The likelihood of pregnancy depends mostly on maternal age rather than paternal age. There is no definitive age when men cannot reproduce because the number, or concentration, of sperm remains relatively constant for them. Even with decreases in semen volume, sperm motility and sperm morphology (the size and shape of sperm), men are able to produce sperm well into their senior years. While some studies have shown that advanced paternal age may be linked to higher miscarriage rates, these data are difficult to interpret due to differences in maternal age and the small numbers of patients in these studies. Nonetheless, while the effects of paternal age on fertility may not be as dramatic as with maternal age, younger is probably better.