Facts About the Uterus Every Person Needs to Know

You may only think about the uterus during pregnancy or menstrual cramps, but this powerful organ is pretty fascinating. Here are the facts you may want to know about.

Your uterus is one of the stars of the show during pregnancy. And when your period hits every month, it reminds you of its presence by cramping and bleeding. But what else is there to know? We asked a couple of ob-gyns and dug through the literature to find some fascinating facts about this powerful little organ.

It's Usually No Bigger Than a Pear

When an individual is not pregnant, the uterus is only about three inches long and two inches wide—roughly the size of a pear, an orange, a smartphone, or your fist—though the exact size varies person-to-person, Christine Greves, MD, an ob-gyn at Orlando Health in Florida, told Health.

According to the National Library of Medicine's resource StatPearls, the uterus is made up of three layers. The innermost lining is the endometrium; this is the layer that prepares for pregnancy each month and sheds if no egg is implanted (which is why you get your period). The middle layer, the myometrium, is a thicker muscular layer that expands during pregnancy to allow for the growing baby and contracts during labor and delivery. And the smooth outer layer is the serosa, which allows the uterus to move in the pelvis when needed.

It Can Expand to the Size of a Watermelon

The uterus is an incredibly stretchy organ. When you conceive, it starts expanding. By the end of the first trimester, the uterus, which started about the size of a pear, becomes about the size of a grapefruit. As the baby grows, so does the uterus, so that by the end of the second trimester, it's about the size of a papaya, and by the end of the third trimester, about the size of a watermelon, according to the American Pregnancy Association. Following birth, a process called involution brings the uterus back to its original size; this takes about six weeks.

Some People Are Born With Two Uteruses

Uterus didelphys, also known as "double uterus," is a very rare congenital condition that gives an individual two distinct uteruses. According to a 2021 study published in the journal Clinical Practice and Cases in Emergency Medicine, the abnormality happens during weeks 12-16 of fetal development when the two tubes that are supposed to join together to form one uterus develop into separate structures.

According to this study, uterus didelphys is one of several Müllerian duct anomalies (MDA), which happen in 0.5-5% of the general population. Uterus didelphys accounts for approximately 5% of all MDAs, so it's very rare. People with uterus didelphys might also have two cervixes and two vaginas.

"Usually when you have two [uteruses], one is going to be more prominent than the other one," Tristan Bickman, MD, an ob-gyn in Santa Monica, California, told Health. According to Dr. Bickman, uterus didelphys isn't typically diagnosed until a woman becomes pregnant, and many times the discovery explains pregnancy complications or fertility issues.

Some Are Born Without One

Mayer-Rokitansky Küster-Hauser syndrome (MRKH) is the name of another rare congenital condition that causes a female to be born without a uterus or vagina, though the external genitalia looks perfectly normal, according to the National Library of Medicine's resource MedlinePlus. People who have MRKH don't get their period, and they can't become pregnant—though they typically have normal ovaries and ovarian function.

If the ovaries are functioning normally, the person may be able to have children through assisted reproduction technology (ART). According to the Centers for Disease Control and Prevention (CDC), "ART includes all fertility treatments in which either eggs or embryos are handled." Since a person with no uterus cannot carry a baby, the eggs would undergo IVF and be implanted into a surrogate with a uterus.

According to MedlinePlus, about 1 in 4,500 female babies are born with MRKH. It's not known what causes the disorder. "It's like being born with one kidney instead of two...who knows why?" said Dr. Bickman.

A Uterus Can Be Transplanted

For those who are born without a uterus or who have had to have it removed and want to attempt to carry a baby, a uterine transplant is a possibility. The uterus comes from someone who has died or someone who is still alive and has had it removed. According to a 2021 study published in the journal Cureus, the entire process, from successful transplant to birth, can take two to five years.

According to this study, before the transplant, the individual must undergo IVF and have embryos frozen. Because the uterus is foreign to the body it's transplanted into, the recipient must take immunosuppressive drugs for the duration of its being in the body to prevent rejection of the new uterus. Embryos are placed into the uterus several months to a year after the uterus transplant. When the person is done having babies, the uterus is removed.

