When trying to get pregnant how often should you have sex? Every day or every other day? Wait for the ovulation predictor kit (OPK) to become positive? Or when the basal body temperature goes up?

To improve your chances of getting pregnant you need to make love on your fertile days. Once a day, every day during your 5-6 fertile days. 

You only have 5-6 fertile days each cycle, the 4-5 fertile days before and the day of ovulation. Having sex more than 5 days before the day of ovulation or having sexual intercourse the day after ovulation will not get you pregnant.

How should you time sex?

Timing intercourse is defined as having sexual intercourse only within a day or two of ovulation. The problem with timing is that has never been shown to improve your pregnancy chances. In addition, many women often do not know the exact day that they will ovulate and studies have shown that even in women with regular cycles, ovulation can possibly happen at any time.

You are probably not improving your chances of getting pregnant if you wait for sexual intercourse until the ovulation predictor kit (OPK) has become positive. The problem with the OPK is that by the time it's positive, you may already have ovulated, and sex after ovulation is unlikely to get you pregnant. Waiting until the BBT temperature has gone up is even worse, as the temperature goes up after ovulation.

How often should you have sex? 

Couples who tried to get pregnant were previously told to have sex once every other day during their fertile days. But studies have shown you can improve your chances if you have sex once a day every day (as long as his sperm are OK) during the fertile 4-5 days prior to, and the day of, ovulation. In those rare circumstances when the man does not have enough sperm (oligospermia), the old rule of once every other day still holds true.

In addition, researchers recommend that the average couple who is trying to conceive should have sex regularly 2-3 times a week, every week because you don't know exactly when ovulation happens.  

Sex during fertile days   % Pregnancy
Sex every fertile day 37%
Sex every other fertile day 33%
Only once during fertile days 15%

Read More:
Due Date Calculator
Ovulation Calendar
Real Body Talk: Is Sex During Your Period Safe?