Med books all say smthng different re sperm lifespan.1 book says max is 4 dys, 1 book says max is 5 dys, 1 bk says max is 6 days, another book says max is 7 days. Which book is correct? Very confused!

It's all the same. In real life, in one woman, nobody knows exactly when all the sperm are dead, when the egg actually pops out of an ovary, when an egg is fertilized, nor when a fertilized egg implants into the uterus... All dates are estimates, so 4 days = 5 days = 6 days = 7 days. They are all the same day in real life. As long as the husband is the only source of sperm, none of this matters.