This is a myth based on census records from 1500s-1700s France. Not joking!! That's where that one study got its data, and we've all just continued to believe it for 40+ years. With modern conditions, healthy women aged 35-40 will have an 80% chance of getting pregnant within one year of trying naturally (which is not much lower than women aged 30-35), and the chances of birth defects only "doubles" from 0.5% at 30 to 1% at 35. With fertility treatments, those risk factors become nearly inconsequential until 45!