Uterine fibroid incidence and growth in an ultrasound-based, prospective study of young African Americans

Abstract

Background: Uterine fibroids are common. Symptoms are debilitating for many, leading to high medical and societal costs. Indirect data suggest that compared with white women, African Americans develop fibroids at least 10 years earlier on average, and their higher health burden has been well documented.

Objective: The objective of the study was to directly measure fibroid incidence and growth in a large, community-based cohort of young African-American women.

Study design: This observational, community-based, prospective study enrolled 1693 African-American women, aged 23-35 years with no prior diagnosis of fibroids. Standardized transvaginal ultrasound examinations at enrollment and after approximately 18 months were conducted to identify and measure fibroids ≥0.5 cm in diameter. Fibroid growth (change in natural log volume per 18 months) was analyzed with mixed-model regression (n = 344 fibroids from 251 women whose baseline ultrasound revealed already existing fibroids).

Results: Among the 1123 fibroid-free women with follow-up data (88% were followed up), incidence was 9.4% (95% confidence interval, 7.7-11.2) and increased with age (P_trend < .0001), from 6% (confidence interval, 3-9) for 23-25 year olds to 13% (confidence interval, 9-17) for 32-35 year olds. The chance of any new fibroid development was greater than twice as high for women with existing fibroids compared with women who were fibroid free at baseline (age-adjusted relative risk = 2.3 (confidence interval, 1.7-3.0). The uterine position of most incident fibroids (60%) was intramural corpus. Average fibroid growth was 89% per 18 months (confidence interval, 74-104%) but varied by baseline fibroid size (P < .0001). Fibroids ≥2 cm in diameter had average growth rates well under 100%. In contrast, small fibroids (<1 cm diameter) had an average growth rate of nearly 200% (188%, confidence interval, 145-238%). However, these small fibroids also had a high estimated rate of disappearance (23%).

Conclusion: This is the first study to directly measure age-specific fibroid incidence with a standardized ultrasound protocol and to measure fibroid growth in a large community-based sample. Findings indicate that very small fibroids are very dynamic in their growth, with rapid growth, but a high chance of loss. Larger fibroids grow more slowly. For example, a 2-cm fibroid is likely to take 4-5 years to double its diameter. Detailed data on fibroid incidence confirm an early onset in African-American women.

Keywords: epidemiology; incidence; tumor growth; ultrasound; uterine leiomyomata.

Figure 1

Variation in fibroid growth by woman and fibroid characteristics Fibroid growth per 18 months associated with categories of baseline characteristics (n = 339 fibroids after excluding 5 outliers). Horizontal lines show 95% CIs. Estimates are adjusted for baseline age and fibroid size and are based on converting model results (change in ln-volume over time) to percent change in volume indexed to 18 months. There was no interaction between age and fibroid size (p=0.70).

Figure 2

Change in fibroid growth with fibroid size Change in fibroid growth rate with baseline fibroid size as estimated by cubic spline analyses. Estimates are based on change in ln-volume for 344 fibroids with nodes at diameter equivalencies of 1, 2, 3, and 4 cm, and indexed to women 26–29 years of age. The graph is truncated at 5 cm to better show the drop in fibroid growth rates in the smaller fibroid-size categories. The shaded area represents the 95% confidence limits around the estimated average growth. The vertical lines show the observed median (solid circle on vertical line) and interquartile range of growth rates for the fibroids in each size category. Percent >400 (above the upper dotted line) are the percent of observed fibroids with growth rates that exceed 400 percent per 18 months. Percent shrinking are the percent of observed fibroids with a decrease in volume of 20% or more. The estimated percent of fibroids lost in each size category during the 18-month follow-up is shown in the box below the graph; methods for estimating loss are described in Supplementary Methods 1.3.