How a brain works, what makes it tick has always intrigued and mesmerized researchers. All progress and regression in our life can be easily attributed to the way our brain works, thinks, deduces, reasons, concludes, etc. Life starts at conception, and until the eighth week it is known as embryo. From the eighth week onwards until the moment of birth, it is known as fetus. Brain development involves the formation of the brain, nervous system and spinal cord, and begins at the embryonic stage itself. The brain development stages start from the first week and go up to the 40th week. Scroll down and see the stages of brain development your baby will go through before it is born. Week 1-2: The egg gets implanted into the uterine wall, and conception is considered as two weeks old. Week 3: Although it is still an embryo in its definition and formation, the backbone, cardiovascular system and the brain begin to form. Week 4: In this phase, the embryo further develops the three brain sections; forebrain, middle brain and hind brain, along with the optical stalk. Week 5: As the brain continues to develop, other organs like the circulatory system begin to function with all four chambers of the heart present. The facial features begin to develop, with a clear distinguished vision (through ultrasound) of arms and legs, complete with fingers. Week 6: This week sees the formation of the brain hemispheres and also some wave activity. The neural tube that connects the brain and spinal cord also closes in this duration. Week 7: By this week, the brain is growing at a rapid rate. Week 8: In this week, the head is quite large, as compared to the rest of the body. The development of the hind brain, responsible for regulating heartbeat, breathing and all concerned muscle movements also begin now. Week 9: The nervous system, by now, is quite developed for proper functioning. Week 10: The genitals begin to develop at this stage, determining the sex of the fetus. Brain development is also very rapid as the brain is forming 250,000 neurons per minute. Week 11: The spinal cord is clearly defined and the spinal nerves start to stretch out from the spinal cord. Week 12: The brain enlarges very little as compared to its birth size and shape, along with the development of the taste buds and vocal cords. Week 13: This week sees the completion of the first trimester. The heart, liver, spleen and many other organs are already functioning. Week 14: By this stage, the baby is taking plenty of nourishment through the placenta. The fetus also begins its practice of breathing - inhaling and exhaling. Week 15: The baby's torso is now growing rapidly and it is completely covered with lanugo; fine hair that protects the skin. The sense of hearing also sharpens at this stage. Week 16: The baby makes its presence known by pulling and tugging on the umbilical cord. Week 17: At this stage, the uterus begins to expand, to give more room to the baby. As the bones are growing and becoming hard, it needs protection. The spinal cord is protected by a barrier made of a substance called 'myelin'. Week 18: The baby develops sensitivity to light and at the same time, the brain is growing rapidly. Week 19: The brain becomes capable of forming millions of motor neurons, enabling the baby to develop and make muscle movements voluntarily. The forebrain further develop into left and right cerebral hemispheres of the brain. The nerve cells required for the processing of all the senses are also developing rapidly. Week 20: Nerve cells at this stage are making complex connections and sensory perception with the brain and the entire body. This development carries on until the age of 5 or 6. Week 21: Rapid growth and development of the fetus slows down in this week. The organs, especially the heart is getting stronger, and fat is getting accumulated in the baby's muscles. Week 22: As the brain is understanding complex sensory perceptions, the fetus becomes capable of distinguishing between different sounds. Week 23: All the nerve cells that so far had formed and developed independently, begin to join to form a complete nervous system. Week 24: At this stage, the brain starts to regulate all body functions, and activates the auditory and visual systems, which heightens the baby's overall senses. Week 25: The spinal cord begins to harden and straighten out. Nervous system is more developed and capable of controlling voluntary breathing. The optic nerve is highly heightened in perceiving light from any direction. Week 26: So far the brain was smooth in its form, but at this stage its tissues begin to develop creased, wave-like formations. The brain waves also get stronger at this stage enhancing auditory and visual sensors. Week 27: As the brain is actively processing many senses, it is quite possible that your unborn will begin to dream. Brain tissues are also developing fast. Week 28: The brain is completely monitoring the breathing and regulating body temperature. It continues to develop creases and fissures. This being the beginning of the third trimester, a blood glucose screening is done to determine gestational diabetes. Week 29-40: The third trimester stage sees that the brain continues to grow and increase rapidly in size, all the while establishing connections between nerve cells. The brain along with the lung is the last organ to develop, and the process is completed, close to the end of the third trimester. A newborn's brain is only about one-quarter the size of an adult's. The environmental, physical and emotional being of the mother, all have a profound effect on fetal brain development. The result of this is mental and cognitive disabilities. Throughout pregnancy, the fetus's brain is vulnerable to disruptions that may cause death, or mental disabilities and other physical developmental delays. Alcohol, smoking and drug abuse have been linked to disabilities, impairments and ADHD. Fetal alcohol syndrome is a disorder that occurs when a pregnant woman ingests alcohol during pregnancy. Alcohol can stunt fetal growth, weight and overall development. Studies have shown that women who smoke have smaller babies that have mental as well as physical retardation. Drugs like marijuana contain the active ingredient tetrahydrocannabinol (THC), that suppresses normal growth of the nerve cells. Alcohol, drug intake and smoking have been strongly condemned by the US Surgeon General, the UK Department of Health and almost all medical departments of the world. Genetics and hereditary, too, play a part in causing damage to a fetus' brain. As the fetus grows, it can inherit a brain dysfunction or an impaired cell leading to mental disorders. Toxins in the environment like cadmium, lead, etc., can get absorbed in the soil, then into our water and food, or even in the air, exposing the fetus to neurological damage. Even exposure to radiation can cause fetal brain damage. Any kind of abuse - mental, physical and sexual - can cause harm to the fetus. Fear and mental trauma can fluctuate blood pressure. The fetus receives all its nourishment from the placenta, which gets disrupted as a result of high or low blood pressure. The placenta sends oxygen to the fetus, which is essential for brain development. Excess or reduced oxygen damages the neurons. A woman falling on her belly or back, or getting hurt, will increase the chances of the fetus' soft skull getting damaged. There are instances where the child has developed mental impairment during the childbirth process. Brain development in a fetus and its associated problems are still being researched. Hence, there are no specific answers to the how's and why's of brain development. However, what does not change is the wonder of a cell, an embryo that develops into a fetus, then a child and finally, an adult going through the passage of time, and physical, mental and cognitive development.