or female antisperm antibodies that are thought to be the cause of infertility.
  • Patients who have a low number of eggs and want to maximize their chances of fertilization.


Preimplantation Genetic Diagnosis (PGD)

Preimplantation Genetic Diagnosis (PGD)

PGD tests a single cell, or blastomere, from a developing embryo for genetic illness or chromosomal abnormalities. As a result, for an increasing number of genetic diseases, PGD permits only embryos that have been identified as being free of the genetic abnormality to be sent back to the uterus for implantation and possible pregnancy.


Who Benefits from PGD?

Benefits from PGD

Both fertile and infertile patients can benefit from PGD technology. PGD is an excellent resource for those patients who:

  1. They are at risk of passing on genetic problems to their children. PGD was created in order to screen embryos for genetic disorders where the precise gene implicated was known. Tay Sachs, Huntington's disease, Cystic Fibrosis, and Muscular Dystrophy are examples of single gene illnesses that are handed down in families from either the mother or the father.
  2. Are at least 35 years old. The sperm and egg both contribute 23 chromosomes to the ensuing embryo, for a total of 46 (23 pairs) chromosomes, including the sex chromosomes. An embryo is deemed genetically defective if it has an extra chromosome or if a chromosome is missing, a condition known as aneuploidy. There is an increased probability of aneuploidy in a pregnancy with advanced mother age (>35 years old). Down's syndrome is the most commonly discussed aneuploidy in women of advanced maternal age. This happens when chromosome 21 has three copies.
  3. Have had repeated miscarriages. PGD is able to test for 7 chromosomes, 13, 16, 18, 21, 22, and the sex chromosomes X and Y. An extra copy of certain chromosomes (#13,16,18,22) can result in miscarriage or birth of a baby with very serious birth defects that are usually incompatible with life.
  4. They were discovered to be "balanced translocation" carriers. This individual looks to be healthy and has a complete set of 23 pairs of chromosomes (46 in total), but two of the chromosomes have traded portions. This does not appear to be a problem in the individual until they begin attempting to conceive. It is probable that the resulting embryo will be a "unbalanced translocation" carrier. Miscarriage is relatively prevalent in this circumstance, although it is possible to have a baby with varied degrees of birth abnormalities.


The Potential of AZH in Advanced Female Age

AZH in Advanced Female Age

It is commonly known that IVF success rates drop with mother age, and numerous theories have been proposed to explain this decrease in fertility. One such possibility is that spontaneous zona hardening occurs as a result of age-related endocrine changes and/or the lack of lysins from surrounding tissues, which may effect on embryos in vivo.

These environmental factors either govern or influence hatching. It is important to recognize that other factors may be reducing fertility in older women, such as decreased oocyte quality and quantity ('ovarian reserve'), an increase in the incidence of karyotypically abnormal embryos due to age-related endocrine changes, and/or the absence of lysins from surrounding tissues, which may act on embryos in vivo. These environmental factors either govern or influence hatching. Nevertheless, there is, to some extent, a decrease in uterine receptivity. Although aided hatching may benefit older women, it is unlikely that it will increase their pregnancy rate to that of younger women.


Methods Used in AZH

Methods Used In AZH

You may assume that assisted hatching always involves making a small “break” or tear in the zona pellucida. But that’s not actually so. There are a few methods available, and every embryo lab approaches this differently.

There are pros and cons to every way, and the skill of the technician matters. Be sure you gather as much information as possible so that you can make an informed decision.

  1. Mechanical hatching: With this technique, the embryologist keeps the embryo steady with the help of a pipette, while using a micro-needle to puncture through the zona pellucida, go just underneath the shell for a bit, and then come out the other end. (Imagine drawing a very thin line just alongside the embryo.) Then, the area between the two punctures is gently rubbed until a small tear occurs. It's difficult to control the size of the opening with this method.
  2. Mechanical expansion of the shell: With this technique, the zona pellucida is not broken open. Instead, hydrostatic pressure is introduced just under the shell, to cause it to expand. The idea for this method comes from the natural expansion of the outer shell during the hatching process.
  3. Chemical hatching: Assisted hatching using acid Tyrode’s has been described in detail by Cohen et al. The embryo is secured on a holding pipette and a microneedle is applied to an area of the zona pellucida overlying either empty perivitelline space or extracellular fragments (i.e. a blastomere-free area). The microneedle is preloaded with Tyrode’s acid before each micromanipulation using mouth-controlled suction. 

The acidic solution is expelled gently over a small area until the zona is breached. Suction is applied immediately after breaching the zona pellucida to prevent excess acid entering the perivitelline space. This technique requires very quick handling in order to avoid unnecessary exposure of the embryo to the acidic solution. The acid may be detrimental to the blastomeres adjacent to the drilled part of the zona pellucida. 

