The uterus, or womb, is a muscular, inverted pear-shaped organ in the female pelvis that is specifically designed to protect and nurture a growing baby. It averages 3 in (7.6 cm) long by 2 in (5 cm) wide. Although, during pregnancy, it expands with the growing embryo and fetus. Embryo is a term used to describe a human in the first eight weeks of development. After that, the human is called a fetus.
During the follicular phase of the menstrual cycle, the lining (or endometrium) of the uterus becomes thick and filled with many blood vessels in preparation for supporting an embryo. If fertilization does not occur within about eight days of ovulation, then this lining is shed in menstrual blood through the cervix. This cycle continues until menopause, when menstruation becomes less frequent and eventually stops altogether.
The cervix is the base of the uterus which extends into the vagina. The narrow passageway of the cervix is just large enough to allow sperm to enter and menstrual blood to exit. During childbirth, it becomes dilated (open) to allow the baby to move into the vagina, or birth canal. However, for most of the pregnancy, the cervix becomes plugged with thick mucous to isolate the developing baby from vaginal events. For this reason, non-reproductive sexual intercourse is usually safe during pregnancy.
The uterus is required for reproduction. With all the male and female aspects contributing to reproduction, a number of diseases, genetic disorders, and other variables can cause infertility, which afflicts 10-15% of couples trying to conceive. Technologies such as in vitro fertilization exist for some couples with infertility due to ovarian, fallopian tube, or sperm problems. However, without a uterus, a human baby can not grow. The uterus plays an integral hormonal and physical role in housing and nourishing the baby.