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Th defects also go up after the age of 35 with the age of the father. A man's fertility starts to decline at 35. At 40, sperm quality declines and there is a higher risk of dwarfism, autism, bipolar disorder, schizophrenia, and other genetic defects. There is also a greater chance of defects when there is an age gap of more than 10 years between the mother and the father. Increased paternal age also increases the chance of miscarriage, premature birth, and low birth weight. It takes a 40 year old man 2 years on average to impregnate a woman in her 20's. A man's peak fertility is in his teens, 20's, and early 30's.