Assisted Hatching
Assisted hatching is a laboratory procedure whereby the shell (zona pellucida) around the 2 or 3 day old embryo is mechanically weakened using a laser to dissolve a portion of the zona pellucida which assists that embryo to ‘hatch’ from the zona more easily, and to allow implantation into the lining of the uterus.
The assisted hatching technique was commenced in 1990 by a team led by Dr Jacques Cohen at Cornell in New York. No large scale trial has ever been conducted, therefore only anecdotal evidence on any improvement in implantation rates after assisted hatching is available.
It should be noted that assisted hatching is currently not commonly used in IVF treatment.
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The photo to the left shows an oocyte with the zona pellucida surrounding it.
Who Considers Assisted Hatching?
Normally, the ‘hatching’ process involves dissolving of the zona pellucida at about 120 hours (5 days) after fertilisation. This allows the embryo to leave the protected environment of the zona pellucida and commence implanting into the lining of the uterus.
Normally, the uterine lining is in a condition which is receptive for implantation for about 48 hours (from 120 to 168 hours after fertilisation). However, in women who undergo ovarian stimulation (as occurs in IVF) this window of implantation is brought forward to between 72 and 120 hours after fertilisation. Thus, if hatching does not occur, or occurs after 120 hours, implantation will be unsuccessful and a pregnancy will not ensue. Those women who would benefit from Assisted Hatching include all those who have either thicker zonae or those who have harder (but normal thickness) zonae.
The Following Conditions May Benefit From Assisted Hatching:
- Older Eggs/Oocytes – eggs from older women generally have thicker zonae than those from younger women.
- Reduced Embryo Energy Levels – In some women who have had repeatedly unsuccessful ART cycles it is possible that the cellular energy level required for normal hatching may be insufficient.
- Asymmetry between the ‘window of implantation’ and the hatching time of the embryo. (If the embryo hatches too late for the uterine lining to accept it.)
- Frozen/thawed embryos – it is evident that the freezing/thawing process can harden the zonae.
Most fertility specialists will not advise patients to use assisted hatching unless it is considered necessary. Where there has been an indication that reduced hatching rates have occurred, which could benefit from assisted hatching, this may be considered useful.
Benefits of Assisted Hatching
It is believed that assisted hatching allows embryos to hatch with greater ease and earlier than would otherwise have occurred. It also assists embryos to hatch which might not otherwise have done so.
Disadvantages of Assisted Hatching
Embryologists should be experienced at the micromanipulative technique of assisted hatching. However, there is limited data on the likely success rates using assisted hatching. In some instances, the creation of a weakness in the zona pellucida will not be enough to improve the chances of adequately hatching.