Many of the phenotypes associated with the large offspring syndrome (LOS) have features reminiscent of some naturally occurring human overgrowth syndromes, such as the Beckwith-Wiedemann syndrome (BWS), that are associated with errors in an imprinted cluster of genes on human chromosome. The major obstetric challenges facing those involved with assisted reproductive technologies (ART) continue to be preterm deliveries, low-birthweight babies and additional complications associated with multiple births. The alterations in genomic imprinting correlate with a varied range of aberrant fetal phenotypes that depend on the species in question, the type of insult during oocyte and/or preimplantation development, the gene(s) affected and the nature of the epigenetic modification. The process is known to involve active demethylation of paternal chromatin prior to syngamy in some but not all species, and may go some way to explain interspecies variation in imprinted gene expression following in vitro fertilisation (IVF) or intracytoplasmic sperm injection (ICSI).