Using data from clinical psychoanalytic research on lesbian couples undertaking Artificial Insemination by Donor (AID), this article explores the position of the third as it appears in the family project of lesbian couples. The third is examined through the analysis of constructions surrounding the image of the anonymous donor, the impact of the medical act of insemination on the women's psychic economy, and the search for other promising bases for triangulation. The complexity of the issue of the third in same-sex parenting is highlighted. Excerpts from clinical interviews with two lesbian couples are used to illustrate and support the authors' hypotheses.