Pregnancy is accompanied by a number of changes that the mother and the baby undergo simultaneously. To understand these changes and developments, it is essential to know the week by week progress of a pregnancy. The duration of a full-term pregnancy is of 280 days or 40 weeks. These 40 weeks are divided into 3 trimesters of 13 weeks each. Signs of pregnancy are experienced at the beginning of the 2nd week, and the development of the embryo or the fetal development begins in the 4th week. Miscarriages and other complications are most common in the first trimester. Let's take a look at the week by week development of a pregnancy. The First Trimester Weeks 1 and 2: The first day of the last menstrual period is considered as the first day of pregnancy. During the first two weeks, the process of ovulation and fertilization takes place. Progesterone and Estrogen is produced in order to prepare the body for ovulation. Then, the ovaries release an egg, and if this egg is fertilized, conception is achieved. Early signs of pregnancy, which last till the 12th week, are experienced, and the baby's due date can be determined. Week 3: During the third week, the growth of the baby is remarkably faster. Rapid division and growth of cells takes place. It then enters the uterus in the form of a blastocyst. Then, the blastocyst attaches itself to the uterus wall. This is known as implantation. The amniotic sac and cavity start developing in this stage. Week 4: The blastocyst is implanted deeper within the endometrium. Once it is implanted, a hormone called human chorionic gonadotropin (hCG) is produced. The ovary stops releasing an egg every month, which is also why menstruation stops. The baby measures around 0.04 inches. Week 5: During this week, the embryo starts getting a definite shape. The baby's facial features, the neural tube, the bones and muscles begin to form. The placenta also develops and nourishment to the baby is given through it. Week 6: The baby's heart starts beating, which can be detected in an ultrasound examination. The brain, nervous system, respiratory system, and the digestive system also start developing. The baby measures around 0.08 inches. Week 7: The umbilical cord is formed during this stage. The leg and arm buds can be seen, and the cerebral hemispheres of the brain also start developing. Organs like the pancreas and appendix develop and an intestinal bulge is noticed. Week 8: The baby's eyelids, optical nerves, and the retina start forming in this week. The ears, nose, and the throat, start developing rapidly. The baby's genitals, in the form of buds, also begin to appear. Week 9: By this stage, the arms and hands have grown, and become longer. The legs continue to grow. There might be some movement owing to the developed muscles. Ears and eyes are visible externally. The baby measures around 0.90 inches, and weighs around 2 grams. Week 10: By the end of this week, the embryonic stage ends, and the fetal stage begins. All the vital organs have been formed by now. Tooth buds start forming, and if it's a baby boy, testosterone is produced by the testes. Week 11: There is an increase in the number of blood vessels in the placenta to support the rapid growth of the fetus. The pancreas, gallbladder, and thyroid start developing along with the baby's reflexes. Week 12: The fingernails and vocal cords are formed, and the kidneys start functioning. The pituitary gland starts producing hormones. Week 13: This week marks the end of the first trimester. The eyelids are closer together, the face appears more human-like, and the placenta is fully developed to provide nutrients, and dispose waste products. The intestines move from the umbilical cord towards the stomach. The Second Trimester Week 14: The baby's mouth and hands are developing rapidly during this week. The thyroid gland starts producing thyroxine. By the end of this week, the baby may start swallowing, breathing, sucking, etc. Week 15: Skin and hair start developing, and the bones grow harder because of calcium retention. The baby measures 3.98 inches, and weighs around 70 grams. Week 16: Movements of the baby can be felt, and various facial expressions can be observed. Muscles of the head and neck become stronger, which enables the baby to hold his/her head erect. Week 17: The mother's lower abdomen starts swelling gradually, and the breasts start getting prepared for milk production. The baby's weight increases, and a lot of growth takes place. Week 18: The bones begin to ossify (harden), the ears continue developing rapidly, and meconium (fecal waste matter) starts collecting in the baby's bowels. Week 19: Vernix caseosa, a white creamy substance, starts covering the baby's body. It forms a protective layer for the glands and sensory nerves. If it's a baby girl, egg cells are formed in the ovaries. Week 20: The baby's movements become stronger, and are more frequent. Hands and nail growth increases, and the uterus expands to make room for the growing fetus. Week 21: The digestive system begins functioning, and the baby may start swallowing the amniotic fluid, which helps the functioning of the digestive system. The baby measures 10.51 inches and weighs around 360 grams. Week 22: The baby's taste buds and nerve buds are fully formed, the sense of touch and the reproductive system continue to develop. Week 23: By this week, the mother's weight increases by 5-7 kg. Sounds are transmitted to the baby, and insulin is produced by the pancreas. Week 24: The lungs develop to be able to produce surfactant, a substance that prevents the air sacs from collapsing after the baby is born. Some amount of weight gain is noticed in the baby. Week 25: Small blood vessels start forming, the color of the skin turns pink, and the baby is able to hear and recognize certain sounds. Week 26: The five senses are fully developed, and the baby may develop a certain sleeping pattern. Growth of hair and eyelashes is observed. This is the last week of the second trimester, and the baby is now 14 inches long, and weighs around 760 grams. The Third Trimester Week 27: The optic nerve has developed along with the sense of sight. The immune system, the forebrain, and the lungs are still under development. Week 28: Development of the baby's brain in the form of grooves and folds takes place. Fats are deposited under the skin, which increases body weight, and makes the baby appear healthier. Week 29: There is a marked increase in fetal activity, and the chances of survival in case of a premature baby are high. Week 30: Convulsions may develop in the brain, and nerve cell activity is rapid. They might also try to mimic breathing movements to aid breathing, post delivery. Week 31: The process of urination initiates, and the baby urinates several times into the amniotic fluid. Weight gain may slow down, however, growth of the organs continues. Week 32: Features like the eyelids, eyelashes, hair, mouth, etc., of the baby are clearly visible. Back pain might be experienced by the mother due to weak stomach muscles. Week 33: The baby is able to detect light, and constriction and dilation of the pupils also occurs. Fats continue to be deposited, and weight gain becomes rapid. Week 34: The baby adapts the position for delivery, and the Vernix caseosa coating thickens further. Week 35: The digestive tract is not developed fully, and there might be a decrease in fetal movement. During the preparation for labor, the baby's position will be headfirst. Week 36: The baby's head is in the mother's pelvis, and the skull bones tend to overlap each other to facilitate passage of the baby through the birth canal. The baby appears to be chubby. Week 37: Strength and coordination is developing faster; the ossified bones help make the baby flexible to pass through the birth canal, and weight gain will decrease by the end of this week. Week 38: Fecal waste starts accumulating increasingly, which will be passed as the first stool after birth. The mother may feel very hungry and tired in this phase. Week 39: All the organs and systems are functioning properly, and the immune system becomes capable of protecting the baby from infections for 6-8 months after birth. The mother's body begins to prepare for labor and childbirth. Week 40: Labor sets in, and frequent contractions indicate the arrival of the baby. The pregnancy term ends in this week with childbirth. A healthy baby weighs around 6 - 10 pounds, and is around 20 inches long.