After culling through several internet articles, here is one that answers your questions.
Evening primrose oil has been used to help induce labor. Because it contains GLA, it’s considered a good source of prostaglandins, which can help soften the cervix and prepare it to open [source: Birthing Naturally]. It’s the same reason some people claim having sex can induce labor. Semen also contains prostaglandins, which stimulates moisture production at the mucus membranes.
Because it may cause contractions, it is recommended that you don’t take evening primrose oil until you are at least 34 weeks into your pregnancy [source: Hudson]. You can take it orally or apply it directly to your cervix using your fingers or by inserting the capsule vaginally. And again, evening primrose oil’s ability to induce labor is still strictly theory. Some midwives swear by it, but most experts say it does nothing more than what the body was already ready to do naturally. Source
As others have stated, please check with your doctor before taking this.