The fetal age in weeks is known as the gestational age. The fetal period is the earliest stage of human development and is the stage of the baby’s development in the mother’s womb before birth, from conception to delivery for a total of about 10 months, or about 280 days. During this period the fertilized egg develops into an embryo, which then continues to develop into a mature fetus.
1. Stages of intrauterine development of the fetus
The union of the egg and the sperm is called fertilization. The normal gestation period is divided into 3 periods.
(1) Germ stage (0-2 weeks): after fertilization, the fertilized egg moves slowly downward from the fallopian tube and finally implants in the endometrium, a process that takes about 10 to 14 days. This process is influenced by the regulation of estrogen and progesterone and a variety of cytokines and the environment of the uterine cavity.
(2) Embryonic stage (3-8 weeks): the embryo is in a state of rapid growth, which is the most important period for differentiation and generation of fetal organs, limbs and other physiological systems, i.e. the critical period, and the most sensitive period for fetal development.
(3) Fetal stage (9-40 weeks): After the process of embryonic organ differentiation is roughly completed, it enters the fetal stage, which generally occurs in the third month after pregnancy.
2.Development of physiological functions during the fetal stage
The physiological functions of the fetus acquire steady development after the embryonic stage. Starting from 3 months, the fetus is able to swallow and urinate, after 6 months, the fetus is able to breathe and cry, and after 7 months, it has the ability to survive outside the womb. During the last 3 months before birth, fetal development slows down. at 8 months, the fetal subcutaneous fat begins to grow and develop, which is important for fetal survival after birth.
(1) Nervous system: The fetal period is the first peak of brain development. The fastest development of the normal fetal nervous system is between mid-gestation and 18 months after birth. The nervous system is most vulnerable to intrauterine growth disorders.
(2) Respiratory system: The fetal respiratory unit is not well developed until 28 weeks, and if the fetus is born before 28 weeks, gas exchange is difficult and survival is difficult. The amount of alveolar surface active substance in the fetal lung does not increase rapidly until 40 weeks, so preterm infants under 37 weeks are at risk for respiratory distress syndrome.
(3) Urinary system: At 36 weeks of fetal life, the general structure of the kidney has been basically completed, but there is still a big gap compared with adults, and the glomerular filtration area and renal tubular volume are relatively insufficient, the latter being more obvious.
3.Motor function development during fetal period
The fetal reflexes and fetal movements are its initial forms of movement, and the human fetal period initially starts with spontaneous movements.
(1) Fetal movement: It is the spontaneous physical activity or peristaltic movement of the fetus in the mother’s body, which can appear when the fetus is 8 weeks old, and the most active period of fetal movement is from 28 to 30 weeks of gestation. There are 3 types of obvious fetal movements.
① slow writhing or writhing, which is most detectable in the 3rd to 4th month of gestation;
(2) strong kicking or rushing, which increases from the 6th month until delivery;
(iii) violent spasmodic movements.
(2) Reflex activity: Babinski reflex and other activities similar to sucking reflex and grip reflex are already present in the fetus at 3 months. after 5 months, the fetus gradually acquires instinctive movements that are important and valuable for its life, such as defense reflex, swallowing reflex, blinking reflex and tense neck reflex.
①Babinski reflex
This reflex can be seen in infants as early as 4-6 months of age. The reflex gradually disappears at about 6-18 months of age, but it can still appear during sleep or coma, and after 2 years of age, the same reflex as that of adults appears in the foot concussion.
②Sucking reflex
It is one of the innate reflexes of mammals and human infants, and is one of the unconditioned reflexes of newborns. When a newborn’s lips are touched with a nipple or a finger, the sucking and wriggling of the lips and tongue will occur accordingly. It disappears on its own 3 to 4 months after birth and is gradually replaced by active feeding movements. However, during sleep and on some other occasions, the infant still shows spontaneous sucking movements for a period of time. If the sucking reflex disappears or is significantly weakened during the neonatal period, it suggests an intracerebral lesion; if it is hyperactive, it is a sign of hunger; its presence after 1 year of age suggests cortical dysfunction.
③Grip reflex
Touching the newborn’s ulnar palm, sole or toes with a finger or wooden stick causes finger-toe flexion activity. The hand grip reflex gradually disappears from 4 to 6 months and is replaced by random movements. Absence of this reflex in the neonatal period or asymmetry on both sides is pathological, and its presence after 6 months also indicates brain disease. Plantar grip reflex disappears from 6 to 12 months, and early disappearance is mostly due to spinal cord dysplasia.
4.Behavioral development in fetal period
(1) Development of vision
The visual organs of infants are basically developed during the fetal period, and the optical properties of both eyes have been formed. 4 to 5 months old fetuses can already respond to visual stimuli with sensitivity. The initial onset of vision occurs in the late fetal stage, and the fetus at 4 to 5 months of age already has the ability to respond to visual stimuli and the corresponding physiological basis.
(2) Hearing development
The sense of hearing is already present in the fetus, and the fetal auditory receptors are basically developed by 6 months of age. The fetus can establish the sense of hearing in the first 3 months of life. Currently, it is believed that the fetus at 5-6 months of age begins to establish its auditory system and can hear external sounds at frequencies below 1000 Hz through the mother’s body. Therefore, it is feasible to implement fetal music education.
(3) Taste, smell and touch
Taste receptors begin to develop at 3 months of embryonic age, and are formed by 6 months of age, and are quite well developed at birth. Fetal olfactory receptors begin to develop at 30 days. By 7 to 8 months of age, the fetus’ olfactory receptors are quite mature. The earliest manifestation of the sense of touch is also in the fetal period. The fetus has the first tactile responses by day 49. At 4 to 5 months of gestational age the fetus has already established the initial tactile response.
(4)Fetal memory
The fetus already has the initial auditory memory ability at the end of gestation (around 8 months).
(5) Fetal speech
In the middle and late 5-8 months of gestation, the fetus already has the initial auditory response, has the primitive auditory memory ability, can roughly distinguish musical sounds, noise and speech, and shows the ability to discriminate and remember language. As the initial stage of the development of infant speech perception from low to high – the auditory stage, it is the period when speech perception first occurs.
(6) The possibility of fetal learning
The fetus can receive external stimuli such as speech and music and gain experience at the end of gestation, and the experience can be maintained until after birth and have a significant impact on its behavior. As memory is one of the conditions for learning, the fetus’ memory development provides the possibility for learning.