Complex sentence: Chromosomal abnormalities are found in more than half of embryos miscarried in the first 13 weeks; half of embryonic miscarriages (25% of all miscarriages) have an aneuploidy (abnormal number of chromosomes).
Split into simpler sentences: Chromosomal abnormalities are found in more than half of embryos miscarried in the first 13 weeks. Half of embryonic miscarriages (25% of all miscarriages) have an aneuploidy (abnormal number of chromosomes).