It only because your blood volume increase.. with you bb get what it needs from you.. therefore it "looks" like you are 貧血. take the 鐵丸 will let your haemoglobin return to normal (but anyway, there wont be a problem after delivery).
there is 2 types of鐵丸, one can cause便秘, but one will not (but more expensive).. so check with you dr. when you have 便秘.
so, dont worry about your bb, you both will be fine.
personally, I dont worry about the iron.. my Haemoglobin is marginal but I remember.. I should have some extra Ferritin(a iron storage in our body) when I do my check-up before pregnant.