Introduction Nuclear transfer of somatic cells and other biotechnologies applied to embryo production in vitro yield very few em- bryos that are competent to become live offspring. Multiple factors can cumulatively impinge on embryonic compe- tence, including methods for the induction of ovulation, in vitro maturation (IVM), fertilization, culture, nuclear transfer, gene injection, cryopreservation and transfer. In general, minimization of in vitro handling associated with these procedures is likely to be beneficial to subsequent development. This is especially true for pig gametes and embryos, as the techniques are less well developed for this species (Nagashima et al., 1995; Wang et al., 1998, 1999; Dobrinsky et al., 2000). Several groups have reported the production of viable piglets after nuclear transfer of somatic cells (Betthauser et al., 2000; Onishi et al., 2000; Polejaeva et al., 2000). Interestingly, all of these groups transferred cloned embryos to final recipients before embryonic compaction. Although piglets were produced, the proportion of embryonic loss directly due to early transfer is unknown. In early studies in mice and sheep, transfer of precompacted embryos with disrupted zonae pellucidae resulted in embryonic loss (Modlinski, 1970; Bronson and McClaren, 1970; Trounson and Moore, 1974). Transfer of zona pellucida-free rabbit blastomeres also results in embryonic loss (Moore et al., 1968). However, nuclear transfer rabbit embryos can survive immediate oviductal transfer (Collas and Robl, 1990; Stice and Robl, 1998). The mechanism by which embryos are lost remains unclear, although it has been speculated that the loss reflects immunological attack (Moore et al., 1968). The objective of the first experiment in Reproduction (2002) 123, 507–515 Research Embryo transfer and pregnancy maintenance strategies in pigs were evaluated with reference to situations in which limited numbers of viable embryos or micromanipulated embryos are available, such as pig cloning. Development of embryos with compromised zona pellucida was com- pared with development of embryos with intact zona pellucida. Micromanipulation had no effect on blastocyst production rates after development in vivo or in vitro, but development in vivo improved the number of embryos reaching the blastocyst stage. Transfer of embryos with compromised zona pellucida resulted in live piglets. Several hormone treatments to maintain pregnancy were tested in a model in which three embryos were transferred into unmated recipient gilts, compared with transfer of three embryos into mated recipients. None of the hormonal treatments resulted in pregnancy rates of more than 25% at term and no more than 9% of transferred embryos survived, in comparison with 50% of the mated recipients successfully carrying 25% of transferred embryos. Lastly, the developmental potential of parthenogenetic embryos was assessed and 62% of transferred embryos resulted in pregnancies, none of which continued beyond day 55 of gestation. After co-transfer of three fertilized embryos with 55–60 parthenogenetic embryos into each of six recipients, two live piglets were delivered. The results from the present study indicate that transfer of zona pellucida compromised embryos can yield litters of normal piglets. In addition, it was demonstrated in a model system involving the transfer of three fertilized embryos into mature gilts that hormonal pregnancy maintenance strategies support a low proportion of embryos to term. Lastly, the present study shows for the first time a comparably effective but novel alternative for pregnancy maintenance in the pig involving the co-transfer of parthenote embryos. © 2002 Society for Reproduction and Fertility 1470-1626/2002 Embryo development and establishment of pregnancy after embryo transfer in pigs: coping with limitations in the availability of viable embryos T. J. King 1 , J. R. Dobrinsky 2 , J. Zhu 1 , H. A. Finlayson 1 , W. Bosma 1 , L. Harkness 1 , W. A. Ritchie 1 , A. Travers 1 , C. McCorquodale 1 , B. N. Day 3 , A. Dinnyés 1 , P. A. De Sousa 1 and I. Wilmut 1 * 1 Department of Gene Expression and Development, Roslin Institute (Edinburgh), Roslin, Midlothian, Edinburgh EH25 9PS, UK; 2 Germplasm & Gamete Physiology Laboratory, ARS, US Department of Agriculture, Beltsville, MD 20705, USA; and 3 Animal Science Unit, University of Missouri, College of Agriculture, Animal Sciences Centre, Columbia, MO 65211, USA *Correspondence Email: Ian.Wilmut@bbsrc.ac.uk