Congenital anomalies (birth defects or congenital malformations) are abnormalities that are present at birth, even if not diagnosed until months or years later. They may be present from conception, as is the case with a chromosome defect (e.g. Down syndrome) or gene mutation (e.g. achondroplasia), and they also include those structural defects that occur in the embryonic period up to the end of the seventh week of gestation (e.g. spina bifida) or in the early fetal period between 8 and 16 weeks gestation, (e.g. orofacial clefts).