2. Prognostic factors in included trials.
| Study ID | Balanced age between groups | Balances no. of embryos transferred | Prognosis: poor/good | FSH levels | Blastocyst transfer | Complete/partial AH | Frozen cycles |
| Abulsoud 2019 | Yes | Yes | Poor | No data | No | Thinning | Fresh |
| Antinori 1999 | AH mean 1.5 years older | Yes | Good and poor subgroups | No data | No | Complete hole | Not stated |
| Balaban 2006 | Yes | Yes | Unselected | < 10 | No | Thinning | Frozen |
| Balakier 2009 | AH mean 1.3 years older | Yes | Good | < 10 | No | Thinning | Fresh |
| Baruffi 2000 | Yes | Yes | Good | No data | No | Thinning | Fresh |
| Carter 2003 | Yes | Yes | Good | < 10 | No | Not stated | Fresh |
| Ciray 2005 | Yes | Yes | Good | < 15 | No | Thinning | Fresh |
| Cohen 1992 | Yes | Yes | Unstated | ≤ 15 and > 15 subgroups | No | Complete hole | Fresh |
| Elhelw 2005 | Yes | No data | Poor | No data | No | Thinning | Frozen |
| Elnahas 2017 | Yes | No data | Good | No data | No | Thinning | Frozen |
| Fang 2010 | Yes | Yes | Not stated | No data | No | Mechanical expansion | Frozen thawed |
| Ge 2008 | Yes | Yes | Mixed | No data | No | Thinning | Fresh and frozen subgroups |
| Germond 2004 | Yes | Yes | Mixed, in subgroups | Between 3 and 12 | No | Complete hole | Fresh and frozen subgroups |
| González‐Ortega 2015 | Yes | Yes | Poor | > 12 | No | Partial | Fresh |
| Hagemann 2010 | Mean age data given only for combined cycles 1 and 2 | Yes | Under 38 years, > 2 previously failed cycles, ZP thickness > 13 micrometers | No data | No | 20 micrometer diameter opening | Fresh |
| Hellebaut 1996 | Yes | Yes | Good | No data | No | Complete hole | Fresh |
| Hurst 1998 | Yes | Yes | Good | < 10 | No | Complete hole | Fresh |
| Isik 2000 | AH mean 1.4 years older | Yes | Unstated | < 10 | Yes | Removal complete | Fresh |
| Isiklar 1999 | No data | Yes | Unstated | No data | Yes | Complete hole | Fresh |
| Jelinkova 2002 | Yes | Yes | Poor | No data | Yes | Removal complete | Fresh |
| Kutlu 2010 | Yes | Yes | Good and poor subgroups | No data | No | Complete hole | Fresh |
| Laffoon 1999 | No data | No data | Good | No data | No | Complete hole | Fresh |
| Lanzendorf 1998 | No | Yes | Poor | No data | No | Complete hole | Fresh |
| Nada 2018 | Yes | Yes | Good | AH 5.4 ± 1.3 No AH 6.0 ± 1.1 | No | Thinning | Fresh |
| Nagy 1999 | Yes | Yes | Unstated | No data | No | Thinning | Frozen‐thaw cycle |
| Ng 2005 | Yes | Higher proportion of controls received 3 embryos | Unstated | < 11 | No | Thinning | Frozen‐thaw cycle |
| Petersen 2005 | Yes | Yes | Poor | No data | No | Thinning | Fresh |
| Razi 2013 | Yes | Yes | Unstated | No data | No | Partial | Fresh |
| Rufas‐Sapir 2004 | No data | Yes | Poor | No data | No | Complete hole | Fresh |
| Ryan 1997 | No data | No data | Unstated | No data | No | Complete hole | Both |
| Safari 2017 | Yes | Yes | Unstated | AH 6.4 ± 2.3 No AH 5.6 ± 2.1 |
No | Thinning | Frozen |
| Sagoskin 2007 | Yes | Yes | Good | < 10 | No | Hole | Fresh |
| Shi 2016 | Yes | Yes | Advanced maternal age | < 10 | No | Zona Thinning | Fresh |
| Stein 1995 | No data | No data | Poor | No data | No | Complete hole | Fresh |
| Tucker 1993 | Yes | Yes | Good | < 15 | No | Thinning | Fresh |
| Tucker 1996 | AH mean 1.8 years older | Yes | Not stated | No data | No | Complete hole | Fresh |
| Utsunomiya 1998 | No data | No data | Poor | No data | No | Thinning | Fresh |
| Valojerdi 2010 | Yes | Yes | Not stated | No data | No | Partially thinned | Vitrified‐warmed embryo |
| Wan 2014 | Yes | Yes | Previously unsuccessful 1 fresh cycle | No data | Yes | Partial | Vitrified‐warmed embryo |
AH: assisted hatching. ET: embryo transfer. FSH: follicle‐stimulating hormone.