Can any healthcare prof explain why chances of pregnancy on first attempt are >20%?

[HotFeet]

Surely if you had sex every day and your fertility and your partner's fertility was fine, the chance of conception is 100%???

I understand it's if you were to try once and only once.... But surely assuming all's good and you're at it every day for a month over your cycle it would be ca-ching and in the uterus bank as it were?!

Not every egg or sperm is capable of producing a viable pregnancy, even in normally fertile people. Not every fertilised egg implants in the uterus.

Not a medical expert but you have to remember there are so many factors involved - the egg and sperm that meet both have to be 100% perfect; the uterine lining must be just right for implantation; the cells need to start dividing in exactly the right way.