The Chronological Development of the Fetus
During the first month of pregnancy, rapid growth of the fertilized egg and the development of an early placenta take place. At the end of the month, the fertilized egg is about fourteen days old and is smaller than a BB shot. The placenta is producing the hormone HCG, which allows the mothers body to make a multitude of changes to support pregnancy. The hormone will now show up in the mother's blood so that a laboratory test at this time could confirm a suspicion of pregnancy.
The fertilized egg is about the size of a marble, but the developing baby is only half that size. In the four weeks since fertilization, it will have developed a complete circulatory system and a placenta, its embryonic heart has begun to beat, and it has the beginnings of a real heart, brain, limbs, ears, a nose, and eyes.
Eight weeks (two lunar months)
The ovum is about the size of a hen's egg; the developing embryo is slightly larger than the yolk. It has developed limbs and external genitalia. The cavity within the placenta is enlarging to accommodate fetal growth and already contains the amniotic fluid in which the fetus floats. The fetal heart can be heard by ultrasound fetoscope and be seen beating by sonogram. The first tiny movements occur.
Twelve weeks (three lunar months)
The ovum is about the size of a tennis ball, while the fetus is about two-thirds that size. The fetus develops fingers, toes, and nails and continues to mature. The tiny fetus is complete and moving.
Sixteen weeks (four lunar months)
The fetus is about seven inches long and weighs about one-fifth of a pound. It has breathing movements and swallowing reflexes. Between the fourth and fifth months, the fetal movements, known as quickening, can be felt by the mother. Usually by the start of the fourth month, the doctor will be using a small machine called a Doppler that translates movement into sound, which will allow anyone in the room to hear the baby's heartbeat.
Twenty weeks (five lunar months)
he fetus is ten inches long and weighs a little more than half a pound. Its body is covered with a cheesy white protective material called vernix caseosa. The facial features are fully developed. Early toenails can be seen. At this time the doctor can perform a directed ultrasound during which the baby can be evaluated closely for many defects.
Twenty-four weeks (six lunar months)
The fetus is about 13 inches long and weighs about 1½ pounds. If born at this time, it might possibly survive a few hours, although the lungs are usually too immature to sustain life. Its skin is shiny and red and is covered with a fine hair known as lanugo.
Twenty-eight weeks (seven lunar months)
The fetus is 15 inches long and weighs about 2½ pounds. Each day, its chances of surviving premature birth are better and better. The baby is getting larger and may be moving less because it has less room. The eyes open now, and good hair development occurs.
Thirty-two weeks (eight lunar months)
The fetus is about 16 inches long and weighs approximately 3 pounds. Its chances for survival are increased. The closer the mother is to term, the better the baby's chances for survival.
Thirty-six weeks (nine lunar months)
The baby is fully formed after thirty-six weeks, measures 18 inches, weighs a little more than 5 pounds, and spends all its time maturing and building up fat tissue to provide it with warmth after birth.
The baby is ready and waiting for a signal that will precipitate labour. It measures about 19 inches and weighs an average of 7 pounds.