Which is the best preventive measure to avoid pregnancy before participating in sex?

IUD: The Mirena (levonorgestrel) iud is the most effective form of contraception, as effective as a tubal ligation.

Depends: If you look specifically at each contraceptive method, the best rates of prevention are found in users of the iud, subdermal implants (ie, Nexplanon or implanon), and injectable contraceptives. This is not just bc they are effective in their own right, but also bc they take much of the "human error" out of the equation., since they dont require you to remember daily to use them.