New research on uterine fibroids and miscarriage suggests that smaller fibroids, which are often not considered by doctors to be troubling, may actually pose a greater risk of miscarriage to women who are pregnant or trying to become pregnant. But researchers at the University of North Carolina at Chapel Hill say more studies on how size and location of fibroids affects pregnancy is still needed.
The study was led by Dr. Katherine Hartmann, Director of the Center for Women’s’ Health Research at the University of North Carolina at Chapel Hill. She said early in the study experts asked, "Why are they messing around with those little fibroids, they just don’t matter? Our reply was you have no evidence anywhere that they don’t matter, nobody’s ever studied them before," said Hartmann.
For their study, Hartmann’s team recruited more than 1,600 women, who were at least 18 years old, from 3 cities in North Carolina who were no more than 12 weeks along in their pregnancies, or who were still trying to become pregnant. All the women were given ultrasounds 3 times during the course of their pregnancies to look for fibroids.
Unlike earlier studies that only looked for fibroids 3cm or larger in diameter — which is roughly the size of a golf ball — this study included fibroids as small as half a centimeter and larger, or about the size of a jelly bean.
"Of all the fibroids that we measured, the median diameter was 2.4cm, so more than half would not have been counted as fibroids in previous studies," said Dr. Hartmann.
The average age of women in the study was 28 years. Women with fibroids tended to be older and heavier. There were 167 women diagnosed with fibroids. About 41% of their fibroids were intramural, which means they were small enough not to distort the surface of the uterus. Those that tend to distort the external contour of the uterus — called subserous fibroids — made up 37% and 14% were submucuous — which are those that stick into the uterine cavity.
The results of Dr. Hartmann’s study show that women with fibroids have a 55% increased risk of miscarriage regardless of their age, weight, smoking status, or ethnicity. Fibroids that were less than 3cm carried with them a 4-fold increase in risk. Dr. Hartmann explained that most of these smaller fibroids were adjacent to the uterine cavity, and it may be that they’re activity in the uterus disrupts the environment for the fetus.
"For the larger fibroids we don’t see any effect at all, which suggests the larger ones don’t really have any effect and this is where we need a whole bunch more data," said Dr. Hartmann.
But she added that it’s probably not the case that the larger ones have no effect at all—they just may be less active. She said she plans to study more women in order to see how fibroids of all sizes and locations impact a woman’s risk for miscarriage. She hopes the research will provide clarity on whether it’s actually the size of a fibroid—versus the location in the uterus—that increases miscarriage risk.
And while 55% sounds like a big number, Dr. Hartmann wants to reassure women that it’s really not that much of an increase over the risk carried by women with no fibroids.
"The odds are still overwhelmingly in your favor that you’re going to have a fine pregnancy," she said. "It’s by no means a sentence to crummy reproductive performance that you have fibroids," said Dr. Hartmann.