Fibroid during Pregnancy

A Fibroid or Fibroids are very often discovered in the womb (uterus) during a pelvic examination or more frequently during a routine ultrasound scan when a woman is pregnant. Medical research has shown that the most uterine fibroids do not grow or shrink during pregnancy. It has been found that about a third of fibroids may enlarge during the first three months (known medically as the first trimester) of pregnancy. Studies have shown that the vast majority of women with fibroids have an uncomplicated pregnancy and childbirth. However it has been found that some women do unfortunately have some problems due to their fibroids. It is known that uterine fibroids are linked to a greater rate of spontaneous miscarriage, placental abruption, early onset of childbirth (preterm labour), malpresentation of the baby (abnormal position of the baby e.g. breech baby), an abnormal or difficult childbirth( labour dystocia), need for caesarean section and bleeding after the baby is born (postpartum haemorrhage).

Fibroids (leiomyomas) are benign (non-cancerous) tumours or swellings made up of uterine smooth muscle which develop in the muscular wall of the womb. They are very common. By the time they are 35 years of age it is estimated that 40-60% of women will have at least one fibroid, and by the time they reach the age of 50 approximately 70-80% will have fibroids. The exact cause of fibroids is still unknown. Making the diagnosis of fibroids during pregnancy is neither simple nor straightforward.

Research has shown that only 42% of fibroid greater than 5cm (labelled as large) and 12% of smaller fibroids (measuring 3-5 cm) can be felt and diagnosed on pelvic examination. Surprisingly, making the diagnosis using ultrasound is also difficult. This is mainly because of the difficulty of distinguishing fibroids from the normal thickening of the lining of the womb (myometrium). It is therefore thought likely by specialists that the frequency of fibroids during pregnancy is actually underestimated.

There is an increasing trend in the Western world for pregnancy to be delayed until women are older. Research has shown that the incidence of uterine fibroids in older women who are having medical treatment for infertility is approximately 12-25%.

Even though they are becoming more common, the exact nature and reasons for leiomyomas causing infertility and problems when a woman is pregnant are not fully understood.

Medical evidence from ultrasound studies which monitored the size of fibroids during pregnancy has shown that most fibroids (60-78%) did not change in size significantly throughout pregnancy. The volume of fibroids was monitored prospectively. Some fibroids did grow (22-32%) and increased in volume and most of this growth took place in the first three months of pregnancy. In particular most growth occurred during the first 10 weeks of pregnancy and it is interesting to note that very little growth took place during the last 6 months of pregnancy (i.e. during the second and third trimesters).

The vast majority of fibroids cause no symptoms. A well described, but rare complication of fibroids is known as “red degeneration”. This is most likely to happen to a pedunculated subserosal fibroid. Red degeneration causes severe localised abdominal pain. Fibroids can also cause pain simply due to their size and larger fibroids (those greater than 5cm) are those that most often cause pain, usually during the second and third trimesters of pregnancy. In fact pain is the most common complication of fibroids during pregnancy.

A study investigated the frequency of red degeneration in fibroids during pregnancy using ultrasound scans. It was found that 9% of fibroids had ultrasound evidence (showed a heterogeneous echogenic pattern or cystic change) of red degeneration. The women were asked about their symptoms and about 70% of these women had experienced severe abdominal pain compared with about 12% of women whose ultrasound scans showed no sign of red degeneration in their fibroids.

Why does red degeneration cause pain?

There are three medical hypotheses which have been proposed to explain why red degeneration of a fibroid causes severe pain.

The first theory is that the rapid speed of growth of the fibroid causes the tissue in the middle of the fibroid to outgrow its blood supply resulting in death of the tissue (known as infarction).

The second theory is that the growing uterus disrupts the blood supply to the fibroid by kinking the blood vessels in some way (there is thought to be a change in the architecture of the blood vessels).  This happens even if the fibroid does not grow.

The third theory is that the pain is due to prostaglandin chemicals produced by damage of the cells in the fibroid. This theory is supported by the fact that NSAIDS drugs (such as ibuprofen or diclofenac) which work by inhibiting prostaglandin quickly and effectively control the pain.

Effect of Fibroids on Pregnancy Outcome

It has been found that between 10% and 30% of women with fibroids have problems or complications during their pregnancy. However, medical specialists have criticised the way that some of these studies have been carried out. (They have commented that there have been problems such as: selection bias, small and differing characteristics of the populations studied, low occurrence of adverse outcomes, varying inclusion criteria and inadequate confounding variables.) The studies carried out so far have found varying and inconsistent correlations between fibroids and the frequency of complications in pregnancy. More research into fibroids during pregnancy needs to be carried out.

It is not known yet precisely how fibroids cause complications in pregnancy. It is hypothesised that mechanical obstruction or impaired distensibility (stretchability) of the uterus may explain some of the problems caused by fibroids.

Early Pregnancy


It is known that the frequency of spontaneous miscarriage is increased in pregnant women who have fibroids. One study found that the rate of miscarriage in women with fibroids was 14% compared with 7% in women without fibroids. Research has found that the size of a fibroid does not affect the risk of miscarriage, but that the number of fibroids may do. Multiple fibroids increase the risk of miscarriage (miscarriage rate found to be 23%) compared with a single fibroid (miscarriage rate was 8%). It is also thought that the position of the fibroid in the uterus is important. Early spontaneous miscarriage happens more often when the fibroids are in the main body of the uterus when compared with fibroids being in the lower segment of the uterus or those fibroids which are intramural or submucosal. It is not known exactly how fibroids cause miscarriage. Possible mechanisms include: increased irritability of the uterus, mechanical compression by the fibroid and/or damage to the blood supply to the growing placenta or foetus.

