Hi!
There are herbs that can be turned into tea for use as contraception. You can find more information about these herbs and their whereabouts in the journal (located in the tent). The primal regression will naturally fade out if you manage to avoid getting caught by feral roamers. Additionally, talking with Cassie daily once you have discovered the Cliffside Camp (and bringing her there) trigger a GG scene and reduce the PR. You can also ensure that you eat at least one berry a day to reduce the PR gain, or you can cook berry jam, which also reduces Olivia's PR. Completing certain quests can also help in reducing PR or increasing Olivia's PR pool capacity. Finally, regarding pregnancy, it is possible for Olivia to contract parasites from the swamp water (once the water temple is completed) which sometimes feed on the foetus and sometimes, she may experience a miscarriage on her own.