In Vitro Fertilization (IVF) is inherently inefficient since several (typically 2-4) good quality preimplantation embryos (fertilized eggs) are usually transferred to the uterine cavity with a resultant overall pregnancy rate (per cycle) of (up to) only 50-75%. Moreover, for those who do get pregnant the singleton (one baby) rate is about 70-75%, twinning (two baby) rate is about 20-25%, and the triplet (or more) rate is less than 5%. The suggestion is that the inherent inefficiency of IVF (and human reproduction in general) is due to difficulty (failure) of implantation into the uterus, with calculated rates of implantation being about 10-20% (per transferred embryo per cycle).
Implantation failure may be due to many factors, including genetic (chromosomal) abnormalities in the (replaced) fertilized eggs, abnormal early embryonic development due to the inability of culture conditions to fully mimic physiologic events and conditions normally present in the fallopian tube, abnormalities in the zona pellucida (shell) of the fertilized eggs that make it difficult for the embryo to “hatch” from this shell prior to its attachment to (and implantation in) the uterine lining, abnormal hormonal preparation of the uterine lining (endometrium) resulting in a decreased (or closed) window of uterine receptivity, and (currently largely unrecognizable) immunologic barriers present in the uterus which may limit embryo adhesion or implantation.
IVF culture conditions that result in a reduced ability for embryos to normally hatch from their zona pellucida (shell) are possibly (at least partially) overcome with the “assisted hatching” technique (microscopic creation of a gap in the acellular zona pellucida). The tools that are used to create such a gap vary from finely drawn pipettes (to mechanically “tear” a hole in the shell), to acid Tyrode’s solution (to chemically digest a hole in the shell), to lasers (to drill a hole in the shell using high power density laser energy). The technique that is used is critically important since a hole (in the shell) that is either too narrow or too large will actually be counterproductive in terms of implantation.
The IVF couples most likely to benefit from hatching include those with older female partners (hatching is often suggested for women over 38 years of age), women with a decreased ovarian reserve (using available biochemical testing such as basal FSH concentration or clomiphene challenge testing), prior unexplained (IVF) implantation failure, frozen embryo cycles, and lower grade (quality) embryos (with increased fragmentation) or increased zona (shell) thickness or hardening (related to the time taken for acid Tyrode’s solution to digest the wall).
The risks of embryo hatching include an increased risk of identical twins (the embryonic cells may be traumatized and become divided into two separate identical pregnancies during their passage through the hole created in the shell), exposure of the embryo to an embryotoxic chemical solution (such as acid Tyrode’s solution) if the acid is applied to the shell for too long a time, and an increased risk of trauma to the embryo during embryo transfer (especially if the transfer is difficult). In available reports, there does not appear to be an increase in any congenital abnormalities (birth defects) for couples that become pregnant after the hatching procedure is performed.