Effects of follicular aspiration and flushing, and the genotype of the fetus on circulating progesterone levels during pregnancy in the mare.

When aspirating ovarian follicles in pregnant mares to obtain oocytes for in vitro fertilisation (IVF), the effect of the manipulation on circulating concentrations of progesterone may be an important consideration in terms of the maintenance of pregnancy. The object of this study was to compare the effects of 3 different forms of transvaginal ultrasound-guided follicle aspiration (Treatment 1, no aspiration, n = 4; Treatment 2, aspirate only follicles > or =20 mm in diameter, n = 7; Treatment 3, aspirate all visible follicles, n = 7) on peripheral plasma progesterone concentrations between Days 21 and 150 of gestation in 9 mares carrying intraspecies horse and 9 mares carrying interspecies mule conceptuses. The 3 follicle aspiration treatments were applied at the peak of each follicular wave as determined by follicular mapping by means of transrectal ultrasonography on alternate days. The plasma progesterone profile in mares undergoing Treatment 1 was in close agreement with those reported previously in pregnant mares. A decline in plasma progesterone levels occurred after Day 53 of gestation in Treatments 2 and 3 mares, indicating that the follicular aspiration procedures did interfere with the formation of secondary corpora lutea. However, the levels in individual mares never dropped low enough to endanger the pregnancy. Mares carrying mule pregnancies exhibited higher mean plasma progesterone concentrations between Days 39 and 45 of gestation than mares carrying horse pregnancies, equivalent levels between Days 46 and 66 despite the lower circulating concentrations of chorionic gonadotrophin (mule CG) in their blood during this period and lower progesterone levels between Days 67 and 150 of gestation. The results indicate that the primary corpus luteum in the pregnant mare may be more sensitive to mule CG than horse CG. Furthermore, the earlier disappearance of CG from the circulation in mares carrying mule fetuses is reflected by an earlier decline in plasma progesterone concentrations in this type of equine pregnancy.