Well at this age the eggs are much older as they have been hanging about in your body all this time, so the risks of having a baby with Downs syndrome and that sort of thing is greatly increased. However there is screening for all sorts of things these days and lots of women of 40 have had perfectly healthy babies so with all today's modern tests there is a good chance that all will be well if the mother is in good health. Personally the thought of having a teenager when I am in my fifties would be worrying though !!!