Same sex twins should give consent for zygosity testing except when medically necessary

Sir

Smith and family propose that zygosity testing of same-sex twin children should only be performed for medical reasons (1). We find that their argument presents only part of the picture.

We (2) and others have proposed that the parents of such twins should be offered the option of zygosity testing. As pointed out previously (3-5), parents of twins are often “informed” about their twins’ zygosity, but sometimes erroneously due to incorrect assumptions, made by some parents and some medical professionals, about placental structures, the twins’ physical appearance, as well as the belief that IVF pregnancies only produce dizygotic twins. Optional zygosity testing soon after birth would avoid such misunderstandings. Such misunderstandings could lead to parents’ not feeling confident enough to answer questions about their twins’ zygosity from family, friends and strangers. Knowing that twins are dizygotic, which is more likely to run in families, also helps parents and close relatives plan future pregnancies.

Twins themselves also benefit from zygosity knowledge for reasons similar to those of their parents. This is because type of twin (monozygotic or dizygotic) can determine the intimacy and perceived “specialness” of their relationships. The International Council for Multiple Birth Organisations (ICOMBO) and the International Society of Twin Studies (ISTS), in their joint Declaration Of Rights, state that, 1) “Parents have a right to expect accurate recording of placentation, determination of chorionicity and amnionicity via ultrasound, and the diagnosis of zygosity of same sex multiples at birth” and that 2) “older, same sex multiples of undetermined zygosity have a right to testing to ascertain their zygosity” (6). Zygosity knowledge has also been described as a “birthright”(3).

For these reasons, as representatives of twins and twin researchers, we recommend that all parents of newborn twins should be informed about the concept of twin zygosity before birth, and that parents of same-sex twins should be given the option to have their twins tested. If parents feel strongly that they would like their twin children to decide for themselves, or if some adult twins do not wish to undergo zygosity testing for personal reasons after considering the concepts involved, then these choices should be respected.

References
(1). Smith SE, Carr LKNS, Smith EM, et al. Same sex twins should give consent for zygosity testing except when medically necessary. BMJ 2016;355:i5584.
(2). Craig JM, Segal NL, Umstad MP, et al. Zygosity testing should be encouraged for all same-sex twins: FOR: A genetic test is essential to determine zygosity. BJOG 2015;122(12):1641.
(3). Keith L, Machin G. Zygosity testing. Current status and evolving issues. J Reprod Med 1997;42(11):699-707.
(4). Vadgama N, Nirmalananthan N, Sadiq M, et al. Identical non-identical twins and non-identical identical twins. British medical Journal 2015;351:h6589.
(5). Cutler TL, Murphy K, Hopper JL, et al. Why Accurate Knowledge of Zygosity is Important to Twins. Twin Res Hum Genet 2015;18(3):298-305.
(6). ISTS I. Declaration of Rights and Statement of Needs of Twins and Higher Order Multiples. Secondary Declaration of Rights and Statement of Needs of Twins and Higher Order Multiples 2014. http://icombo.org/wp-content/uploads/2010/11/Declaration-of-Rights-2014.pdf.