Home pregnancy tests work by detecting elevated hCG levels in the woman's urine. Maternal and embryonic structures interlock to form the placenta, the nourishing boundary between the mother's and embryo's systems. The umbilical cord extends from the placenta to the embryo, and transports food to and wastes from the embryo. Fetal DevelopmentIn humans, pregnancy usually lasts about 280 days. The baby develops for about 266 days, calculated from the time of fertilization to birth. The period of time from fertilization to birth is divided into trimesters, each about three months long. Each trimester brings significant advancement in the development of the embryo. During the first trimester the three embryonic tissue layers form. Cellular differentiation begins to form organs during the third week. After one month the embryo is 5 mm long and composed mostly of paired somite segments. During the second month most of the major organ systems form, limb buds develop. The embryo becomes a fetus by the seventh week. Beginning the eighth week, the sexually neutral fetus activates gene pathways for sex determination, forming testes in XY fetuses and ovaries in XX fetuses. External genitalia develop. During the second trimester the fetus increases in size during this trimester, and bony parts of the skeleton begin to form. Fetal movements can be felt by the mother. The fetuss body metabolism cannot yet maintain a constant body temperature, and its lungs have not matured enough to provide a regular respiratory rate. During the last trimester the fetus increases in size. Circulatory and respiratory systems mature in preparation for air breathing. Fetal growth during this time uses large parts of its mother's protein and calcium intake. Maternal antibodies pass to the fetus during the last month, conferring temporary immunity. Sometimes in the ninth month, the fetus rotates its position so that its head is down, partly as a result of the sh...
