Evening Primrose OilAviva Romm, midwife and Herbalist, says in "the Natural Pregnancy book" --That for late pregnancy, you can take 500mg daily for last four weeks. Do not take more unless cervix is "unripe" at due date, in which case you can double your dose.
In my practice, I don't have anyone do anything like that unless they went way overdue before, or are first timers that are totally unripe at their due date. Nature knows what it's doing, I personally don't feel the need to mess with a process that works well on it's own. EPO is pretty gentle and harmless though. It will give you prostaglandins that are the building blocks for the hormones in labor, but won't kick you into labor.
Aviva's notes on the topic in her book, which address EPO and more complex "partus preparators" are along the same line of thinking as me, she addresses herbal remedies that can be taken before labor, but says eloquently, "the use of herbs to prepare the body for labor begs the question of why one would use an herbal preparation to prepare the body for something it naturally knows how to do".
Whatever you decide to do, remember to talk to your care giver about it so they know where your mind is about it as well as what you are doing on a physical level....