The halachic concepts involved in the laws of niddah are directly related to female reproductive anatomy. Familiarity with basic biological concepts is important for understanding these laws.
The uterus, or womb, is shaped roughly like an upside down pear. The lining of the uterus is called the endometrium. This lining grows anew each month and is shed when you get your period. The lower, narrow section of the uterus is called the cervix.
The ovaries are two small glands located on either side of the uterus but not attached to it. They produce the eggs that are released during ovulation, as well as the hormones that govern the menstrual cycle such as estrogen and progesterone.
Branching out of the wider part of the uterus towards the ovaries are the fallopian tubes. At ovulation, the fingerlike extensions on the ends of the tubes help gather the egg released from the ovary. The egg travels down the fallopian tube to the uterus.
The cervix hangs down into the top portion of the vagina. The opening of the cervix, through which menstrual blood exits, is called the os.
The vagina is a muscular tube about three inches long. It has an amazing ability to stretch, and can accomodate a full-size baby.
The hymen is a thin membrane that covers part of the opening of the vagina. There is a hole in the middle through which menstrual blood can flow. This membrane is stretched during marital relations and generally disappears after a few times.
The area surrounding the vaginal opening is called the vulva. Click here for a diagram . The vulva includes two sets of labia (lips). The outer lips are called the labia majora; the inner, thinner ones are the labia minora. The clitoris is found under the hood of the labia minora. The opening of the urethra, the tube through which urine exits the body, is directly in front of the vaginal opening. Behind the vagina is the anus, where stool exits. The area of skin between the vagina and the anus is known as the perineum.