Re: Why is only one egg released in a female

That's a good question. The fact is, sometimes but rarely two or three eggs are released which can lead to fraternal twins. But the usual case for humans is as you indicated. The answer has to do with how ovulation is produced and controlled.

In part, it has to do with the base of the brain ... two neighboring anatomical parts of the brain called the pituitary and thehypothalamus. The hypothalamus releases a substance called a "gonadotropin releasing hormone (GnRH)" that in turn causes the pituitary to release its hormones (called "pituitary gonadotropins "). The pituitary gonadotropins cause specialized cells (Graafian follicles) in an ovary to form the egg, and then to release it. Right after the first egg is released, the remaining cells quickly become specialized by forming a new tissue on the ovary called the "corpus luteum" (translated from Latin, means "yellow body"). This tissue produces large amounts of the hormone progesterone which does many things throughout pregnancy.

From the earliest days of pregnancy onward, increased amounts of progesterone in the woman's blood shuts down the hypothalamus such that it stops secreting GnRH. Without GnRH, the pituitary cannot secrete the hormones that produce the egg and its release from the ovary. So this explains in general how after the first egg is formed and released a second (and third) usually does not occur. After the baby is born, tissues which form progesterone during pregnancy are eliminated or disappear ... so that the amount of progesterone in the blood becomes a fraction of what it was during pregnancy. Thereupon, the hypothalamus once again secretes GnRH, the pituitary secretes the gonadotropins, and ovulation resumes its natural approximately 28 day cyclic pattern; until an egg is released which becomes fertilized and again causes additional ovulation to stop.