Study design Between October 2004 and December 2006, ovulatory women undergoing infertility treatment were asked to participate in this RCT subject to the following inclusion criteria: maternal age below 36 years, need for ICSI and having motile sperm (sperm parameters: more than 1 x 106 sperm cells/ml) and both partners having a normal karyotype. Following written informed consent, eligible couples were randomly assigned to ICSI followed by blastocyst transfer either with or without PGS on the eight-cell stage embryo. Randomization was performed at the outpatient clinic, and each patient could be included only once. The consent form mentioned that patients allocated to PGS group would agree to donate embryos for research if these were chromosomally abnormal or unsuitable for transfer or cryopreservation. The study was approved by the Ethics Committee of the University Hospital. The patients were aware of the treatment allocation and were informed at the moment of transfer concerning the chromosomal abnormalities observed and about the morphological quality of the embryos.