The answer is not the same for all mothers, but would vary depending among other things on their age, dietary history (read: toxin load), and level of Iodine
sufficiency at conception. A fully Iodine
sufficient woman who has no bromide, fluoride, etc. load would probably need very little for bearing an optimal child, whereas one who drank fluoirdated water for 20 years and mountain dew and ate junky or even semi-junky food probably ought to undergo a 1-2 year proper cleansing diet, including minerals which most people aren't even aware exist. Mercury fillngs in the teeth are another factor. Indeed, there are many factors in the complex equation of life, hence no simple answer. But, a little bit is better than nada, or so the song goes.