The zona dissolves on contact with the acid, hence, the embryo is removed immediately and rinsed several times to remove any trace of the acid. Cruciate thinning of the zona pellucida with acid Tyrode’s has been described in mice and humans. 

    4. Drilling: With drilling, vibratory movements are used to create a conical opening. This technique uses something known as Piezo technology.

    5. Laser-assisted hatching: By generating an opening, laser-assisted hatching can make it simpler for the embryo to "hatch" or break through its outer layer or "shell" (a membrane also known as the zona pellucida). This layer can become excessively thick and/or solid in some cases, with the freezing and thawing process being one of the contributing elements.

The easier the embryo hatches, the higher its chances of adhering or implanting into the uterine wall.

Pregnancy cannot occur unless the embryo hatches and implants, and laser-assisted hatching can help with both of these phases. Laser-assisted hatching may be the safest and most successful way. However, not every embryology lab is equipped with this equipment. Chemical hatching is increasingly popular. With any of these approaches, the embryologist's expertise and experience level can make a significant impact.

Under a microscope, an embryologist sends a short, intense laser beam to create a crack in the shell through which the embryo can emerge. This is often performed three days following fertilization during an IVF or Intracytoplasmic Sperm Injection (ICSI) cycle, when the embryo has begun to cleave (divide). It simply takes a few seconds and causes no harm to the embryo. The embryo is then returned to the patient's uterus to connect to the lining and continue to develop.




Any manipulation or interference with an embryo carries some risk. One danger of assisted hatching is that the embryo will be fatally harmed. This injury might occur before or after embryo transfer. Pregnancy would not occur in either instance.

Another danger of aided hatching is that the embryo's natural hatching process is disrupted, and the embryo fails to fully hatch from the zona pellucida. Meanwhile, twinning, notably monozygotic twinning, can develop via aided hatching.

Monozygotic twins are identical twins born from a single egg and sperm. Twinning is already raised after traditional IVF therapy, and research suggests that aided hatching may increase that risk even further. While all multiple pregnancies are dangerous, monozygotic twin pregnancies are substantially more dangerous for the mother and kids. Twinning is still rare, happening fewer than 1% of the time.

You might question if assisted hatching raises the chance of birth abnormalities. A major retrospective research of over 65,000 assisted reproduction deliveries discovered that aided hatching was "marginally related" with a higher risk of congenital abnormalities, but that the increased risk might have been attributable to other causes.




The key question, of course, is whether it's effective. Does assisted hatching make it easier to bring a kid home? The solution is a little more complex. A Cochrane analysis of 31 trials including 1,992 pregnancies and 5,728 women indicated that aided hatching very modestly increased clinical pregnancy rates. 4 However, the rate of live births did not improve.

Because the aim of any fertility therapy is to take home a baby, not simply a positive pregnancy test, live birth rates are more crucial to examine than clinical pregnancy rates.

Unfortunately, most of the research on assisted hatching has only reported clinical pregnancy rates, and not live birth rates. Those studies that did look at live birth rates didn’t find an advantage. Consequently, more research needs to be done.

Another study found that when assisted hatching was done on “good quality” embryos, pregnancy rates went down.5 The results varied depending on age group when assisted hatching was done on fair- to poor-quality embryos. These results would imply that assisted hatching not only won't help those with a good prognosis, but it may harm their chances of success.



Despite the ASRM's cautions against using assisted hatching on a regular basis, some clinics still provide it to every patient. The cost of aided hatching may already be "included" in the overall IVF fee at some clinics.

The cost of assisted hatching at clinics can range from $200 to $700 on average. There are also a few clinics that will provide the technology "for free" if they believe it will be beneficial.



Assisted hatching is a method that can be done on individuals undergoing in vitro fertilization (IVF) therapy. When embryos are generated by IVF, they are covered by a hard outer layer of cells known as the zona pellucida. Consider the embryo's outer layer to be its "shell." To implant into the uterus and grow into a pregnancy, an embryo must break out of its "shell."

Assisted hatching is a technique in which assist the embryo "hatch" from its "shell" by making a tiny fracture in the zona pellucida. It is thought that assisted hatching can aid embryo implanting in the uterus, resulting in better pregnancy rates in some individuals.

Generally, assisted hatching is undertaken on the third day of embryo development. A laser is used by embryologists to produce a very small hole in the zona pellucida. Previously frozen and thawed embryos can also be assisted hatching.

Assisted hatching is not indicated for all patients, however it may be beneficial in women over the age of 37 or who have previously failed IVF.

There is a small increase in the probability of identical twins in assisted hatching embryos. An embryo can be injured by the assisted hatching method very rarely.