For women, the possibility of pregnancy begins in the ovaries. A woman is born with 1 to 2 million eggs, more than a lifetime's supply. The eggs begin dying off almost immediately and she never produces more. The average egg lives only about 24 hours, so it has to be fertilized soon for pregnancy to happen. If it doesn't get fertilized on time, it either dissolves or is absorbed by the body. When an egg is fertilized, its own genetic material and the genetic material of the sperm that got to the egg first combine to create a new cell that starts dividing rapidly. A woman isn't actually pregnant until that new bundle of cells (called the embryo) travels down the fallopian tube and attaches to the wall of the woman's uterus. The average pregnancy lasts for 38 weeks from the date you conceive (called conception). However, doctors usually date your pregnancy from the first day of the woman's last menstrual period. Using this method, a pregnancy is said to last 40 weeks, or 280 days.
