The uterus or womb is part of the reproductive system of the female body. The uterus is the place a baby grows for nine months during pregnancy.  It is a pear-shaped organ inside a woman. It is behind the bladder and in front of the rectum. The uterus weighs 70 grams. The endometrium is the tissue that lines the uterus. A hormone called estrogen makes the endometrium thick with blood and fluid. This uterine lining gives the growing baby what it needs to grow. The endometrium leaves the uterus as the monthly flow of blood (menstruation). The endometrium will form again. This happens every 28 days. The number of days can be different for each women. The fertilized egg will move through the fallopian tube and into uterus. It will then attach to the endometrium.

Anatomy 
The uterus is part the female reproductive system. Other structures and organs that are part of the reproduction system are the vagina, ovaries and fallopian tubes.

The uterus has four main parts. The fundus is the upper part of the uterus. It has a rounded shape. Another part of the uterus is the body. The uterotubal angles are the parts connected to the Fallopian tubes. The bottom part of the uterus is the cervix. The uterus has three layers. The outer layer is called the perimetrium. It is a thin layer that surrounds the outside of the uterus. The perimetrium is made of tissue made of epithelial cells. The middle layer is the myometrium. Most of the uterus is made up of the myometrium. The layer on the inside is the endometrium. The endometrium is made of secretory, ciliated, and basal cells. The uterus is not in the same place for all women. It is tilted forward in most women. Other women have a uterus that tilts up or backwards.

The uterus is supplied by blood vessels. Blood travels to the uterus through arteries. The big arteries are the internal iliac arteries. The big arteries branch off into smaller arteries called the ovarian artery and the uterine artery. The smaller arteries give blood to all the layers of the uterus. The blood returns to the heart and lungs through the veins.

The uterus is held in place by ligaments. These are the anterior, posterior, lateral, uterosacral, and round ligaments. The position of the uterus can vary depending on the contents of the bladder.

The uterus changes during the life of a woman. It is inactive during childhood and old age. During the time that a woman or girl can have a baby it is a very active organ. It does many things. It is where menstruation begins, it is the place where the fertilized ovum attaches, it is the place where the baby grows, it is the strong muscle that pushes the baby out during birth.

Tumors 
Cancer can form in the uterus. But this is not common. There are two types of uterine cancer.  These are endometrial cancer and uterine sarcoma. Endometrial cancer can grow from cells lining the uterus. Uterine sarcoma is a rare cancer that grows from cells in the smooth muscle of the uterus. Other tumors can grow in the uterus that are not cancer. One of these is a Uterine fibroid.

Abnormal uterus 

A girl can be born with a uterus that is not normal. When a woman has uterus that is not normal she can also have other organs that are not normal. She may not know this until she wants to have babies. Having a baby can be hard. If a woman has a uterus that is not normal, she may not be able to have babies. Many times the uterus can be corrected by surgery.

Pregnancy 
The uterus changes during pregnancy. It grows with the baby. It also has fluid and the placenta inside. It starts small but it becomes very large. It can hold as much as five to twenty liters. The uterus is made of smooth muscle called the myometrium.  The cells of the myometrium grow during pregnancy. Strong fibrous tissue grows on the outside of the uterus. The myometrium grows at first but becomes thinner at the end of the pregnancy. A doctor or nurse can feel the baby through the thinning uterus.

Uterine prolapse 
The uterus can move down and be seen through the vagina. This can happen after a woman has a baby. Older women have this problem more than younger women. Other things that make women have their uterus drop down:

 Getting older
 A lower amount of estrogen after menopause
 Being overweight
 Coughing a lot
 A tumor
 Pushing too hard to have a bowel movement
 Weak pelvic muscles

Surgery 
The uterus is removed by surgery for many reasons. Removing the uterus is called a hysterectomy. Surgery to remove the uterus may need to be done after having a baby. This is because the woman may be bleeding heavily. The uterus may need to removed because there is cancer growing in it.

Other animals 
A uterus is present when an animal gives birth to live offspring. Therefore, all mammals have a uterus except monotremes. However, humans are almost alone in having a single uterus. The other mammal which has a single uterus is the chimpanzee, our nearest living relative. Other mammals have either wholly separated uteri or uteri which are fused along part of their length.

Animals which lay eggs have an oviduct, and there are quite a few reptiles which are viviparous or ovoviviparous.

 Cattle have two uteri.
 The uterus of the horse is made of two short uterine horns. The uterus of the horse is affected by hormones. Horses can have an infection of the uterus. The horse can also develop uterine cysts. In an older female horse, there may be scarring in the uterus after they have their baby. Damage to the uterus in the horse may make it unable to have babies.
 The uterus in a female dog has two uterine horns.
 The female cat has two uteri that join one cervix. Marsupials have two uteri that connect to separate vaginas.

References

Anatomy of the female reproductive system