Diestrous Ovulations in Pregnant Mares as a Response to Low Early Postovulatory Progestogen Concentration

Abstract

Spontaneous prolongation of the luteal phase has been described in horses, but the underlying causes are still unclear. The present study investigated details of gonadotrophin and progestogen secretion in pregnant mares (n = 11) with or without experimentally reduced early postovulatory luteal function. From days 0 to 3 after ovulation, they were treated with the prostaglandin F2α (PGF2α) analogue cloprostenol or left untreated. After conceptus collection on day 34, they were assigned to the opposite treatment. Mares were affiliated to the group primary corpus luteum (n = 6) or diestrous corpus luteum (n = 5) depending on diestrous corpus luteum (CL) detection in the PGF pregnancy. For statistical comparisons, a p-value < 0.05 was significant. There was an effect of treatment (p < 0.01), but not of group on progestogen concentration. The concentration of LH was higher in PGF-treated than in untreated pregnancies (p < 0.05), but did not differ between groups. The FSH concentration did not differ between groups nor treatments. The total luteal tissue area was greater in mares with a diestrous ovulation during the PGF treatment pregnancy. Low progestogen concentration in the early postovulatory phase diminish the negative feedback on the hypothalamic-pituitary axis in early pregnancy and, thus, stimulate a luteal tissue response. Detection of secondary CL at the time of pregnancy examination in mares may reflect that early post-ovulatory progestogen concentrations were low.

Lethal variants of equine pregnancy: is it the placenta or foetus leading the conceptus in the wrong direction?

“…Work over the last decade has questioned this early evidence (Betteridge et al 2018) suggesting a precipitous drop in progesterone occurs more frequently preceding early pregnancy loss than previously thought. Further, more nuanced effects of early luteal progesterone levels on a number of pregnancy related parameters, including day 14 pregnancy rates (Hollinshead et al 2022), endometrial function (Beyer et al 2019), as well as early embryonic and foetal growth and organ development (Willmann et al 2011;Okada et al 2020) suggest much needs to be done to understand the role of progesterone levels in pregnancy loss.…”