How the embryo and fetus develop throughout a pregnancy
The moment of conception is the beginning of the dramatic and exciting development of new human life. The potential fate of this meeting of sperm and egg in the outer part of the fallopian tube may be foretold within the first few hours, or days, following fertilization.
The cells then fuse and immediately start to divide, so that after 4days there is a mass of cells called the blastocyst. This develops little fingerlike structures, known as chorionic villi, which are necessary for the embedding of the developing embryo. After 7days, the embryo reaches the uterus. The villi start borrowing into the lining of the uterus, and the egg receives a new source of nutrition by opening into the mother’s blood vessels.

The first trimester
The mass of cells begin to form into a fetus and amniotic sac. As the fetus develops, a cord goes out to the yet-to-be-formed placenta. Amniotic fluid is put into production, and the amniotic and chorionic membranes appear.
Normal pregnancy dating refers to the first day of your last period. But, to be more accurate, I am using a method that refers to the moment of conception (day 14 of that cycle). So, where I say 2 weeks after conception, that would be the same as the 4th week of normal pregnancy dating, or the time of your first missed period.
By the 2nd week after conception, the pregnancy is large enough to be seen by naked eye. The ovary still contains the tiny corpus luteum, which is supporting the conception.
By the 3rd week, the embryo is 2 millimeters long. Its major organs, such as the spine, nervous system, head, and trunk, are just starting to take shape. By the 4th week, the head is formed, and the chest, abdomen, brain, and spinal cord are complete. Limb buds begin to appear, and by the end of this week, the heart is formed and circulation begins.
By the 6th week the heart starts beating strongly. All the major internal organs, including the lungs, are formed. Growth of the eyes and ears is now taking place. The embryo is 2.2 centimeters (nearly an inch) long. This is the very worst time for the mother to be contact with German measles (rubella), as the fetal eyes and ears would be directly involved. An ultrasound scan should now be able to show the fetal heart pulsing, and the viability of the pregnancy can be assessed.
By the end of the 7th week, the embryo is about 3 centimeters long (just over 1.25 inches) and weighs 2 grams (less than an eight of an ounce). Although the baby can now move,  you will not feel it for some time yet.
By the 8th week, the umbilical cord is formed, but there is still not a placenta in working order. The embryo will be 4.5 centimeters (1.75 inches) long and will weigh 5 grams (less than a quarter of an ounce).
By the 9th week after conception, the embryo is recognizable as a human being. Its eyes are completely formed and it will now be classified as a fetus. It will be about as long as your little finger, 5.5(2.25 inches) centimeters long, and will weigh 10 grams (one-third of an ounce).
All the major organs are formed within 9 weeks, which is known is the period of organogenesis. From this time on, therefore, the fetus is not subject to major congenital catastrophes, although it may be affected by environmental hazards or premature delivery.
In the 10th week the face is completely formed, and the external genital organs are forming. The baby is 6.5 (2.5 inches) centimeters long, and weighs 18 grams (nearly two-thirds of an ounce).
By the 11th week from conception, the sex of the baby may possibly be seen on an ultrasound scan (if it is a boy at least). The fetus will be about 7.5 centimeters (3inches) long and will weigh 30 grams (about an ounce).
By the 12th week after conception marks the end of the first trimester, when all the organs are formed, but the baby is still immature. The fetus, as we have seen, could not live independently outside the uterus,
The uterus is now so enlarged that it begins to protrude out of the pelvis, and your doctor should be able to palpate it (feel the uterus with his or her hands) abdominally. The uterus also now contains about 100 milliliters (lees than a quarter of a pint) of amniotic fluid.
The object of the pregnancy from this point on is to help the baby further mature so that it can survive outside the womb.

The second trimester
By the 13th week, bodily changes became evident in the fetus. The neck has lengthened, and its head no longer rests on its chest. The abdominal wall closes, concealing the intestines, which until then have been on the outside.

In the 14th week the baby’s joints start moving; fingernails and toenails are grown. A fine hair called lanugo covers the entire body. The baby is also covered with a greasy substance called vernix, which protects its skin from watery amniotic environment. The fetus is now 16 centimeters (6.25 inches) long and weighs 35 grams (1.25 ounce).
From the 15th to the 18th week, the fetus grows rapidly in both its length and weight. It has hair on its head, and, because of increased muscle development, it will begin to make some very active movements that the mother can feel. There is still a relatively a large amount of amniotic fluid in which the baby is swimming. The fetus is now 25.5 centimeters (10 inches) long and its weight begins to jump to around 340 grams (nearly 12 ounce)
By the 22nd week after conception, the baby is about 35.5 centimeters (14 inches) long and weighs about 570 grams(1.25 pounds.)
By the 26th week, the baby’ head is only slightly larger in proportion to its body size, so it takes on the appearance of a more normal baby. It should be about 37 centimeters (14.5 inches) long and weigh around 900 grams (nearly 2 pounds).

From the 26th week, the fetus can be regarded as viable or capable of independent life in the outside world, although highly dependent on neonatal care. Birth from this time on is called premature. Legally the baby must be registered, and if it dies, the baby will require burial. Neonatal care gives an almost 70 percent chance of survival at this stage, but there are many, many risks along the way in the next few weeks.

The third trimester
At the beginning of this trimester, the baby is still covered with a greasy vernix, and its lings are not mature, which of course offers the main problem for the neonatologists.
By the 30th week after conception, the baby is perfectly formed. The head in proportion to the body, and the baby has a good chance to survival if born. The baby is about 40.5 centimeters ( 16 inches) long and weighs about 1.6 kilograms (3.5 pounds).
At 36 weeks, the baby is considered fully mature. Its organs are not only formed but are work normally. The baby can survive if born now, or if induced or delivered by C-section, and it should not have a problem with its lungs because by now they are mature. The baby is about 46 centimeters (18 inches) long and weighs about 2.5 kilograms (5.5 pounds).
By the 38th week after conception ( the classic 40 weeks of full-term birth), the baby is ready to be born. The fine body hair called lanugo has disappeared, and the baby has early head hair. The nails are properly grown. It will have put o a lot of body fat in the past month to give a chubby, healthy appearance at birth.
At full-term a baby as long as 50 centimeters (nearly 20 inches) and will weigh an average of 3.5 kilograms (7.5 pounds), though of course all babies vary in length and weight.