Donor sibling relations among adult offspring conceived via insemination by lesbian parents

study question: how do adult offspring in planned lesbian-parent families feel about and relate to their donor (half) sibling(s) (DS)? summary answer: a majority of offspring had found DS and maintained good ongoing relationships, and all offspring (regardless of whether a DS had been identified) were satisfied with their knowledge of and contact level with the DS. What is known already: the first generation of donor insemination offspring of intended lesbian-parent families is now in their 30s. coincident with this is an increased use of DNA testing and genetic ancestry websites, facilitating the discovery of donor siblings from a common sperm donor. few studies of offspring and their DS include sexual minority parent (SMP) families, and only sparse data separately analyze the offspring of SMP families or extend the analyses to established adult offspring. study design, size, duration: this cohort study included 75 adult offspring, longitudinally followed since conception in lesbian-parent families. quantitative analyses were performed from online surveys of the offspring in the seventh wave of the 36-year study, with a 90% family retention rate. the data were collected from march 2021 to november 2022. participants/materials, setting, methods: participants were 30- to 33-year-old donor insemination offspring whose lesbian parents enrolled in a US prospective longitudinal study when these offspring were conceived. offspring who knew of a DS were asked about their numbers found, characteristics or motivations for meeting, DS terminology, relationship quality and maintenance, and impact of the DS contact on others. all offspring (with or without known DS) were asked about the importance of knowing if they have DS and their terminology, satisfaction with information about DS, and feelings about future contact. main results and the role of chance: Of offspring, 53% (n = 40) had found DS in modest numbers, via a DS or sperm bank registry in 45% of cases, and most of these offspring had made contact. the offspring had their meeting motivations fulfilled, viewed the DS as acquaintances more often than siblings or friends, and maintained good relationships via meetings, social media, and cell phone communication. they disclosed their DS meetings to most relatives with neutral impact. the offspring, whether with known or unknown DS, felt neutral about the importance of knowing if they had DS, were satisfied with what they knew (or did not know) of the DS, and were satisfied with their current level of DS contact. this study is the largest, longest-running longitudinal study of intended lesbian-parent families and their offspring, and due to its prospective nature, is not biased by over-sampling offspring who were already satisfied with their DS. limitations, reasons for caution: the sample was from the USA, and mostly white, highly educated individuals, not representative of the diversity of donor insemination offspring of lesbian-parent families. wider implications of the findings: while about half of the offspring found out about DS, the other half did not. regardless of knowing of a DS, these adult offspring of lesbian parents were satisfied with their level of DS contact. early disclosure and identity formation about being donor-conceived in a lesbian-parent family may distinguish these study participants from donor insemination offspring and adoptees in the general population, who may be more compelled to seek genetic relatives. the study participants who sought DS mostly found a modest number of them, in contrast to reports in studies that have found large numbers of DS. this may be because one-third of study offspring had donors known to the families since conception, who may have been less likely to participate in commercial sperm banking or internet donation sites, where quotas are difficult to enforce or nonexistent. the study results have implications for anyone considering gamete donation, gamete donors, donor-conceived offspring, and/or gamete banks, as well as the medical and public policy professionals who advise them. study funding/competing interest(s): no funding was provided for this project. the authors have no competing interests. trial registration number: N/A.