**Core Concept**
During pregnancy, the body undergoes various physiological changes to support the growing fetus, including increased calcium requirements to support fetal bone mineralization and maternal bone health. The recommended daily intake of calcium increases during pregnancy to meet these demands.

**Why the Correct Answer is Right**
The increased demand for calcium during pregnancy is primarily due to the transfer of calcium from the maternal circulation to the fetus, which is essential for fetal bone development. The recommended daily intake of calcium is approximately 1,000 mg in non-pregnant women, but this increases to 1,300 mg during the second and third trimesters of pregnancy. This increased calcium requirement is essential for the development of the fetal skeleton, which is rich in calcium and phosphorus.

**Why Each Wrong Option is Incorrect**
**Option A:** 600 mg is below the recommended daily intake for non-pregnant women and is insufficient to meet the increased demands of pregnancy.
**Option B:** This option is not a valid choice as it is incomplete.
**Option C:** This option is not a valid choice as it is incomplete.
**Option D:** This option is not a valid choice as it is incomplete.

**Clinical Pearl / High-Yield Fact**
It's essential for pregnant women to maintain adequate calcium intake to support fetal bone development and prevent complications such as preeclampsia and gestational hypertension.

**Correct Answer: D. 1,300 mg**