How Does Normal Human Reproduction Work?

Normal human female reproduction depends on the correct functioning of four components of a woman’s body: the brain, the ovary, the fallopian tube, and the uterus. At the time of her birth, a woman’s ovary contains all of the eggs that she will ever have. These eggs are contained within fluid-filled sacs called follicles.

Every month, the brain sends out a signal from the pituitary gland (a gland located at the base of the brain) stimulating the follicles to grow. Not surprisingly, this hormone is called follicle-stimulating hormone (FSH). Under the influence of FSH, a group of follicles begins to grow, but by the fifth day of the reproductive cycle a single dominant follicle has already been selected. This dominant follicle may be either on the right ovary or the left ovary.

As it grows, the follicle produces an important steroid hormone called estrogen. Estrogen causes the lining of the uterus (endometrium) to thicken in anticipation of the eventual implantation of an embryo.

By mid-cycle, this follicle has grown to a diameter of 20 to 22 mm. At this time, the brain releases a second hormone, called luteinizing hormone (LH), from the pituitary gland. LH is the trigger that induces ovulation.

Approximately 36 hours after the LH surge, the follicle releases the egg. It is the job of the fallopian tube to trap the egg. If the fallopian tube fails to catch the egg, then pregnancy cannot occur.

During intercourse, tens of millions of sperm are deposited in the woman’s vagina when her male partner reaches orgasm and ejaculates. While the egg is safely held within the fallopian tube, these sperm swim from the vagina, into the cervix, through the uterus, and up into the fallopian tube, where fertilization occurs. (see Figure 1). Normally, the growing embryo travels through the fallopian tube for 5 days after fertilization, at which point it reaches the uterus. (An embryo that remains trapped within the fallopian tube is called a tubal pregnancy or ectopic pregnancy, and can be a life-threatening condition.) The embryo divides many times along the way, and by the time it reaches the uterus, it has grown to hundreds of cells and is called a blastocyst. Once the egg is released from the ovary, the follicle (now called a corpus luteum) continues to produce estrogen and begins to produce a new hormone: progesterone. Progesterone induces changes in the estrogen-primed endometrium, allowing implantation of the embryo and thus permitting pregnancy to occur. In the absence of a pregnancy, the levels of estrogen and progesterone both fall 2 weeks after ovulation and a menstrual period ensues, shedding the lining of the uterus. Menstrual flow lasts approximately 3 to 5 days in most women.

Overall, human beings are not very fertile, with maximum pregnancy rates of only 20% to 25% per cycle during the years of peak fertility (the second and third decades of life).