Editing In vitro fertilisation (section)

===Preimplantation genetic screening or diagnosis===
{{Main|Preimplantation genetic diagnosis}}
[[Preimplantation genetic screening]] (PGS) or preimplantation genetic diagnosis (PGD) has been suggested to be able to be used in IVF to select an embryo that appears to have the greatest chances for successful pregnancy. However, a systematic review and meta-analysis of existing [[randomised controlled trial]]s came to the result that there is no evidence of a beneficial effect of PGS with cleavage-stage biopsy as measured by [[live birth rate]].<ref name=Mastenbroek2011>{{cite journal | vauthors = Mastenbroek S, Twisk M, van der Veen F, Repping S | title = Preimplantation genetic screening: a systematic review and meta-analysis of RCTs | journal = Human Reproduction Update | volume = 17 | issue = 4 | pages = 454–466 | year = 2011 | pmid = 21531751 | doi = 10.1093/humupd/dmr003 | doi-access = free }}</ref> On the contrary, for those of [[advanced maternal age]], PGS with cleavage-stage biopsy significantly lowers the live birth rate.<ref name=Mastenbroek2011/> Technical drawbacks, such as the invasiveness of the biopsy, and non-representative samples because of [[mosaic (genetics)|mosaicism]] are the major underlying factors for inefficacy of PGS.<ref name=Mastenbroek2011/>

Still, as an expansion of IVF, patients who can benefit from PGS/PGD include:
* Those who have a family history of inherited disease
* Those who want [[prenatal sex discernment]]. This can be used to diagnose [[monogenic disorders]] with [[sex linkage]]. It can potentially be used for [[sex selection]], wherein a fetus is aborted if having an undesired sex.
* Those who already have a child with an incurable disease and need compatible cells from a second healthy child to cure the first, resulting in a "[[saviour sibling]]" that matches the sick child in [[HLA type]].<ref>{{cite web | vauthors = Britten N | date = 7 May 2011 | url = http://www.telegraph.co.uk/health/healthnews/8499394/Saviour-sibling-cures-sick-older-brother.html | title = Saviour Sibling Cures Sick Older Brother | work = The Daily Telegraph, Health News | archive-url = https://web.archive.org/web/20110510061827/http://www.telegraph.co.uk/health/healthnews/8499394/Saviour-sibling-cures-sick-older-brother.html | access-date = 8 May 2011 | archive-date = 10 May 2011 }}</ref>

PGS screens for numeral chromosomal abnormalities while PGD diagnosis the specific molecular defect of the inherited disease. In both PGS and PGD, individual cells from a [[pre-embryo]], or preferably trophectoderm cells biopsied from a [[blastocyst]], are analysed during the IVF process. Before the transfer of a pre-embryo back to a person's uterus, one or two cells are removed from the pre-embryos (8-cell stage), or preferably from a [[blastocyst]]. These cells are then evaluated for normality. Typically within one to two days, following completion of the evaluation, only the normal pre-embryos are transferred back to the uterus.  Alternatively, a blastocyst can be cryopreserved via vitrification and transferred at a later date to the uterus. In addition, PGS can significantly reduce the risk of multiple pregnancies because fewer embryos, ideally just one, are needed for implantation.