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If the oocyte is successfully fertilized, the resulting zygote will begin to divide into two cells, then four, and so on, as it makes its way through the uterine tube and into the uterus. There, it will implant and continue to grow. If the egg is not fertilized, it will simply degrade—either in the uterine tube or in the uterus, where it may be shed with the next menstrual period.

Ovaries, uterine tubes, and uterus

This diagram shows the uterus and ovaries in the center. To the left is a micrograph showing the ultrastructure of the ovaries and to the right is a micrograph showing the ultrastructure of the uterus.
This anterior view shows the relationship of the ovaries, uterine tubes (oviducts), and uterus. Sperm enter through the vagina, and fertilization of an ovulated oocyte usually occurs in the distal uterine tube. From left to right, LM × 400, LM × 20. (Micrographs provided by the Regents of University of Michigan Medical School © 2012)

The open-ended structure of the uterine tubes can have significant health consequences if bacteria or other contagions enter through the vagina and move through the uterus, into the tubes, and then into the pelvic cavity. Unlike the male reproductive system, this is not a "closed system". If an infection occurs, it could quickly become life-threatening. The infection can easily move from the open end of the uterine tubes into the pelvic cavity. The spread of an infection in this manner is of special concern when unskilled practitioners perform abortions in non-sterile conditions. Many STDs increase a woman’s risk for pelvic inflammatory disease (PID) , infection of the uterine tubes or other reproductive organs. Even when resolved, PID can leave scar tissue in the tubes, leading to infertility.

The uterus and cervix

The uterus    is the muscular organ that nourishes and supports the growing embryo (see [link] ). Its average size is approximately 5 cm wide by 7 cm long (approximately 2 in by 3 in) when a female is not pregnant. It has three sections. The portion of the uterus superior to the opening of the uterine tubes is called the fundus    . The middle section of the uterus is called the body of uterus    . The cervix    is the narrow inferior portion of the uterus that projects into the vagina.

Several ligaments maintain the position of the reproductive organs in the abdominopelvic cavity. The ligaments of the female reproductive tract can be divided into three categories: Broad ligament which is a sheet of peritoneum, associated with both the uterus and ovaries. Uterine ligaments are ligaments that support the uterus. The third type, ovarian ligaments support the ovaries.

The wall of the uterus is made up of three layers. The most superficial layer is the serous membrane, or perimetrium    , which consists of epithelial tissue that covers the exterior portion of the uterus. The thick middle layer, or myometrium    , is a layer of smooth muscle responsible for uterine contractions during childbirth and menstruation. The innermost layer of the uterus is called the endometrium    . It is the endometrium that grows and thickens in response to increased levels of estrogen and progesterone. It is the endometrium that sheds during menstruation. The endometrial tissue dies and blood, pieces of the endometrial tissue, and white blood cells are shed through the vagina during menstruation, or the menses    . The first menses after puberty, called menarche    , can occur either before or after the first ovulation.

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Source:  OpenStax, Mrs browne's reproductive modules. OpenStax CNX. May 26, 2015 Download for free at https://legacy.cnx.org/content/col11800/1.1
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