Generally speaking, the rate of chromosome abnormality is proportional to the pregnant woman's age. For example, a woman over forty has a 40 times greater chance of having a child with Down's syndrome than does a woman in her twenties.
一般說來，孕婦年齡愈大，染色體異常的機率越高。以先天性蒙古癡呆症的發生率而言，四十歲以上的產婦是廿至廿九歲的四十倍。