Further analysis demonstrated that 95% of the NGF population variation is due to age alone for ages up to 25 years. The remaining 5% is due to factors other than age e.g. smoking, BMI, parity and stress. We can speculate that as chronological age increases, factors other than age become more important in determining the rate at which NGFs are lost through apoptosis.
http://plosone.org/article/info:doi/10.1371/journal.pone.0008772#article1.body1.sec3.p1

Those couples having difficulty conceiving should be told of 5% risk factors which are preventable. Should be an awareness campaign re. age and fertility. Primary care providers could counsel their patients at an earlier age-late twenties- about decline of ovarain reserve.