R. P. de Arruda, “Are there relationships between the luteal size, luteal morphoechogenicity by ultrasound and plasmatical progesterone concentrations in recipient mares?”, Braz. J. Vet. Res. Anim. Sci., vol. 38, no. 5, pp. 233–239, Jan. 2001, doi: 10.1590/S1413-95962001000500007.