Arruda, Rubens Paes de, et al. “Are There Relationships Between the Luteal Size, Luteal Morphoechogenicity by Ultrasound and Plasmatical Progesterone Concentrations in Recipient Mares?”. Brazilian Journal of Veterinary Research and Animal Science, vol. 38, no. 5, Jan. 2001, pp. 233-9, https://doi.org/10.1590/S1413-95962001000500007.