Essential Oils to avoid while Pregnant 

Oils Considered NOT Safe During PregnancyAniseed, Angelica, Basil, Black pepper, Camphor, Cinnamon, Chamomile, Clary Sage (often used during labor by midwives safely), clove, fennel, fir, ginger, horseradish (should not be used by anyone), Jasmine, Juniper, Marjoram, Mustard, Mugwart (should not be used by anyone), Myrrh, Nutmeg, Oregano, Peppermint, Rosemary, Sage, Thyme, Wintergreen.
I would personally recommend checking with a doctor or midwife before using an essential oils during pregnancy.
Peppermint essential oil may decrease milk supply while nursing, and as such, I avoid it topically while nursing.

DO NOT CONSUME OILS  only for TOPICAL use, not to be consumed if also trying to get Pregnant.