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Fertilization occurs in the ampulla of the fallopian tube, followed by early cell divisions as the zygote develops into a blastocyst. Implantation begins about 6–7 days later. In early pregnancy, human chorionic gonadotropin (hCG) from syncytiotrophoblasts supports the corpus luteum; by 10–12 weeks, the placenta becomes the main source of pregnancy hormones. Progesterone helps maintain uterine quiescence, estrogen supports uterine growth and breast development, and human placental lactogen influences maternal energy use. Maternal adaptations include changes in cardiac output, glomerular filtration, breathing, vascular resistance, and blood composition. Fetal circulation relies on the ductus venosus, foramen ovale, and ductus arteriosus to route blood around the liver and lungs.