Several micronutrients are important for the health of the developing fetus, especially in areas of the world where insufficient nutrition is common. Women living in low and middle income countries are suggested to take multiple micronutrient supplements containing iron and folic acid. These supplements have been shown to improve birth outcomes in developing countries, but do not have an effect on perinatal mortality. Adequate intake of folic acid, and iron is often recommended. In developed areas, such as Western Europe and the United States, certain nutrients such as Vitamin D and calcium, required for bone development, may also require supplementation. Vitamin E supplementation has not been shown to improve birth outcomes. Zinc supplementation has been associated with a decrease in preterm birth, but it is unclear whether it is causative. Daily iron supplementation reduces the risk of maternal anemia. Studies of routine daily iron supplementation for pregnant women found improvement in blood iron levels, without a clear clinical benefit. The nutritional needs for women carrying twins or triplets are higher than those of women carrying one baby.