Anatomy of Pregnancy

An understanding of the anatomy of the reproductive tract is helpful during pregnancy. Your doctor or midwife will liberally sprinkle medical terms throughout their conversations with you. Our Glossary may help with some of the translation.

Although pregnancy produces profound effects throughout the body, the reproductive tract and pelvic area are the primary “targets” of change. In fact, health care providers routinely talk about “target tissue” when they discuss changes in the body. For example, the hormones produced by the ovaries have as their target tissue the following: lining of the uterus (the endometrium), the glands of the cervix, skeletal tissue (bone), the brain (certain parts more than others), the walls of blood vessels, and the breasts.

Many of the anatomical terms your doctor or midwife will use describe the “external genitalia”, or genitals. These are the parts of your reproductive tract which you can see from the outside.

Beyond the introitus of the vagina (“introitus” means opening) is the “lumen” of the vagina. Lumen means cavity or channel. At the upper end (or back) of the vaginal lumen is the the beginning of the uterus. The vaginal end of the uterus is called the cervix. The cervix hangs down in the vagina a bit. If you put your fingers in the vagina, your fingers will be surrounded be the soft tissue on the walls of the vagina. The vaginal walls will be about as soft as your cheek. If you feel as far up as your fingers can go, you will hit something about as hard as the end of your nose……this is the cervix, the lower end of the uterus.

In the middle of the cervix is a hole, called the “os” (“os” means mouth). The os leads to the central cavity of the uterus. The os has two parts, the external os, and further up, the internal os. In first-time mothers, the external os is usually tight and closed (except for a dimple opening). In women who have had babies, the external os is frequently more open, and often shows creases (scars) from having dilated during childbirth.

Between the external os and the internal os (a distance of about an inch) is an area rich in glands which secrete mucus……these microscopic glands are responsible for a woman’s vaginal discharge. Under the influence of reproductive hormones (mainly estrogen and progesterone) these glands alter the type of mucus which is produced. When the body is rich in estrogen and low in progesterone, a clear, watery, slippery mucus is produced. This happens around the time of ovulation. Sperm love this type of mucus. When estrogen is low and progesterone is high, a thick, sticky, white mucus is produced. Sperm cannot penetrate this type of mucus. During pregnancy both estrogen and progesterone are high, and so is the amount of mucus produced.

Beyond the cervix is the central cavity of the uterus. Lining this cavity is the endometrium. The endometrium is like a lawn which the non-pregnant body mows once a month. The lawn trimmings flow out as menstrual blood if you did not get pregnant in the previous cycle. When you do get pregnant, an abundance of progesterone fertilizes the endometrium and allows it to grown thick and lush. Your fertilized egg lies down in this thickened endometrium and implants itself. The thickened endometrium during pregnancy is called the decidua.

The top of the uterus is called the fundus. The middle portion of the uterus and the fundus contain muscle fibers. When these muscle fibers contract they exert force downward toward the vagina. When all the muscle fibers of the uterus work together they are capable of pushing out just about anything which is in the central cavity of the uterus.

The uterus is held in place by ligaments which keep it suspended in the hole formed by the pelvic bones. Ligaments are tough, rope-like structures which hold the body together. The ligaments of the lower uterus tie the cervix to the sacrum. The sacrum is the bone just above the tailbone (which is called the coccyx) and just below the spinal column. The sacrum is actually five fused spinal vertebrae. So the sacrum is both part of the backbone and the back part of the pelvic bone. The ligaments which attach the cervix to the sacrum are probably the reason that some women experience back pain during labor. Two other ligaments (one on the right and one on the left) connect the top part of the uterus to the pelvic muscles and the pubic bone. These are the round ligaments. They can spasm (contract suddenly) during pregnancy and cause a sharp pain on either sider of the lower abdomen. This is called round ligament pain.

Your bladder is fixed to the front side of your uterus. Going up from the bladder to the kidneys are two tubes (one on the right and one on the left) called the ureters. The ureters carry urine from the kidneys to the bladder. Going down from the bladder to the external genitalia is the tube called the urethra. This short tube (about 2-3 inches long) carries urine from the bladder to the outside of your body. During pregnancy, hormones soften all of the urinary tract tissue. This causes three things which increase the chance of infection. The softened urethra allows bacteria to travel the short 2-3 inches into the bladder. The softened bladder allows more residual urine to remain in the bladder after you urinate. And the softened ureters allow bacteria in the bladder to travel up to the kidneys. In addition to all this, the kidneys allow more sugar (glucose) to pass into the urine. Bacteria grow better in this high-sugar urine.

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