**Core Concept**
During pregnancy, there is an increased demand for iron and folic acid due to the expansion of maternal blood volume and the growth of the fetus. Iron is essential for the production of hemoglobin, while folic acid is crucial for preventing neural tube defects in the developing fetus.

**Why the Correct Answer is Right**
The correct dose of iron during pregnancy is 100 mg per day, as recommended by various obstetric guidelines. This is because the increased demand for iron during pregnancy can lead to iron deficiency anemia if not adequately met. Folic acid supplementation is also crucial, and a dose of 500 mcg per day is recommended to prevent neural tube defects. This is because folic acid plays a critical role in DNA synthesis and cell division, and a deficiency can lead to congenital abnormalities.

**Why Each Wrong Option is Incorrect**
**Option A:** 500 mg iron +100 meg folic acid is excessive and may lead to gastrointestinal side effects such as nausea and vomiting. Additionally, the dose of folic acid is too low to prevent neural tube defects.
**Option C:** 100 mg iron +100 meg folic acid is incorrect because the dose of folic acid is too low to prevent neural tube defects.
**Option D:** 20 mg iron +100 meg folic acid is insufficient to meet the increased demand for iron during pregnancy, and the dose of folic acid is too low to prevent neural tube defects.

**Clinical Pearl / High-Yield Fact**
It is essential to note that folic acid supplementation should be started at least one month prior to conception and continued throughout pregnancy to prevent neural tube defects.

**✓ Correct Answer: B. 100 mg iron + 500 mcg folic acid**