A study says shapely sperm is the best indicator of a man's infertility. The reserch rewrites the standards for analyzing semen, which are based primarily on numbers of sperm cells in a sample. The study says current standards under or over estimate many men's fertility. A well shaped sperm has, among other things, an oval head and whippy tail. Researchers looked at sperm samples from fertile and infertile couples. The scientists say the most reliable determinant of fertility was well-shaped sperm cells. But the lead researcher says no measurement is infallible.The researcher is at the University of Rochester Medical Center. The study is in the New England Journal of medicine.