In all mammals, including humans, fetal gestation occurs internally, in the womb. It is here that the fetus develops from a fertilized egg to a fully formed infant capable of surviving outside the mother's body. However, while in the womb (“in utero”) the fetus is entirely dependent on its mother's body to get the substances it requires to survive and develop. These substances are delivered via the blood of the mother.
Oxygen is the key component to survival that the fetus receives from its mother. The fetus cannot breath air, initially because it has not developed a respiratory system and later, when it has, because it is fully enclosed in the womb, suspended in the amniotic fluid that protects it. Indeed, its lungs will contain this fluid. So oxygen is delivered in the blood that passes between mother and fetus.
Essential nutrients also pass from the mother to the fetus via the blood. These range from calcium for developing bones, iron for the circulatory system and a range of vitamins that effect skin, hair, eye and brain development. It is essential that the mother eat healthily during her pregnancy. This will enable the fetus to develop well. A balanced, nutrient-rich diet also means that the mother will not put on too much or too little weight during pregnancy, either of which cases could effect the development of the fetus.
All cells in a human being contain the genetic code, the DNA, that makes them unique. It is the fusion of the genetic codes of a mother and father when an egg is fertilized that creates a unique fetus. This genetic code is also present in the cells of the mother's blood that passes to the fetus. This will effect the cell composition of the fetus and, thus, the resemblance of the child to the parents.
The mother and fetus do not share a blood supply directly. Blood is diverted from the mother's arterial system to the placenta. This is an organ that is attached to the lining of the womb while the mother is pregnant. The fetus is linked to the placenta by the umbilical cord, attached to its thorax. This cord has two arteries; one delivers the oxygen- and nutrient-rich blood to the fetus, while the other returns depleted blood, which contains waste material such as carbon dioxide. This depleted blood reenters the mother's bloodstream where it can be recycled.
While the primary direction of nutrients, oxygen and genetic material in the umbilical relationship is from mother to fetus, the possibility exists that some of the cells of the fetus may travel into the mother's physiological system. A report published in 2010 in Scientific American magazine stated that a small number of cells may migrate from the fetus and could have an effect on the mother's immune system and her risk of contracting certain diseases later in life.