Many believe that pregnancy lasts nine months, but most pregnant women or those who have had babies, known that pregnancy actually lasts about 40 weeks. The medical profession gauges the true development of a baby using the first four weeks of pregnancy. Although physicians consider 40 weeks as a normal length of pregnancy, some women may experience fewer or more weeks before their baby is born. Development of the baby tends to be the same regardless of the length of pregnancy.
Month One
During the first month of pregnancy, a woman has officially been carrying a fertilized egg for two weeks. The fertilized egg, called a zygote, undergoes rapid cell division until a blastocyst forms and implants into the wall of the woman's uterus to receive nutrition and protection.
Month Two
The embryonic period begins during the second month. The Mayo Clinic describes the embryo as consisting of three layers that make up the outermost layer of the baby's skin, the central and peripheral nervous system, eyes, inner ear, and many connective tissues. Overall, the baby is about 1/2 inch long.
Month Three
In the third week of the third month, the embryo becomes a fetus. The genitals have started to form. Baby begins moving and an ultrasound can see and hear the baby's heartbeat. The baby is over two inches long and weighs about 14 grams.
Month Four
The genitalia becomes recognizable on an ultrasound. The baby begins forming bones, blood cells in the spleen and the neck become more defined, reports the Mayo Clinic. The fetus is over four inches long and weighs almost 4 oz.
Month Five
The fetus begins developing layers of fat to provide warmth and protection after birth. Hearing has developed for baby. Female reproductive organs become more refined, such as the development of the uterus. Identifying the gender of the baby becomes easy for an ultrasound technician during a routine 20-week ultrasound.
Month Six
Baby begins to swallow in the sixth month in preparation for life outside the womb. A fine layer of hair, called lanugo, forms over the baby's body. Real hair, including the eyebrows, begins to sprout. Male testes start the descent from the abdomen and female ovaries are in place. Baby's length is over eight inches and weight surpasses 1 lb.
Month Seven
Features become more refined. The lungs, immune system and liver continue maturing. Baby becomes less wrinkly. Length reaches 10 inches and weight is over 2 lbs.
Month Eight
Refinement continues for baby. The eyes are wide open, the testes in males have made their way to the scrotum and the female clitoris has become significantly prominent. The baby's lungs have yet to form, but she already begins to practice breathing by taking in amniotic fluid and expelling it.
Month Nine
Sensory abilities have improved for the baby during this month. A white coating, called vernix caseosa, thickens around the baby's skin. Weight gain will become rapid from now until birth. After the 37th week, the baby is considered full-term and can survive if delivered. Baby is now over 13 inches long and weighs nearly 6 lbs.
Month Ten
The baby's chest grows outward to accommodate the developing lungs. Baby still inhales amniotic fluid. The baby weighs about 7 lbs. and is more than 14 inches long. Labor and delivery will take place some time around the 40th week.
