Introduction 

Uterine fibroids, also called leiomyomas or myomas, are non-cancerous tumours that grow in the muscle of the uterus. They are common in women of reproductive age. Research suggests that up to 10 per cent of pregnant women may have fibroids. Most fibroids do not cause symptoms and are found by chance. However, during pregnancy, fibroids can sometimes cause problems. As the uterus grows to support the baby, fibroids may also grow or change, which can lead to complications. Because of this, regular monitoring and individualised care are important to support both maternal and fetal health.

Managing fibroids during pregnancy is important for achieving the best possible outcomes. Healthcare providers need to consider several factors, including the size, location and number of fibroids as well as the mother’s symptoms and how far along the pregnancy is. While many women with fibroids have healthy pregnancies, some may face complications such as miscarriage, early labour or difficulties during delivery. This article will discuss how fibroids can affect pregnancy, what risks to be aware of and the best ways to manage them to help ensure the safety of both mother and baby.

Types and characteristics of fibroids

Uterine fibroids, also known as leiomyomata, are the most common benign tumours found in persons assigned female at birth of reproductive age. They arise from the smooth muscle cells of the uterus (myometrium) and are primarily influenced by estrogen and progesterone levels. These growths vary in size, number and location. While some persons assigned female at birth remain completely without symptoms, others experience symptoms such as abnormal uterine bleeding, pelvic pressure or pain, urinary disturbances and back pain. 

Fibroids are typically classified into three main types based on their location within the uterus: 

• Subserosal fibroids grow outward on the outer wall of the uterus. These are generally considered the most concerning in pregnancy due to their potential to disrupt the uterine lining, affecting embryo implantation and fetal development
• Intramural fibroids embedded within the uterine wall
• Submucosal fibroids, which protrude into the uterine cavity

Beyond location, the size of the fibroids plays a crucial role in determining the severity of symptoms and the risk of complications, especially during pregnancy. Smaller fibroids (less than 5 cm) are less likely to interfere with fertility or cause significant discomfort. 

In contrast, larger fibroids (greater than 5 cm) are more likely to lead to issues such as pain, inflammation or even degeneration when their growth exceeds their blood supply. Fibroids can also be pedunculated, meaning they grow on a stalk, further impacting nearby organs depending on their placement. 

Diagnosis typically begins with a pelvic exam and is confirmed through imaging techniques like transvaginal ultrasound, which has a high sensitivity for detecting fibroids. In certain cases, MRI or hysteroscopy may be used for more detailed evaluation, especially when planning surgical treatment. 

While fibroids are non-cancerous, they remain the leading reason for hysterectomy in the U.S., with significant impacts on healthcare costs and women's reproductive health.¹ 

Impact of fibroids on fertility and conception 

1. How fibroids can affect fertility

While most women with fibroids can conceive naturally, certain types of fibroids may reduce the chances of becoming pregnant. They include the following:

• Submucosal fibroids grow into the inner cavity of the womb and can interfere with the implantation of a fertilised egg. They may also block the fallopian tubes, making it harder for the sperm and egg to meet
• Intramural fibroids are located within the muscular wall of the uterus, may change the shape of the womb and impact its lining, potentially lowering the chances of pregnancyespecially in assisted reproductive procedures like IVF
• Subserosal fibroids grow on the outer surface of the uterus and typically do not affect fertility
  

Studies have shown that women with fibroids that distort the shape of the uterus have lower pregnancy and live birth rates. In some cases, removing fibroids, particularly submucosal ones, has been shown to improve fertility outcomes when no other causes of infertility are found.

