During pregnancy many hormone levels increase. Prolactin is increased. This regulates lactation and during pregnancy it causes enlargement of the mammary glands. The parathyroid hormone increases causing increases of calcium uptake in the gut and reabsorption by the kidney. Cortisol, aldosterone, and placental lactogen increase as well.
The zygote is the initial cell formed during reproduction. A human zygote exists for about four days, and becomes a blasocyst on the fifth day. "In humans the morula develops and travels to the uterus around 3 to 4 days after fertilization, and at about 4 to 5 days after fertilization a fluid-filled space called the blastocoel cavity appears and the morula becomes a blastocyst." Once a blastocyst the cells may adhere to the uterine wall and become implanted.
"The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination and gas exchange via the mother's blood supply."
In humans the placenta usually weighs about one pound and is dark reddish in color. It connects to the fetus by an umbilical cord that contains two arteries and one vein.