Morulae and blastocysts obtained from ewes 5, 7 and 9 days after oestrus were transferred to 120 recipients whose oestrous cycles were either synchronized or ± 2 or 3 days out of phase relative to the onset of oestrus in the donors.
When synchronization was exact 75% of all recipients became pregnant. A high proportion of pregnancies were, in general, also obtained where the onset of oestrus in the recipient differed from that of the donor by ±2 days. With a difference of ±3 days, however, only 8 % of ewes became pregnant. The survival of 5-day morulae transferred to 7-day recipients was lower than when 9-day blastocysts were transferred to 11-day recipients.
The results are discussed with reference to possible factors associated with early embryonic mortality.
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