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One of the safest oils for a breastfeeding mother to use is lavender (Lavandula angustifolia). Other oils that may be considered are geranium (Pelargonium graveolens), Roman chamomile (Chamaemelum nobile), and fennel (Foeniculum vulgare var. dulce).
For a list of essential oils that should be used sparingly by pregnant or nursing mothers, check out this post on safe essential oils during pregnancy and breastfeeding. Quick note: If you want a printable list of these oils, I have created the option to download them at the bottom of the post.