Thank you for your query regarding iron supplementation.
In a iron replete women with hemoglobin more than 12 gm percent, routine iron supplementation is not required. However considering that most women in our country have reduced iron stores elemental iron 100 mg for 100 days starting between 16 -20 weeks is apt. A hemoglobin done in antenatal period shall give an idea of iron requirement. It is important to note that women should be free from nausea and vomiting in pregnancy before iron supplementation is started.
For calcium supplementation the requirement increases in the 2nd and 3 rd trimesters, due to increased placental transport to the baby in this period. In addition to tablet supplementation it is important to incorporate foods rich in iron and calcium in food.
Also, a continued supplementation of folic acid beyond a period of organogenesis till delivery, prevents the development of folate deficiency anemia.
Hope that was helpful.