Wow, I've never heard anything about eating burdock during pregnancy or breastfeeding. Here in Japan it is a very common food and generally known as a healthy food. . .in season, it's served daily in most homes.Here's a site
that says it's okay, even recommended during pregnancyAnother one
that says it's okay during pregnancy
Another thing I found
: "Burdock root—It is an aperient: A mild stimulant for the bowels; a gentle purgitive.
It is a cholagogue: An agent for increasing the flow of bile into the intestines.
It is a diaphoretic: An agent that promotes perspiration.
It is a diuretic: An agent that increases the secretion and expulsion of urine.
Burdock root contains arctiin, biotin, copper, essential oils, inulin, iron, manganese, sulfur, tannins, zinc, vitamins B1, B6, B12, and E.
It purifies the blood, restores liver and gallbladder function, and stimulates the immune system. Helps skin disorders such as boils and carbuncles and relieves gout symptoms.
Burdock root, dandelion, ginger, and nettle enrich mother’s milk"
When researching, it seems most of the sites that do mention burdock being contraindicated during breastfeeding or pregnancy are talking about an overdose using powdered burdock or burdock tea. If you are putting it in soup, are you talking about the fresh vegetable burdock (long, thin, sticklike vegetable usually pretty dirty here in Japan) or are you talking about putting some powdered burdock in the soup? I personally would think the vegetable would be fine (especially since it is eaten so often here by older babies, toddlers, children, adults, elderly) but I might be wary of the powdered form that might be in higher concentration.