Effects of sub-physiological plasma progesterone concentration in the early luteal phase on embryonic and fetal development, pregnancy outcome and foal characteristics

Abstract

High progesterone concentration in the early luteal phase supports pregnancy and postovulatory progestin treatment stimulates conceptus growth (Willmann et al., Theriogenology 2011;75,421) whereas reduced progesterone concentration changes histotroph composition (Beyer et al., Theriogenology 2019;125:236). Embryos obtained on day 34 from mares with reduced progesterone concentration were less developed than embryos from control pregnancies (Okada et al., Reprod Fert Develop 2020;32:1156). Sub-physiological progesterone concentrations in early pregnancy thus delay embryonic development at least until placentation. In this study, subsequent fetal growth, pregnancy loss and foal characteristics were investigated in pregnancies with reduced progesterone concentrations in the early luteal phase (PGF; n=7) and control pregnancies (CON; n=7) in the same mares. Estrous mares were inseminated every 48h until ovulation (day 0). Group PGF received 62.5µg of the prostaglandin F2α analogue cloprostenol daily on days 0-3 to induce sub-physiological progesterone concentrations whereas CON pregnancies remained untreated. Mares were assigned to PGF and CON treatments in alternating order and received the opposite treatment in the following year. Blood was obtained and conceptus size determined by transrectal (≤day 100) and transabdominal (>day 100) ultrasonography at regular intervals. IgG concentration was determined in colostrum at foaling and in foal plasma at 18-22 hrs after birth. Foals were weighed,measured, submitted to a clinical examination and the neutrophil / lymphocyte (N / L) ratio was determined as an indicator of maturity. Time from birth to first standing and first suckling was recorded. Data were analyzed with the SPSS statistics software, using two-way repeated measures ANOVA and χ2 analysis. Progesterone concentration was reduced in the first two weeks of PGF pregnancies (p<0.001; day 7 CON 35±3, PGF 11±2 ng/ml). All foals from CON pregnancies were healthy and mature, whereas 3/7 PGF pregnancies were lost (one embryonic death, one abortion, one non-viable foal, p=0.05). Size of the conceptus (day 49: PGF 6.6±07, CON 7.7±07 cm, p=0.006), embryo proper (day 54; PGF 4.4±0.8, CON 5.8±0.6 cm, p=0.015) and fetal orbita diameter (day 73 PGF 0.7±0.1, CON 1.0±0.1 cm, p=0.007) were smaller in PGF than in CON pregnancies. Size differences between PGF and CON pregnancies decreased over time and were no longer present at foaling. The viable PGF foals also did not differ in N / L ratio and time to standing and first suckling from CON foals In conclusion, reduced plasma progesterone concentration in the early luteal phase delays fetal growth beyond placentation and increases pregnancy loss. When viable foals are born, however, they are not compromised. Results indicate that pregnancies with sub-physiological progesterone concentrations will benefit from early progestogen substitution.