In recent years there has been a growing realization that the period between conception and birth is a crucial one for the development of the individual. If prenatal conditions are favorable, the child will grow normally and get a good start in life. If the conditions are unfavorable, he may be seriously and lastingly handicapped from the moment he enters the world. Moreover, it is impossible to attain a full understanding of human development without knowing what goes on during this essential period of life.Information about prenatal development has been gathered from many sources: examination of embryos and fetuses removed from the mother’s body, studies of animals, the use of instruments to detect movements and esponses to stimuli during the gestation period, mothers’ reports of fetal movements, and photographs taken at various stages of prenatal life. The developmental process starts with the fertilization of the ovum by the sperm, and proceeds with such remarkable rapidity that within the short space of 280 days, a single microscopic cell develops into an indescribably complex organism composed of 200 billion cells. What is more, this growth process proceeds according to a timetable that is orderly and predictable to the minutest detail. It even has a built-in safety factor which enables the organism to survive if birth occurs as much as one hundred days before the normal term.The prenatal period is divided into three stages. During the period of the ovum, an interval of two weeks, the fertilized egg lives off its own yolk, dividing many times until it forms a cluster of cells about the size of a pinhead. This cluster descends through the Fallopian tube and implants itself in the wall of the uterus on about the tenth day, deriving its nourishment from the mother’s body from then on. During the period of the embryo, extending to the end of the second lunar month, growth and development are extremely rapid. By the end of the third week the heart starts to beat even though the organism is only a quarter of an inch long. The liver secretes during the seventh week, and by the end of two months the facial features, fingers and toes are well formed. At the same time the “accessory apparatus” develops, consisting of the placenta, the umbilical cord, and the amniotic sac. These structures provide the embryo with nourishment and protection. From a psychological point of view it is important to recognize that there are no nerves in the umbilical cord, and therefore no transfer of thoughts from the mother to the embryo is possible.The third prenatal period, the period of the fetus extends from the end of the second lunar month to birth. No new features appear during this period, but growth and development are steady and sequential, following two principal directions: the cephalocaudal or head- tail direction and the proximodistal direction, from the center of the body outward. In other words, the head develops more rapidly than the legs, and the internal organs reach full functioning before the hands and fingers. In fact, the heart, stomach, and other internal organs are well developed by the fifth month, and by the sixth or seventh month the fetus has reached the age of viability, which means that it has a chance of survival if birth occurs at that time.There is a fairly definite timetable for activity as well as for physical development during the fetal period. The umbilical cord often becomes twisted in the second or third month, indicating that the unborn child turns around at that time. Peristaltic movements also start very early, probably by the seventh week. Most of the basic reflexes—swallowing, palmar, plantar, flexion, and Babinski—are established in the fourth and fifth months, and by birth all the others are present. Another type of reaction, known as mass activity or generalized movements, starts in the head region as early as the third month under appropriate stimulation. These movements become spontaneous a little later, and by the fourth and fifth month the head, arms and legs are capable of moving independently. In some fetuses these movements occur as much as 75 per cent of the time, but in others as little as 5 per cent; and some fetuses squirm while others kick or have hiccups.The various movements increase in strength and number up to the ninth month, and are most violent when the mother is fatigued. There is also evidence of fetal movements if the mother is frightened or suddenly becomes angry. Overactive fetuses tend to become underweight, since they use up their energy instead of storing it in the form of fat. Infants who have been active as fetuses tend to develop motor co-ordination earlier than average, but those who have been less active usually have less difficulty adjusting to the postnatal environment directly after birth.