through sperm donation have a birth defect rate of almost a fifth compared to the general population. Samples required per donor offspring The number of donor samples (ejaculates) that is required to help give rise to a child varies substantially from donor to donor, as well as from clinic to clinic. However, the following equations generalize the main factors involved: For intracervical insemination: N is how many children a single sample can help give rise to. Vs is the volume of a sample (ejaculate), usually between 1.0 mL and 6.5 mL c is the concentration of motile sperm in a sample after freezing and thawing, approximately 5-20 million per ml but varies substantially rs is the pregnancy rate per cycle, between 10% to 35% nr is the total motile sperm count recommended for vaginal insemination (VI) or intra-cervical insemination (ICI), approximately 20 million pr. ml. The pregnancy rate increases with increasing number of motile sperm used, but only up to a certain degree, when other factors become limiting instead. With these numbers, one sample would on average help giving rise to 0.1–0.6 children, that is, it actually takes on average 2–5 samples to make a child. For intrauterine insemination, a centrifugation fraction (fc) may be added to the equation: fc is the fraction of the volume that remains after centrifugation of the sample, which may be about half (0.5) to a third (0.33). Only 5 million motile sperm may be needed per cycle with IUI (nr=5 million) Thus, only 1–3 samples may be needed for a child if used for IUI. Using ART treatments such as IVF can result in one donor sample (or ejaculate) producing on average considerably more than one birth. However, the actual number of births per sample will depend on the actual ART method used, the age and medical condition of the female bearing the child, and the quality of the embryos produced by fertilization. Donor sperm is less commonly used for IVF treatments than for artificial insemination. This is because IVF treatments are usually required only when there is a problem with the female conceiving, or where there is a 'male factor problem' involving the female's partner. Donor sperm is also used for IVF in surrogacy arrangements where an embryo may be created in an IVF procedure using donor sperm and this is then implanted in a surrogate. In a case where IVF treatments are