Men in their early 40s who wish to have children should not delay, experts have said.
Even leaving fatherhood to age 45 compared to 41 could influence a man's ability to get a woman pregnant, according to a new study.
Experts at the Huntington Reproductive Medicine Centre in Brazil found that the chance of fathering a child fell by 7% for every year above the age of 41.
Their analysis was based on 570 cycles of IVF treatment at the centre between March 2008 and April 2011.
Researchers controlled for the age of the women - a major factor in why pregnancies can fail - by looking at IVF cycles using donor eggs from young, healthy women aged 18 to 30.
The results showed that a man's age had "a significant impact" on the chance of a couple conceiving.
For fathers aged around 41, the chance of achieving pregnancy in IVF was about 60%. But this fell to about 35% in the group where men were aged around 45.
The researchers suggested the pregnancy rate may fall off at an even faster rate for men over 45.
The sperm of older fathers was also more likely to look abnormal, which has been linked to fertility problems, the study found. Men whose sperm appeared normal had a 22% increased chance of fathering a baby compared to those with less healthy looking sperm.
Dr Paula Fettback, who presented the findings at the American Society for Reproductive Medicine (ASRM) conference in Orlando, said men should not delay fatherhood.