Prenatal development is the development of human individuals before they are born. The foetus is the organism 12 weeks after conception until birth. The embryo is the developing organism during the period when organs are forming. A neonate is an infant less than a month old. Postnatal development is the development of a human individual after he is born. It is now possible to study foetuses using ultrasound and fMRI.

The ectoderm is the outermost of three primary germ layers of an embryo. The central nervous system develops from the ectoderm. The other two layers are the endoderm and the mesoderm. The endoderm develops into the neural plate, a thickening of cells that will give rise to the brain. The neural plate folds into a neural tube, which is a hollow structure in the embryo that gives rise to the brain and spinal column. The first movements of the foetus are reflexes that occur using the spinal cord, without the use of the brain.

The cerebral cortex is the area of the brain that is associated with complex tasks, such as memory and language. In the first two or three months of pregnancy, there is relatively little development in this area. The cerebral hemispheres develop from the forebrain at about 9 weeks and rapidly increase in size and will later become highly specialised areas in the brain. After 6 months, sulci (deep narrow grooves of the outer surface of the brain) and gyri (ridges on the outer surface of the brain) have appeared. This shows that the brain has developed a lot since infolding is necessary to accommodate for the total brain area. At around 15 weeks, there is a lull in movement, because inhibition has appeared in the brain and this leads to a period of reorganisation of behaviour. After 27 weeks, the cerebral cortex is mature, but the brain continues to grow in size until adulthood, mostly because of myelination, the process by which myelin is formed around neurons. Myelin causes faster neurotransmission. Foetuses at about 24 weeks have the ability to learn and have a very basic form of memory.

Foetuses’ behaviour becomes progressively more organised as the pregnancy proceeds. At 34 weeks, they don’t continuously move but have distinct patterns of rest and activity. There are now two dominant patterns of activity: quiet sleep or active sleep. In the active sleep, the foetuses will be responsive to sensory stimuli. Early neuronal networks are being stimulated during active sleep. Foetuses at the end of pregnancy no longer spend a lot of time in active sleep, because their brain has matured and more inhibitory pathways have developed. Term foetuses are more active when the mother is not, for example, when she is asleep.

The emergence of the senses follows a set mammalian pattern of development. The rooting reflex is the reflex that causes new-borns to respond to one of their cheeks being touched by turning their heads in that direction.

1. Touch
   This is the first sense that is developed and is shown by the rooting reflex.
2. Chemosensory system
   This system is the second thing that is developed and included gustatory (taste) and olfactory (smell) senses. Substances such as cigarette smoke can pass into the bloodstream of the foetus. During the fourth month, foetuses begin to inhale amniotic fluids, which is the fluid surrounding the foetus. It is possible that foetuses learn some food preferences in utero. New-borns are selectively responsive to their own amniotic fluid and their own mother’s colostrum, the breast fluid that precedes true milk.
3. Vestibular system
   This is the sensory system that contributes to balance and spatial information. Foetuses move around a lot. They are constantly subjected to passive motion. The system is actively being stimulated and this stimulation is very important for normal growth and development. Preterm infants that are deprived of the vestibular stimulation show lags in neurobehavioural development. Kangaroo Care, skin/to/skin contact between the mother and the pre/term infant favourably affects the maturation of the autonomic nervous system. This system starts to work at about 14 weeks.
4. Visual system
   There is little visual stimulation for the developing foetus. It is barely stimulated during gestation, but it is functioning at birth. The eyes start to develop at about 5 weeks of pregnancy. The visual pathway is developed simultaneously with the eyes. At the end of the first trimester, the nerve fibres of the eyes have crossed. The striate cortex is concerned with many aspects of basic visual function. At birth, the visual system continues to develop, as it is still relatively immature.
5. Auditory system
   This development begins at about 6 weeks of pregnancy. The cochlea forms one coil at 9 weeks. By 10 weeks, sensory cells are present in the semi-circular canals. Cochlear function is considered to begin at around 24 weeks. After this time, the foetus can begin to detect vibroacoustic stimulation. The sounds that reach the foetuses are low-frequency sounds. The uterine sound environment is rich. Very loud sounds will result in a very fast heart rate.

Transnatal learning is learning that occurs during the prenatal period which is remembered during the postnatal period. Foetuses show habituation and dishabituation, a form of learning. Foetuses can probably learn about sounds they hear in utero. Early postnatal responding is influenced by prenatal sounds. This is shown by the fact that neonates prefer a muffled version of their mother’s voice, something they would hear in the womb. Infants respond differentially for as long as four months after birth to specific sounds that they experienced prenatally.

Perinatal is the period just before and after birth. Abnormal numbers of chromosomes are usually caused by an error in the separation of chromosomes into appropriate daughter cells during meiotic division. Risks for chromosome anomalies increases as maternal age increases. The age of the father plays a role in the likelihood of schizophrenia and autism.  DNA methylation abnormalities in the sperm of ageing fathers may be transmitted to offspring and confer risk for a range of neurodevelopmental and psychiatric disorders. Autosomal genetic disorders are disorders resulting from a mutation in a gene in one of the non-sex chromosomes, such as dwarfism, and risk for this increases as the age of the father increases.

Severe usage of alcohol during pregnancy can lead to cognitive and behavioural impairments and mild to severe physical anomalies. Alcohol has detrimental effects on the development and function of the placenta. Alcohol cannot be filtered by the foetus’ liver yet, so the foetus receives the unfiltered alcohol. Smoking is also a severe risk for foetuses, as it can cause a growth restriction. It is also associated with behavioural problems and cognitive weaknesses of the infant. Prenatal exposure to psychoactive substances prescribes to treat maternal conditions may result in a higher risk for adverse neonatal outcomes.

Specific nutritional requirements must be met for healthy foetal development. An adult disease that is associated with specific prenatal challenges during particular time windows is called developmental programming.

Maternal psychosocial stress during pregnancy has been linked to negative birth outcomes, such as alterations in foetal neurobehavioural development. It is possible that some stress is beneficial for the new-born, but the amount of stress must not be excessive.

A new-born has several reflexes that will disappear during the first year, such as the toe-curling reflex and the stepping reflex. The more active a foetus was, the more fussiness and inconsistent behaviour the infant will show.