Why female above 40years increases chance of downs syndrome?

However, most of the babies who haveDown Syndrome (about 75%) are born to mothers who are 35 or younger. ... Because the chances of having a baby with Down Syndrome increase with the age of the mother, many health care providers recommend that women over age 35 have pre-birth testing for the condition.

Harry Fisch and others, over the period 1983 to 1997, investigated 3,429 cases of Down Syndrome in relation to father’s age.

The researchers found that fathers over 40 years of age, when having kids to women aged over 35 years, had twice the Down syndrome risk factor, when compared with men 24 years of age and younger also having kids to women aged over 35 years. If the women were under 35 years of age, the father’s age had no effect on whether it would be a Down Syndrome pregnancy.

For mothers over 40 years of age, the researchers discovered that an increase of 50% in Down syndrome risk factor was caused by the advanced age of the father.

So, the biggest cause of Down Syndrome pregnancy is the advanced age of either or both the mother and father.