Effect of flushing of blastocysts on Days 10–13 on the life-span of the corpora lutea in the pig

Blastocysts were flushed out of both uterine horns of gilts on Days 10, 11, 12 or 13. In mated non-pregnant gilts flushing had no effect on progesterone profile or cycle length (20.8 +/- 0.4 versus 20.6 +/- 0.6 days in the preflush cycle, N = 6, mean +/- s.e.m.). Flushing the blastocysts out of the uterine horns on Day 10 resulted in a cycle with a normal progesterone profile and a normal length (21.2 +/- 0.4 days, N = 5). Flushing on Days 11, 12 or 13 resulted in a normal cycle or in maintenance of the CL for 3-13 days as indicated by elevated progesterone concentrations and an increased interoestrous interval of, respectively, 22.0 +/- 1.2 versus 19.8 +/- 0.6 days (Day 11; N = 6), 24.8 +/- 1.4 versus 21.0 +/- 0.6 days (Day 12; N = 5; P less than 0.05) and 26.3 +/- 2.3 versus 20.5 +/- 0.4 days (Day 13; N = 6; P less than 0.05). There was a positive relationship between the change in interoestrous interval and the interval between the first observed standing oestrus and flushing of the blastocysts (rs = 0.350; n = 22; P less than 0.1). There was a large variation in the diameter of the blastocysts flushed on the same day. Only in those gilts in which the blastocysts were greater than or equal to 8 mm or filamentous were the CL maintained for 3 or more days. These results indicate that a first signal for maternal recognition of pregnancy is generated on Day 12 and that blastocysts greater than or equal to 8 mm are required for prolongation of CL function for 3 or more days. Since CL function is only extended for a maximum of 13 days (mean 7.4 +/- 1.0), a second signal seems necessary to maintain the CL for the whole period of pregnancy.