Is it true that the chances of having a child with a chromosomal abnormality increases as a womans age increases over 40 year?

Yes. It actually increases to a small degree with every year after peak fertility in the mid 20's. At 40 the risk of a trisomy 21 down syndrome reaches 1% while at 45 it reaches 2%. This number is added to the random risk of 4% that any pregnancy has of an unexpected event (cleft palate, premi, other defect).