Pregnancy rates and births after unilateral or bilateral transfer of bovine embryos produced in vitro.

Late morulae and blastocysts produced in vitro were nonsurgically transferred to heifers by unilateral (n = 184) or bilateral (n = 94) transfer. Of the recipients, 58% had serum progesterone values greater than 1.4 ng ml-1 on day 21 and rectal palpation on day 35 showed that 50% (138 of 278) were pregnant. The embryonic mortality rate between days 21 and 35 was estimated to be about 14% and between days 36 and 90 to be about 12%. Of the animals, 8% aborted between days 91 and 250 of pregnancy. No difference was observed in pregnancy rates between unilateral transfer of one (47%) or two embryos (49%) and bilateral transfer (53%), or in the twinning rate between bilateral transfer (42%) and unilateral transfer of two embryos (33%). The pregnancy rate was 54% with embryos evaluated as morphologically excellent or good, 51% with fair embryos and 26% with poor ones. A higher pregnancy rate (60%) was obtained after embryo transfer when the synchrony between recipient and embryo was -1 day.