Can ovulation occur at any time during the menstrual cycle? 

The exact day of ovulation can vary from one cycle to the next cycle. Once ovulation occurs, the hormone progesterone is produced by the corpus luteum in the ovary, suppressing the release of all other eggs until the following cycle.

Can ovulation happen more than once?

Sometimes, ovulation occurs more than once. When this happens, it usually occurs within 12-24 hours of each other. Once a woman fully ovulates, it is virtually impossible for her to ovulate again until the following cycle.

Can pregnant women ovulate and can it happen during pregnancy?

In general, as soon as the embryo implants into the uterine lining and you are pregnant, chances are that you stop ovulating. The pregnancy hormones generally prevent ovulation from happen after implantation. But like most other things in medicine, it's not 100% but close. Superfetation is an extremely rare event when a woman ovulates, gets pregnant, implantation happens, and she ovulates again later on and gets pregnant with a second fetus.