31 weks pregnant and notice stretch mark already can i start putting coconut oil now even still prenant? does it not harm the baby inside?

Sure: Coconut oil does not harm - women have tried many creams/ butters - but honestly it is all about genetics - if you are going to get stretch marks there is nothing you can do about it.

The general rule: The chemicals in over the counter creams, lotions or ointments you put on your skin are not absorbed enough into your blood to effect a fetus. They will also not effect breast milk (unless placed on the feeding nipples).