A micromanipulation technique in which the shell around the egg (the zona pellucida) is opened or thinned to facilitate the embryo hatching process. Human blastocysts must hatch from the zona pellucida by day 6-7 after insemination in order to implant into the uterine wall. The clinical use of assisted hatching followed studies showing that cleaving embryos with a good prognosis produce a compound that thins the outer shell in preparation for hatching. In addition, fertilized embryos with artificial gaps made in their shells were shown to have high rates of implantation. Assisted hatching involves drilling through the zona using an acid solution to partially thin an area of the zona (~50 µm across), and in some cases create a small hole all the way through the zona. Several studies have reported the beneficial effects of assisted hatching in terms of higher pregnancy rates in IVF .
Assisted hatching may be performed on cleavage stage embryos from day 2-4 (2-12 cells). Extra care is required at the early stages not to make too large a hole that could result in cell loss prior to cell junction formation (at around the 8-cell stage). Assisted hatching may also be performed on compacted embryos and day 5-6 expanded blastocysts.
Assisted hatching at Reproductive Sciences Center is somewhat selective in that hatching is not performed on all cases and is particularly recommended for the following categories of patients:
(a) advanced maternal age (>38 years),
(b) elevated basal FSH levels,
(c) embryos that have uniform thick zona layers (> 15 mm),
(d) a previous failed IVF cycle, and
(e) all frozen embryos.
Recently there has been an upward trend in the number of cases with assisted hatching being performed at Reproductive Sciences Center. Periodic reviews of data have suggested higher pregnancy rates are associated with assisted hatching. The actual technique used has been slightly modified to reduce the risk of damage or cell loss, particularly for embryos with thin shells. For such embryos, zona thinning across a larger area of the outer zona layer avoiding complete penetration through the inner layer may be preferable.
Recent fertility research studies have suggested a possible link between assisted hatching and monozygotic twinning (identical twinning) which may be caused by part of the inner cell mass (the embryonic part of the blastocyst) being trapped in the artificial hole created by the assisted hatching . As yet this has not been observed at our center and may be linked to the hatching technique (i.e. size of hole created in the zona).