the donor number from the fertility clinic. If that donor had donated before, then other donor conceived people with the same donor number are thus genetic half-siblings. In short, donor registries match people who type in the same donor number. Alternatively, if the donor number is not available, then known donor characteristics, e.g. hair, eye and skin color may be used in matching. Donors may also register, and therefore, donor registries may also match donors with their genetic children. The largest registry is the Donor Sibling Registry- with more than 25,000 members, the DSR has matched almost 7,000 donor conceived people with their egg and sperm donors, as well as with their half siblings. Alternate methods of providing an information link between the donor and recipient (both agreeing to stay registered on the DSR) are often provided for in the legal document (referred to as the "Egg Donor Agreement".) Embryo donation An alternative to egg donation in some couples, especially those in whom the male partner cannot provide viable sperm, is embryo donation. Embryo donation is the use of embryos remaining after a couple's IVF treatments have been completed, to another individual or couple, followed by the placement of those embryos into the recipient woman's uterus, to facilitate pregnancy and childbirth. Embryo donation is more cost-effective than egg donation on a "per live birth" basis. Another study has found that embryos created for one couple, using an egg donor, are often made available for donation to another couple if the first couple chooses not to use them. Psychological and social issues Quality of Parent-Infant Relationships Quality of parent-child attachment in early infancy has been recognized as a crucial factor of a child's socioemotional development. The formation of a quality and secure attachment is largely influenced by parental representations of the parent-child relationship. Concern regarding relationship quality and attachment security in egg donor families is understandable and typically stems from the absence of genetic material shared between the mother and child. In recent years, researchers have begun to question if lack of genetic commonality between mother and child inhibits the ability to form a quality attachment. In a recent study, quality of infant-parent relationships was examined among egg donor families in comparison to in vitro fertilization families. Infants were between the ages of 6–18 months. Through use of the Parent Development Interview (PDI) and observational assessment, the study found few differences