Bleeding in early pregnancy

The risk of bleeding during early pregnancy is affected by the location of the fibroid in the womb. If the placenta has implanted close to where the fibroid is then the risk of bleeding is increased significantly to 60% from a risk of 9% if there is no contact between the fibroid and the placenta.

Late Pregnancy

Preterm labour and preterm premature rupture of membranes

Uterine fibroids increase the risk of premature labour and childbirth. The risk of preterm labour when fibroids are present is 16% in contrast with a risk of 8% when no fibroids are present. The rate of preterm delivery is 16% in contrast with a risk of 11% when there are no fibroids present.

Having multiple fibroids and having fibroids that are in direct contact with the placenta have been shown to be independent risk factors for premature onset of labour.

It is interesting that fibroids are not an independent risk factor for preterm premature rupture of the membranes (PPROM) of the amniotic sac around the baby. In fact recent research has suggested that fibroids may actually decrease the risk of PPROM.

Placental abruption

Data from medical studies suggests that placental abruption is three times as common in those women who have fibroids during pregnancy. Independent risk factors for placental abruption are: submucosal fibroids, retroplacental fibroids and fibroids with a volume of >200 cm3

A study which looked retrospectively found that placental abruption occurred in 57% of women who had retroplacental fibroids, in contrast with 2.5% of women who had fibroids in other areas of their womb. It is thought that one way that fibroids make placental abruption more likely is that there is reduced blood flow to the fibroid and the surrounding uterine tissue. This then leads to reduced blood flow (partial ischemia) and decidual necrosis in the tissue of the placenta over the underlying leiomyoma.

Placenta praevia

Only 2 research papers have been published about the relationship between placenta praevia and fibroids. They both indicated that the rate of placenta praevia is doubled even when adjustments were made for previous surgical procedures such as myomectomy and caesarean section.

Foetal abnormalities (anomalies) and foetal growth retardation

Medical research has found that the growth of the baby is not affected by the existence of uterine fibroids. Studies have suggested that women with fibroids were slightly at greater risk of having a baby whose growth had been slowed, these studies were not adjusted for gestational age of the foetus or the age of the mother, and these studies have been criticised by specialists in this area.

A large fibroid or fibroids can rarely press on and distort the cavity of the uterus and may then lead to a foetal abnormality.

The congenital abnormalities that have been shown to occur with large submucosal fibroids include: torticollis ( an abnormal twisting of the neck), dolichocephaly (lateral compression of the baby’s skull) and limb reduction defects.

Labour and Delivery

Malpresentation, caesarean section and labour dystocia

The rate of malpresentation of the foetus in women with fibroids is 13% in contrast with a rate of 4.5% in women with no fibroids. Independent risk factors for malpresentation are multiple fibroids, large fibroids and fibroids in the lower uterine segment.

Many medical studies have found that having a uterine fibroid increases the risk of caesarean section. A systematic review discovered that having fibroids increased the rate of caesarean section by a factor of 3.7 (from 13.3% – no fibroid to 48.8% – fibroids present). This is partly due to an increase in labour dystocia. Predisposing factors for caesarean section are: multiple fibroids, foetal malpresentation, large fibroids, submucosal fibroids and leiomyomas in the lower uterine segment.

However, obstetricians usually advise that the presence of uterine fibroids is not a contra-indication to a trial of labour.

Postpartum haemorrhage

Medical evidence and studies have provided conflicting data about the rate of postpartum haemorrhage (bleeding), however pooled cumulative figures have suggested that the rate of haemorrhage following childbirth is greater in those women who have uterine fibroids (risk – 2.5%) when compared with women with no fibroids (risk – 1.4%).

A fibroid may distort the architecture of the womb and also interfere with contractions of the uterine muscle (myometrium) leading to loss of muscle strength (atony) of the uterus and postpartum haemorrhage. These factors may also explain why fibroids increase the rate of hysterectomy in the weeks following childbirth (during the puerperium).

Retained placenta

A medical study has shown that the rate of retained placenta of the uterus is more frequent in those women who had a fibroid, but only if it was located in the lower segment of the womb. But pooled cumulative data has suggested that the risk of having retained placenta is more frequent in all women with leiomyomas regardless of the position of the fibroid (1.4% – fibroid present – compared with 0.6%- no fibroid present).

Uterine rupture following myomectomy

This is complication is extremely rare. One study looked at 120 women who had a baby at term having had a previous abdominal myomectomy (in which the uterine cavity was not entered) and there were no cases of uterine rupture. It is uncertain whether the same is true with laparoscopic myomectomy. Researchers report that there are many case reports of uterine rupture occurring during childbirth in women who had previously had a laparoscopic myomectomy. Recent research has shown that uterine ruptures take place before the start of labour at the site of the previous myomectomy carried out by laparoscope. Fortunately the risk of uterine rupture following laparoscopic myomectomy is reported to be low at 0.5% to 1%.

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