A defective blastocyst (e.g. chromosomal abnormalities) is a possible cause; a non-receptive endometrium is another cause. Blastocyst embryo transfer into the uterine cavity is performed about 5-6 days after egg collection. Transfer of one or two blastocysts is recommended to avoid high-order multiple pregnancies. Supernumery blastocysts can be frozen for future use.