As with any organ transplant, a uterus transplant is not without risks and complications, including blood clots, infections, and organ rejection.

Some Uteruses Are Shaped Like a Heart

A heart-shaped uterus is called a bicornuate uterus. Instead of being pear-shaped, the uterus has two bumps sticking up that make it resemble a heart.

According to StatPearls, approximately 0.4% of the general population has a bicornuate uterus. It's a congenital abnormality that often has no symptoms, although a bicornuate uterus can cause heavy period bleeding and very painful period cramps. People with a bicornuate uterus are also at a higher risk for miscarriage, premature birth, and breech birth (being born butt- or feet-first instead of head-first).

Some Uteruses Are "Tilted"

For most people, the uterus sits in the pelvis tipped slightly toward the front of the body. But according to MedlinePlus, for one in five women, it tilts backward.

A retroverted (aka tilted or tipped) uterus is something someone can be born with, or it may be the result of scar tissue in the pelvis caused by endometriosis, an infection, or loosening of the pelvic ligaments during menopause, per MedlinePlus.

While a retroverted uterus is often asymptomatic, it can be associated with several issues, including pelvic pain, dyspareunia (painful sex), minor incontinence, fertility difficulty, and difficulty inserting tampons, according to StatPearls.

During pregnancy, a retroverted uterus can cause a condition called incarcerated uterus in which the uterus becomes "trapped" in the pelvis. According to a 2019 study published in the journal BMC Pregnancy and Childbirth, the incarcerated uterus is rare, affecting about one in 3000 pregnancies, and can result in serious pregnancy complications.

Your Uterus Plays a Role in Your Sex Life

Some women say that their uterus helps make sex more pleasurable. "I have had some patients be very hesitant about a hysterectomy because the uterus is an important part of their sensation of their orgasm—and not many women can say that," said Dr. Greves.

Science doesn't definitively explain what's going on here, but it might have to do with the cervix, the passageway that connects the uterus to the vagina. Some women enjoy feeling their partner's hand or penis against their cervix, and there's even something called a cervical orgasm, too. Since the cervix and uterus are connected, cervical stimulation can trigger pleasurable sensations on or near the uterus.

The pleasure could also come from the uterine contractions that happen after an orgasm, which seems to encourage sperm to get closer to the egg for fertilization, added Dr. Greves.

Uterine Cancer Is on the Rise

When people think of the cancers that strike women, they tend to list breast cancer, ovarian cancer, and cervical cancer, said Dr. Greves.

According to the American Cancer Society (ACS), breast cancer is the most common cancer among American women, except for skin cancers. But uterine cancer is the most common cancer of the female reproductive organs, per the ACS, with the most common type of uterine cancer being endometrial cancer (cancer of the lining of the uterus). "I've diagnosed a few cases just in the past couple of months and it's sad," said Dr. Greves.

According to a 2018 study published in the journal Cancer Epidemiology, Biomarkers & Prevention, uterine cancer is the fourth most common cancer in the US among all cancers and appears to be on the rise.

The reason for the rise isn't established, but there may be a connection to body fat. According to the National Cancer Institute, women who are overweight or obese are two to four times as likely as healthy-weight women to develop endometrial cancer (the most common type of uterine cancer).

What's the connection between body fat and this form of cancer? According to Dr. Greves, it could be because people with higher body fat percentages tend to have increased estrogen levels, which increases one's risk of certain estrogen-dependent cancers. According to the National Cancer Institute (NCI), estrogen is a hormone made by the body that "helps develop and maintain female sex characteristics and the growth of long bones." It's also included in some forms of birth control and can be used to help symptoms of menopause, osteoporosis, and menstrual disorders.

The ACS says that about 90% of women with endometrial uterine cancer have abnormal vaginal bleeding. This includes bleeding more heavily during periods, bleeding between periods, or bleeding after menopause. There can also be abnormal vaginal discharge, pelvic pain, or unexplained weight loss. Some people can even feel a mass. While some of these symptoms can be caused by something else, it's imperative that you contact your healthcare provider so they can diagnose the issue.

From menstruation to pregnancy to sex, the uterus is at play in many aspects of our lives. If you have anything that seems abnormal for you, see your healthcare provider.

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