Human development in the womb is a complex process. Lasting 40 weeks on average, it can actually be completed in anywhere from 38 to 42 weeks. In this relatively short time, a single fertilized egg grows and develops into a complete human baby. The development of a human baby in the womb is divided up into three trimesters.
The first trimester, which occurs over the first three months, or weeks 0 to 13 as counted from the mother's last menstrual period. During the first few weeks of this time, the egg is fertilized, starts dividing and implants into the mother's uterus. After implantation, the ball of cells changes into an embryo as the heart, circulatory system and nervous system begin to form. Some of the cells from the original ball of cells become the supporting tissues, such as the amniotic sac that surrounds the baby and the placenta and umbilical cord that pass blood between baby and mother. By the end of the first trimester, the limbs, facial features, genitals and all major body structures have formed and the baby has begun to move, although it can't be felt yet. The baby transitions from being called an embryo to a fetus at this stage.
The second trimester starts at week 14 and lasts until week 26. The baby's growth in the womb takes it from about 6 inches and 4 oz. at the start of the second trimester to about 14 inches and 2 1/2 lbs. by the end. Details such as fingerprints, eyelashes and fingernails form during this trimester. The baby forms protective coverings during this trimester, such as a fine coating of hair called lanugo and a cheese-like substance called vernix that protects the fetus from the surrounding amniotic fluid. The movements of the baby become stronger as he practices stretching and contracting his muscles and moves around inside the womb.
From week 27 until the end of pregnancy is considered the third trimester. At this stage, all of the major development has been completed and the baby could potentially survive outside the womb, even at the start of the third trimester. Over the course of this trimester, the brain continues to develop, the bones start to calcify and the lungs mature. The fetus will occasionally develop hiccups as he practices breathing by moving amniotic fluid in and out of his lungs. The baby will also put on fat rapidly, which fills out his skin and makes it less red and wrinkled, and he will open his eyes, which have been closed since the early part of the second trimester. At the end of the third trimester, the baby will turn head down and prepare for his upcoming birth. By the day of birth, he will weigh between 6 to 10 lbs. and be between 19 and 21 inches long.