Several studies have shown an increased risk for monozygotic twinning after
fertilization treatments. We present the clinical and sonographic characte
ristics of two monozygotic twin cases following blastocyst transfer. It is
suggested that delayed transfer of the embryo in the blastocyst stage is a
contributing factor to monozygotic twinning. (C) 2001 Elsevier Science Irel
and Ltd. All rights reserved.