A typical span of pregnancy is 9 months, usually divided into three 13-week segments called trimesters. At the beginning of the first trimester a sperm cell and ovum unite to create a zygote. They zygote then develops into a multicelled blastocyst that implants on the wall of the uterus about 1 week after fertilization. By 9-10 weeks, the fetal heartbeat can be heard through a stethoscope. By the beginning of the second month the fetus is .5-1 inch long, grayish and crescent shaped. During the second month, the spinal canal and rudimentary arms and legs for, as do the beginnings of eyes, fingers and toes. During the third month, internal organs, such as the liver, kidneys, intestines, and lungs begin limited functioning in the 3-inch fetus. During this time the pregnant women undergoes many changes as well. Hormone changes affect almost every system in the body and can trigger symptoms such as headache, weight gain, upset stomach among other things. Morning sickness occurs in the majority of pregnant women during the first trimester as well.