Fetal development begins seconds after the egg is fertilized and the fertilized egg contains its complete and distinct DNA. Every gene that the baby will carry for its lifetime is immediately present. The pregnancy is composed of 40 weeks of development and growth divided into three important stages: conception, embryonic development and fetal development, according to the University of Maryland Medical Center. Though the pregnancy is more vulnerable to illness or outside harm in the stages that lie within the first trimester, each stage is critical for a healthy pregnancy. The fetus, the uterus and other supporting maternal tissues and hormones work together toward the birth of a baby.
Conception
Conception is the first stage of fetal development and occurs at about week two of pregnancy, well before many mothers are even aware that they are pregnant. The womb is prepared to receive the fertilized egg with a layer of blood-vessel rich tissue to nourish and protect the developing fetus. The mother's hormone levels are delicately balanced to help orchestrate the changes needed for a pregnancy. The fertilized egg, or zygote, is formed by the fertilization of the female ovum by the male sperm. It then travels down the fallopian tubes, which lead from the ovaries to the uterus, to implant into the womb, where it will develop and grow rapidly for the next eight to nine months.
Embryonic Development
The embryonic stage is also within the first trimester. This stage has the most changes in development, though the fetus will still not be visible on an ultrasound, and occurs from conception to around week 11 of the pregnancy. During this time, the cells of the implanted zygote undergo rapid division to form organ systems, the skeletal system as well as its own placenta that will continue to nourish it. By the fifth week, the embryo is only about 1/3 inch long but is beginning to form limb buds. At week six, the heart has already started beating and the embryo may have a different blood type than its mothers. It also already has a full skeletal system. By the eighth week, the stomach is able to produce digestive juices, and kidneys begin to function. Finger and toes will have appeared by week 11, and the baby will be able to frown, squint and suck its thumb.
Fetal Development
The fetal stage begins at around week 11. The organ systems of the body, including the brain and spinal cord, continue developing, and the heartbeat can be heard by week 16. The lungs are also developing at this stage, and the baby is able to inhale and exhale amniotic fluid. The fetus is now about 6 to 10 inches long and can bend its knees and push against the uterus. Its main job in this stage after fine-tuning its organ development is to add a layer of fat beneath its fragile skin and grow rapidly in size. By 20 weeks, the baby can react to noises and vibrations outside the mother, and by 22 weeks, it can open its eyes. From weeks 29 to the end of gestation, the baby continues growing and develops its immune system to enter the outside world.
