Younger students sometimes think that the fetus eats pieces of mashed potatoes and hamburger sent to it by its mother. It
true that the fetus is completely dependent on its mother for food but the main source of energy is glucose, not bits of pizza. It may seem a monotonous diet for nine months but glucose, amino acids and fatty acids alone are enough to cause the tremendous increase in size of the fetus. The glucose diffuses from the maternal bloodstream into the placenta where it may be stored as glycogen (animal starch) and then into the umbilical vein of the fetus.
Insulin which is made by the pancreas of the fetus as early as three months is necessary for the fetal cells to metabolize the glucose. The amount of insulin produced is directly correlated with the concentration of glucose. Therefore a diabetic mother with a high glucose level causes her fetus to produce extra insulin which in turn causes a large uptake of glucose by fetal cells and accumulation of fat. In other words, the baby will be very large at birth, often weighing over ten pounds.
Even though all its nutritional needs are taken care of by the mother, the fetus has to start developing and exercising its own digestive system so it will be able to eat when it is born. The fetus practices by swallowing amniotic fluid as early as eleven weeks. At the same time peristalsis and absorption occur in the small intestine.
Swallowing amniotic fluid is useful for two reasons in addition to the exercise it gives. The fetus helps control the volume of fluid by swallowing it. (Urine contributes significantly and increasingly to amniotic fluid volume from early in the second trimester.) It may ingest up to 750 ml per day in late pregnancy according to Crelin (1973). If for some reason the fetus is unable to swallow, the volume of fluid could even triple. This abnormal condition is called hydramnios or polyhydramnios. The second reason for swallowing amniotic fluid is that it contains a tiny amount of protein so that the fetus actually derives a little nourishment.
When a fetus drinks the amniotic fluid it also ingests lanugo (fetal hair) and discarded cells. All of this debris accumulates in the large intestine as meconium, a sticky greenish substance that is excreted during the first few days after birth.
The baby stores reserves of glycogen in its liver and muscles preparatory to birth. After it is born it rests and lives on these reserves (and an occasional bottle of sugar water) until the mother’s colostrum and then the milk come in a few days later. During this quiescent time its body is adjusting to its new environment (Brazelton, 1972).
The term baby is well adapted to sucking. It pulls the nipple far into its mouth so that ridges on the hard palate hold it in place. Its broad flat tongue then makes a channel for the milk.
The baby is able to breathe comfortably through its nose during prolonged periods of sucking, an impossibility for an adult. There is an anatomical explanation. The newborn’s larynx is located so much higher in its throat than that of an adult that the opening (covered by the epiglottis) is above the stream of milk and the baby can breathe and eat at the same time without choking. At about three years of age, the larynx moves down into the throat along with the back of the tongue which is fastened to it (Crelin, 1973).