2. Effects of fibroids during pregnancy

Fibroids during pregnancy don’t always cause problems, but when they do, it’s often related to their size or location:

• Pain and discomfort: Some women experience abdominal pain due to the fibroid stretching or degeneration as the uterus grows
• Risk of miscarriage: In rare cases, fibroids can increase the risk of early pregnancy loss, especially if they interfere with blood supply or space within the womb
• Preterm labour: Larger fibroids may raise the risk of going into labour early
• Delivery complications: If fibroids are large or positioned low in the uterus, they can block the birth canal, requiring a caesarean section for safe delivery

It's important to note that many pregnancies with fibroids proceed without complications. It is essential to distinguish between cases that require preconception treatment and those that don't. If fibroids are without symptoms and do not significantly distort the uterine cavity, they may not require surgical management before pregnancy. Counselling women on these aspects helps set realistic expectations and informs decisions around fertility planning.² 

Monitoring fibroids during pregnancy

Routine ultrasound examinations are essential in pregnant women with known fibroids to monitor fibroid size and location throughout gestation. In the first trimester, fibroids are documented for baseline characteristics. During this period, the uterus and pregnancy are still small, making it easier to identify associated fibroids. Subsequent scans assess any growth, changes in vascularity, or new symptoms that could impact the pregnancy.

As pregnancy progresses, fibroid detection becomes more difficult due to anatomical changes in the uterus and the presence of the fetus and placenta, which can obscure imaging.

Pregnancy influences the size of fibroids, with many studies suggesting growth during the first trimester. However, in some cases, the maximum increase in fibroid size occurs in the second trimester. Ultrasound evaluations are typically performed at the initial prenatal visit, during the first-trimester morphological scan (12–13 weeks), at 28 weeks, and at birth. Growth assessment is based on the maximum dimension of the largest fibroid when multiple fibroids are present.

Although fibroids often increase in size, many do not change significantly during pregnancy. In some cases, fibroids remain constant in size throughout gestation. Monitoring focuses on identifying changes in fibroid size and location that could impact pregnancy, labour, or delivery. This approach allows clinicians to anticipate potential complications and guide management accordingly.² 

Management of fibroids during pregnancy

The main goals of fibroid treatment are to reduce symptoms, shrink the fibroids, protect fertility if desired, and prevent complications. Only women with symptoms usually need treatment. Vitamin D has been shown to help reduce fibroid size and improve symptoms. While hysterectomy is still common, many less invasive treatments now exist. The choice of treatment depends on symptoms, age, fibroid size and location, and whether future pregnancy is planned.

Medical management of fibroids during pregnancy

Most medicines used to treat fibroids are not safe during pregnancy because they can affect the ability to get or stay pregnant. However, in some cases, a medicine called a GnRH agonist might be used before surgery to help improve low blood levels (anaemia) by reducing heavy bleeding. Since most treatments aren't safe for the baby, doctors usually focus on managing symptoms and keeping both mother and baby stable until after the pregnancy.

Surgical management of fibroids during pregnancy

Surgery to remove fibroids during pregnancy is not common and is only done when absolutely necessary, such as if a fibroid is very large or causing serious problems. Open surgery (laparotomy) may be needed in rare cases, but it comes with more risks, like bleeding and a longer recovery. Other types of surgery, like laparoscopy or robotic surgery, are less risky but still not often done during pregnancy. In most cases, doctors avoid surgery during pregnancy unless there is no other option.

Other management of fibroids during pregnancy

Other treatments like blocking the blood supply to fibroids (uterine artery embolisation) or using focused ultrasound are not usually recommended during pregnancy. These methods may affect the chances of having a healthy pregnancy later, even though some women have had successful pregnancies after these treatments. Another treatment called radiofrequency ablation can shrink fibroids over time, but it’s also not typically done during pregnancy. Most doctors prefer to wait and treat fibroids after the baby is born, unless serious symptoms develop.³ 

Summary

Uterine fibroids are common in women of reproductive age and can occasionally complicate pregnancy depending on their size, location, and number. While most pregnancies with fibroids are uneventful, some may pose risks such as pain, miscarriage, or delivery issues. Careful monitoring through regular ultrasounds, individualised care plans and symptom management are essential. Treatment during pregnancy is typically conservative, with more definitive options explored postpartum to ensure both maternal and fetal well-being.