Pregnancy begins when a sperm fertilizes an egg. It is difficult for humans to get pregnant compared to other animals. An embryo develops into a fetus over several months before birth. During pregnancy, the fetus receives oxygen and food through the placenta as it grows inside the mother's uterus, causing changes in her body like increased urination and sensitivity to smells. Pregnancy and childbirth involve stages like testing, ultrasound exams, childbirth preparations, labor, and delivery with or without medical assistance.