Calcium tablets are recommended for pregnant women starting at 20 weeks of pregnancy, but the specifics are recommended to be individualized according to the pregnant woman’s usual diet and pregnancy comorbidities. The average calcium requirement for adult non-pregnant women is 800 mg/d, which can be supplemented by 300 mg/d of milk, moderate amounts of nuts, soy products, and dark green leafy vegetables in the daily diet. For women with balanced dietary intake before 20 weeks of pregnancy and pre-pregnancy, the basic content of calcium, protein and trace elements in the body is sufficient and calcium tablets can be supplemented from 20 weeks of pregnancy, when an increase of 200mg/d calcium is recommended. In addition, for women with inadequate dietary intake before 20 weeks of pregnancy or pre-pregnancy or with severe obstruction of nutritional intake from pregnancy vomiting, calcium supplementation needs to be started from early pregnancy, or even before pregnancy, and needs to reach an average intake of 800mg/d calcium. Therefore, for most pregnant women with a balanced diet before and during early pregnancy, calcium supplements can be taken from 20 weeks of pregnancy to meet the increased calcium needs of maternal and fetal development and to promote the health of the pregnant woman and her fetus.