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.
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