Low progesterone concentration in early pregnancy and later foal development

We have long been aware of the importance of adequate progesterone concentrations early in pregnancy to maintain the conceptus, embryo, and implantation. In this study, we find sub-physiologic values early in pregnancy affect the development and health of the growing fetus and foal till after parturition. Developing programs that monitor early progesterone levels and supplement suboptimal levels makes good sense.
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Low progesterone concentration in early pregnancy is detrimental to conceptus development and pregnancy outcome in horses
Anim Reprod Sci. 2023 Sep 16;257:107334. doi: 10.1016/j.anireprosci.2023.107334. Online ahead of print.
Authors
Lisa-Hélène Wagner 1 , Jörg Aurich 2 , Maria Melchert 2 , Carolina T C Okada 1 , Camille Gautier 1 , Martim Kaps 1 , Svenja Claaßen 1 , Christine Aurich 3
Affiliations

1 Artificial Insemination and Embryo Transfer, Department for Small Animals and Horses, Vetmeduni Vienna, Vienna 1210, Austria.
2 Obstetrics, Gynecology and Andrology, Department for Small Animals and Horses, Vetmeduni Vienna, Vienna 1210, Austria.
3 Artificial Insemination and Embryo Transfer, Department for Small Animals and Horses, Vetmeduni Vienna, Vienna 1210, Austria. Electronic address: christine.aurich@vetmeduni.ac.at.

PMID: 37725863
DOI: 10.1016/j.anireprosci.2023.107334

Abstract

High progesterone concentrations in the early luteal phase support pregnancy, whereas subphysiological progesterone concentrations delay embryonic development at least until placentation. In this study, fetal growth and development of pregnancy was investigated in pregnancies with prostaglandin F2α-induced low progesterone concentrations (PGF) in the early luteal phase and control pregnancies (CON) in the same mares (n = 12). Mares were inseminated and in PGF pregnancies received the prostaglandin F2α analogue cloprostenol (62.5 µg) on days 0-3 after ovulation to induce subphysiological progesterone concentrations; CON pregnancies remained untreated. Mares were assigned to PGF or CON treatments in alternating order and received the opposite treatment in the following year. Blood was collected and conceptus size determined repeatedly by transrectal (≤day 101) and transabdominal (>day 101) ultrasonography. After birth, foals were weighed, measured and submitted to a clinical examination. Treatment PGF resulted in fewer pregnancies than CON treatment. All foals born from CON pregnancies were healthy and mature, whereas 4/7 PGF pregnancies were either lost (one embryonic death, one abortion) or resulted in the birth of compromised foals (P = 0.018). Size of the conceptus (e.g., diameter day 49: PGF 6.6 ± 0.7, CON 7.7 ± 0.7 cm, P = 0.006) and embryo proper (e.g., crown rump length day 54; PGF 4.4 ± 0.8, CON 5.8 ± 0.6 cm, P = 0.015) differed between treatments. These size differences decreased over time and at birth PGF foals did not differ significantly from CON foals. In conclusion, reduced progesterone concentration in the early luteal phase leads to delayed conceptus growth beyond placentation and increased pregnancy loss.

5. Conclusions
Reduced plasma progesterone concentration in the early luteal phase results in increased early embryonic loss and delays fetal growth beyond placentation. Treatment with synthetic progestins early in pregnancy may thus be justified in mares with reduced endogenous progesterone synthesis in the early luteal phase. It is therefore recommended to determine blood progesterone concentration on day 5 after ovulation before such a treatment is initiated.

Keywords: Abortion; Fetus; Growth retardation; Pregnancy; Progesterone.