By day four the zygote has left the fallopian tube and entered the uterus, where it can benefit from the nutrients its mother will provide.
Because the new life is so genetically distinct, it must release a special protein to prevent a defensive response by the mother’s immune system. Then the embryo can implant in the uterine lining, (5) where the lifeline between mom and baby begins to form. This process is complete by day twelve after fertilization. (6)
The embryo also produces a hormone called human chorionic gonadotropin (hCG) about eight days after fertilization. This hormone brings the menstrual cycle to a halt, allowing the pregnancy to continue. Present in the mother’s blood and urine, hCG is the substance detected in most pregnancy tests.
At this time the placenta – a mix of tissue from both mom and embryo – will form. The placenta allows oxygen, nutrients, and water from the mother’s blood to feed the embryo, and waste like CO2 removed, without intermixing the blood’s genetic material. This circulation begins by the end of the third week. (7)
The placenta keeps the fetal body temperature several degrees warmer than mom’s, as well as many more functions that provide for the baby’s well-being until birth. (8)