Basic Reproductive Anatomy

uterus diagramThe uterus is a muscular organ that lies within the pelvis between the bladder and rectum.  It is shaped like upside-down pear and is about the size of a fist in a non-pregnant woman.  The uterus is composed of the fundus (dome shaped portion above the tubes), the body (largest tapering central portion) and the cervix (opening into the vagina).

If a woman inserts a speculum into her vagina and opens it, she will see her (beautiful!) cervix, which looks like a little pink doughnut about 3-6 inches from the opening her vagina.  The cervix is the lowermost part of the uterus that extends into the vagina and connects the uterus to the top of the vaginal wall (that’s right – the vagina is a sealed cavity and doesn’t open into the abdomen – you can’t lose a tampon or condom within it).  The cervix opens to the vagina via an internal os and an external os, though the external os is the only one visible upon cervical self-exam – it is the little hole in the center of the doughnut. The inner canal of the cervix, which you also can’t see upon speculum exam, is lined with glands that produce secretions that vary in consistency and quality throughout the cycle.  The quality (color, consistency, texture) of this fluid gives us accurate information about our current fertility or possibly an infection.

The uterus is made of three layers of tissue.  The outer layer is called the perimetrium (or serosa) which becomes the broad ligament on either side (shown as the yellow sheet-like structure in the image above). The bulky middle layer of the uterus, called the myometrium, is composed of three muscle layers, which are thickest at the fundus and thinnest at the cervix.  Pound for pound, the uterus is the strongest muscle in the female body.  Amazingly, the thin muscle of cervix thins and dilates with the rhythmic contractions of labor allowing the os to open to 10 cm (much like a head being pushed through a tight turtleneck shirt).  The incredibly powerful muscles of the fundus push the baby from within the uterus into the vagina during labor and as it is birthed through the vulva.  The innermost lining of the uterus is called the endometrium and it creates a thick, specialized tissue each cycle that is shed during menstruation.  It is this velvety nourishing tissue that becomes the site where the placenta grows during pregnancy to nourish the fetus.

Branching from the top of the uterus, there are two oviducts (also called uterine or Fallopian tubes) that open to body of the uterus.  The oviducts are muscular passageways that help sweep the egg from the ovary to the uterus at ovulation.  At the end of the oviducts are finger-like structures called fimbriae, which move over the surface of the ovary to engulf the egg released during ovulation- this looks like a sea anemone moving in the ocean.

The fimbriae nestle two ovaries, which are organs about the size of almonds.  Each ovary holds many follicles, or tiny sacks that contain immature eggs, which are not visible to the naked eye.

It is estimated that women are born with about 1-3 million immature eggs, called oocytes or follicles, that live within the ovaries. Unlike a man who produces his sex cells (sperm) every day, a woman is born with a certain number of sex cells that gradually die over time beginning in infancy and continuing through menopause. When a woman first gets her period at puberty, only about 400,000 follicles remain in her ovaries. With each menstrual cycle, a thousand follicles are lost and (usually) only one follicle will actually mature into an ovum (egg), which is released into the oviduct, marking ovulation.  Depending on how many of her reproductive years are spent pregnant or not ovulating due breastfeeding, it is estimated that between only about 400 of the original 1-3 million will ever mature into ova.

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