Either one or two 60-h morulae were transferred to one uterine horn of Polish, Dutch or Strain A rabbits in which the mean number of ovulations was 4·3, 6·4 and 10·8. The pregnancy rate, as determined by palpation on day 10, varied from 45–65% (mean 53%) in recipients of one egg to 69–71% with two eggs. Only 20% of the does carrying one foetus maintained pregnancy to term compared with 87% of those with two implants. If pregnancy failed this usually occurred between days 15 and 20. In a further experiment, in which additional eggs were transferred to the contralateral horn, it was shown that all single conceptuses were inherently capable of surviving to term.
It is concluded that irrespective of breed and ovulation rate, two conceptuses are normally required to prevent regression of corpora lutea on about day 17. Treatment with follicle-stimulating hormone in mid-pregnancy or removal of the barren uterine horn on day 13 failed to maintain pregnancies involving only one conceptus.
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