How long does it take for a prenatal vitamin with iron to start working in your body? When you start taking a prenatal vitamin with iron, it generally takes some time for the iron levels in your body to increase. The exact timeframe can vary depending on several factors, including the severity of iron deficiency, individual metabolism, and the dosage of iron in the prenatal vitamin. https://www.quora.com/How-long-does-it-take-for-a-prenatal-vitamin-with-iron-to-start-working-in-your-body/answer/Vikash-Sharma-2756