Pamela,
     I have never found any evidence of breastfeeding during pregnancy being linked to congenital defects.  In some places, breastfeeding during pregnancy and tandem breastfeeding - once rare - are relatively common, espec among Mums in mother-support groups.
    With two toddlers, I'd be inclined to encourage them, *well before* the baby is born, to reduce their breastfeeds to just certain times of the day, e.g. nap time, bedtime.   The new baby needs first priority (I don't need to tell you!) and it makes good sense for her/him to have a number of feeds per day *without* sharing with the twins.  It makes good sense to involve the two toddlers in "helping" you with the baby, e.g. bringing a clean anppy (diaper), bringing you a drink, helping emphasis that they are the big ones, who can do so many other things the baby can't do yet..
    Of course, your twins might have weaned by the time of the birth, making this discussion academic!
      All the best,
                     Virginia

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