It has long been thought that biological clocks were strictly a female worry as men can father children even at an advanced age.
But research is increasingly confirming that men should not leave it too late to have children.
Just as many older men are slower than they were in their youth, researchers have found that embryos produced with the sperm of older men are slower to grow.
Another study, presented at the American Society for Reproductive Medicine in San Antonio, also found that sperm from older fathers accumulates more mutations.
IVF scientists in Boston found embryos created with sperm from men over the age of 50 divide more slowly and take longer to reach the early stage known as a blastocyst, when the embryo is ready to be implanted in the womb.