Eighty percent of your baby's energy needs are met by carbohydrates, mainly in the form of glucose, and almost 20 percent from protein. Fat is not used as an energy source but it is used for growth. All mineral, vitamin, and calcium needs are met from your own reserves and your diet. Two possible exceptions are iron and folate, a water-soluble vitamin that occurs naturally in foods; folic acid is the synthetic version. Folate does not easily cross the placenta to reach and nourish your baby. Your own iron reserves may already be low if you eat little or no red meat or if this pregnancy has followed on quickly from your last one. Your baby needs iron (and folate) to make red blood cells and because only a small percentage of iron in your diet is absorbed, iron supplementation is often recommended at this stage.
While all of your baby's gut structures were present by 20 weeks, it is not until this stage that all the enzymes needed for digestion are activated and the absorptive surface of the gut is established to a degree that would enable your baby to feed if she were born now.