Despite the numerous studies carried out on the endangered European mink (Mustela lutreola) for conservation purposes, the reproductive biology of this species is largely unknown. In the wild reproductive parameters such as litter size are difficult to observe, particularly for cryptic species such as mustelids. We compared the reliability of nonstained and stained placental scar counts in farmed American mink (Neovison vison) with known litter sizes and then applied the best methodology to free-ranging related species—European mink, European polecat (Mustela putorius), and feral invasive American mink—for a comparative study of embryonic litter size in western Europe populations. The staining method allowed us to improve the detection of placental scars and to increase the reliability of the method by reducing the observer effect. Nevertheless, this analysis must be performed by 7 months postpartum, before the regeneration of uterine tissues. In free-ranging animals the mean embryonic litter size, estimated by stained placental scar counts and embryo counts, was significantly lower in European mink compared with polecats and American mink, and in polecats compared with American mink. Small litter sizes in European mink could be a factor limiting population growth rates in the species. Our results constitute a first step toward demographic analyses aimed at modeling the population dynamics of these species.