There are no data on whether parents of twins will disclose mode of conception to researchers or to their children, who will be informants in adulthood. We sent 1600 questionnaires about this via the Victorian branch of the Australian Multiple Birth Association, to be returned anonymously. Parents were asked how their twins were conceived and whether those who used assisted conception would disclose this to researchers studying assisted conception, twin pregnancy or twin children, or to their children. Comments were invited. Altogether 975 (61%) questionnaires were returned and 389 (40%) indicated use of some form of assisted conception: 75 (19%) ovarian stimulation alone, 165 (42%) In Vitro Fertilisation, 132 (34%) Intracytoplasmic Sperm Injection, and 17 (4%) Gamete Intrafallopian Transfer, with 20 reporting use of donor eggs and thirteen donor sperm. Of those using assisted conception, the proportion reporting that they would not, or may not, tell researchers was 5% for assisted conception studies, 6% for twin pregnancy studies, and 7% for studies of twin children, while 7% reported that they would not, or may not, tell their children. From the comments (from 374/975; 38%) it was clear that questions about mode of conception can be offensive to some parents of twins, unless there is a need to know. Further, the question ‘are your twins natural?’ should be avoided. We believe the question ‘Did you need medical help to conceive your twins’, followed up with specific questions, is more acceptable.