Because of this potential interference, it may be absolutely fine, if not advisable to get a little extra zinc. So long as this is only a once in a while occurrence, it seems like a safe practice, but again, you should speak with your doctor to be sure. Plus it may leave you feeling better so you can eat and drink better, get the exercise you need and that important sleep. The additional zinc should not cause the difficulties of secondary deficiencies or depressed immunity if it is not a daily practice. You are wise to ask the question, and would be even wiser to double check with the doctor.
Herbal supplements are best to leave alone during pregnancy.
Because there is no standard of identity, nor any guarantee of purity, and because potency is unpredictable, it is best to pass them up. Six ounces of yogurt a day is proven to boost immunity, if eaten daily. It takes a little while for the effect to occur if you are not now a yogurt eater, so the sooner you start, the better your will be able to avoid colds, etc. when you do become pregnant. Other foods that may stimulate immunity include garlic, foods rich in beta carotene, and a low fat, vegetarian diet. Thanks for your question.