Editing In vitro fertilisation (section)

===Sex ratio distortions===

Certain kinds of IVF have been shown to lead to distortions in the [[sex ratio]] at birth. [[Intracytoplasmic sperm injection]] (ICSI), which was first applied in 1991, leads to slightly more female births (51.3% female). [[Blastocyst transfer]], which was first applied in 1984, leads to significantly more male births (56.1% male). Standard IVF done at the second or third day leads to a normal sex ratio.{{Citation needed|date=April 2023}}

[[Epigenetic modifications]] caused by extended culture leading to the death of more female embryos has been theorised as the reason why blastocyst transfer leads to a higher male sex ratio; however, adding retinoic acid to the culture can bring this ratio back to normal.<ref>{{cite journal |display-authors=6 |vauthors=Tan K, An L, Miao K, Ren L, Hou Z, Tao L, Zhang Z, Wang X, Xia W, Liu J, Wang Z, Xi G, Gao S, Sui L, Zhu DS, Wang S, Wu Z, Bach I, Chen DB, Tian J |date=March 2016 |title=Impaired imprinted X chromosome inactivation is responsible for the skewed sex ratio following in vitro fertilization |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=113 |issue=12 |pages=3197–3202 |bibcode=2016PNAS..113.3197T |doi=10.1073/pnas.1523538113 |pmc=4812732 |pmid=26951653 |doi-access=free}}</ref> A second theory is that the male-biased sex ratio may due to a higher rate of selection of male embryos. Male embryos develop faster in vitro, and thus may appear more viable for transfer.<ref>{{cite journal |vauthors=Lou H, Li N, Zhang X, Sun L, Wang X, Hao D, Cui S |date=July 2020 |title=Does the sex ratio of singleton births after frozen single blastocyst transfer differ in relation to blastocyst development? |journal=Reproductive Biology and Endocrinology |volume=18 |issue=1 |pages=72 |doi=10.1186/s12958-020-00623-x |pmc=7362517 |pmid=32669110 |doi-access=free }}</ref>