Illustration of a uterus with a photo of a person wearing lab coat and stethoscope

What Happens to the Uterus During Pregnancy?

Get Started With TRIO

During pregnancy, the body goes through a series of miraculous changes. One of the biggest transformations can be seen in the uterus. It can expand many times its pre-pregnancy size by the end of the third trimester (1).

This article examines how the uterus adapts during pregnancy to support and protect the fetus, as well as tips for keeping the uterus healthy.

What is the Uterus?

The uterus is a hollow, pear-shaped organ found in the bodies of people assigned females at birth. In a non-pregnant person, this structure is situated in the pelvis between the bladder and rectum.

The uterus is made up of three distinct layers that help support its function. The innermost layer is called the endometrium. This is a hormone-responsive lining that thickens over the course of the menstrual cycle in anticipation of receiving a fertilized egg.

If pregnancy occurs, the endometrium provides the embryo with nutrients and support. If a pregnancy does not take place, the endometrium breaks down and the body sheds it as a period (2).

Over the course of a pregnancy, the uterus expands and adapts to accommodate the growing fetus. The uterus also becomes home to the placenta and other pregnancy-related structures, such as the umbilical cord and amniotic sac.

The Size and Position of the Uterus During Pregnancy

During pregnancy, a fetus transforms from a microscopic embryo into a fully developed newborn. Because the fetus undergoes such a dramatic growth spurt, it’s essential that the uterus can keep up. According to the American Pregnancy Association, the size and position of the uterus changes during pregnancy in the following ways (3):

Before Pregnancy: The uterus is the size of an orange and is found in the pelvis.

The First Trimester: The uterus is the size of a grapefruit. The growing uterus may put pressure on the bladder, causing the pregnant person to feel like they constantly need to pee!

The Second Trimester: The uterus is the size of a papaya and starts to grow upwards into the abdominal cavity. This can cause some discomfort since the organs are displaced from their usual position. Many people start noticing a pregnancy bump during the second trimester as the uterus expands.

The Third Trimester: The uterus is the size of a watermelon! Extending from the pubic area to the rib cage, the uterus now takes up a lot of space in the body. Some pregnant people may start feeling short of breath as their lungs have less room to expand.

After Birth: The uterus slowly returns to its pre-pregnancy size, shape, and position. For pregnancies with more than one fetus, the uterus stretches more quickly to keep up with the space demands of the babies within.

Functions of the Uterus During Pregnancy

The uterus plays many important roles during pregnancy, including:

Growing with the Fetus: A key role of the uterus is to grow alongside the fetus to ensure they have enough room to develop.

Implantation: One of the first stages of pregnancy is implantation. During this event, the uterus accepts a fertilized egg, which burrows into the uterine lining. The fertilized egg receives nutrients and support from the uterus as it develops.

The Placenta: The uterus supports the placenta, which is an essential structure during pregnancy. It allows for the flow of oxygen and nutrients to, and waste products from, the fetus.

Protection from Infection: A mucus plug develops in the cervix, which stops bacteria and other sources of infection from reaching the uterus (4). This ensures that the uterus is a safe space for the fetus to grow and develop.

Childbirth: When it’s time for the baby to meet the outside world, muscles in the uterus contract to help them (and the placenta) leave through the vagina. These contractions are a telltale sign that labour has begun.

Measuring the Uterus During Pregnancy

Many pregnant people have their fundal height measured during prenatal appointments to check fetal growth. The fundal height is the distance between the pubic bone and the top of the uterus, and this measurement should roughly correspond with the number of weeks pregnant someone is. For example, someone 28 weeks pregnant should have a fundal height of around 28 centimetres.

If the uterus measures smaller or larger than it should be, it isn’t necessarily a cause for concern. This could happen if the parents are small in stature, or if the due date is slightly off. However, a small or large fundal height could also suggest a pregnancy complication and may require further testing (3).

If you are pregnant and have questions about your fundal height, please speak with your physician.

How to Keep Your Uterus Healthy

Having a healthy uterus is important for reproductive wellness and can help ensure a pregnancy runs as smoothly as possible. To keep your uterus healthy, you can:

Practice a Healthy Lifestyle: Eating a diet rich in fruits and vegetables while consuming less red meat, and taking part in regular exercise, could reduce your chance of developing fibroids (5, 6). Fibroids are non-cancerous growths in the uterus that can interfere with fertility and pregnancy.

Quit Smoking: Smoking has been shown to reduce the receptivity of the uterus and can affect fertility.

Have Regular Cervical Screenings: The cervix is the narrow end of the uterus that connects the uterus to the vagina. It’s important to have routine pap tests to screen for early signs of cervical cancer.

Attend Your Gynecological Appointments: The uterus can develop certain conditions, like fibroids. Symptoms may include heavy menstrual flow, urinary frequency, pain, and infertility. It’s important to keep up to date with your gynecological appointments so they can monitor your uterine health. If you notice pelvic pain or experience heavy or irregular periods, please reach out to your healthcare provider.


If you have questions or would like to know how TRIO can support you, please get in touch. Our fertility specialists are here to help.