An average pregnancy lasts 40 weeks and is divided into three periods of time called trimesters. During each of these stages a baby undergoes incredible growth to transform itself from a fertilized egg into a human being.
First Trimester
During the first 12 weeks of pregnancy, cells multiply and evolve from a fertilized egg to a blastocyst (an inner group of cells with another group of cells forming an outer shell) to an embryo (defined when the cells begin to take on specific functions) until it finally become a fetus (the baby is considered a fetus until it is born). The umbilical cord and placenta also develop during this first trimester of pregnancy, carrying oxygen and nutrients essential to growth from mother to fetus and waste materials back from fetus to mother. Some of the cells from the placenta will form the amniotic sac that protect the baby while letting it float in amniotic fluid.
Many organs begin to form about three weeks after fertilization, including the brain and heart. The spinal cord as well as major blood vessels also form during these early weeks. By week 10, PregnancyPeriod.com states, almost all organs are completely formed, with the exception of the lungs, brain and spinal cord, which will continue to develop during the second and third trimester. By the end of the first trimester the fetus has grown to fill the entire uterus.
Second Trimester
During the second trimester, the baby becomes covered in lanugo, a coating of fine hair over most of its body, along with vernix, a white cheesy substance that covers and protects the skin from amniotic fluid during the course of the pregnancy. By week 14 doctors can identify the sex organs of the baby, and as its growth continues it will be large and strong enough to be felt moving and kicking, around weeks 16 to 20. In fact, by the end of the second trimester the baby will weigh almost 2 lbs. and begin having periods of waking and sleeping similar to those of newborns. As the placenta's tiny hair-like structures called villi expand deeper into the uterine wall during this period, more nutrients and waste materials are passed back and forth between baby and mother. By 20 weeks the placenta is fully formed. During this period the fetus learns to suck and swallow, and even develops fingerprints, according to AmericanPregnancy.org. Sweat glands, the liver and the pancreas are able to secrete fluids by the end of the second trimester.
Third Trimester
During the last trimester of pregnancy the lungs will grow and develop air sacs to prepare for survival outside of the womb. By week 24 a baby can likely survive outside the womb with medical assistance. The brain develops quickly during this trimester. During these last few weeks the baby really packs on the pounds, storing fat in preparation for birth. In fact, from week 30 until it is born, the baby will gain approximately 1/2 lb. per week to reach an average of 8 lbs. at 40 weeks. Although the bones are fully developed at this point, they are soft and pliable. By week 32 the lanugo disappears from the skin, and the bones continue to absorb calcium and other vital vitamins and minerals necessary for future growth. At 38 weeks the fetus is considered full term and is ready to greet the world.
