I've been told my a friend who is a medical student that 35 is based on the graph of risk factors to age; at 35 the line begins to incline at a slightly sharper rate; there's a more dramatic rise at 40. But to say that statistically pregnancies at 35 are slightly riskier than at 34 is not to say that every 35+ mom is high risk, and most doctors and midwives will judge based on the individual. I'm still classed as low risk at 35, and my nuchal screening test came back with my risk factors equivalent of a 20 year old.