The uterus is a hollow, muscular organ in which a fertilised ovum becomes embedded and develops into a fetus. Its major function is protecting and nourishing the fetus until birth.

During pregnancy, the muscular walls of the uterus become thicker and stretch in response to increasing fetal size during the pregnancy. The uterus must also accommodate increasing amounts of amniotic fluid (the waters surrounding the fetus, contained in a bag of fetal membranes), and the placenta (the structure that delivers nutrients from the mother to the fetus).

The uterus has four major anatomical divisions, shown below:

• Body: the major portion, which is the upper two-thirds of the uterus.
• Fundus: the domed area at the top of the uterus, between the junctions with the two fallopian tubes.
• Endometrial cavity: the triangular space between the walls of the uterus.
• Cervix: the narrow neck at the upper end of the vagina.

The wall of the uterus has three layers of tissue, two of which are shown above:

• The perimetrium: the outermost thin membrane layer covering the uterus. (It is not important for you to know this term for obstetric care.)
• The myometrium: the thick, muscular, middle layer in the figure above.
• The endometrium: the thin, innermost layer of the uterus, which thickens during the menstrual cycle. This is the tissue that builds up each month in a woman of reproductive age, under the influence of the female reproductive hormones.