Starting Volume of Fibroid
Mavrelos et al. found that the size of fibroids at presentation significantly influenced their subsequent growth rates [10••]. Small fibroids, defined by a mean diameter of less than 20 mm, increased in size by a median of 51.3% (IQR, 9.3–210.3) per year, whilst large fibroids (diameter of more than 50 mm) increased by a median of 40.7% (IQR, 14.1–67.0) per year. Intermediate fibroids (between 20 and 50 mm) only increased in size by a median 16.8% (IQR, − 7.4–63.0) per year (P = 0.007). Fibroids classified as either small or large demonstrated a growth rate nearly three times more than fibroids classed as intermediate in size at presentation [10••].
Similarly, Baird et al. found that the size of the fibroid at initial assessment was a significant variable in predicting growth rates [11]. However, the authors found that large fibroids (≥ 50 mm) were significantly less likely to have short-term variability in growth when compared to small fibroids (< 50 mm) (P < 0.001) [11].
Peddada et al. did not find a significant difference in the growth rates of fibroids when fibroids < 14.0 cm3 were compared with larger fibroids (P = 0.48) [12]. Similarly, Tsuda et al. had the same conclusion when comparing fibroids larger or smaller than 31.9 cm3 [8].
Type of Fibroid
Tsuda et al. found that submucous fibroids were less likely to increase in size. 15.4% (2/13) of submucous fibroids increased in size per year on ultrasound, compared with 27.4% (17/62) of intramural fibroids and 30.7% (8/26) subserous fibroids (P < 0.05). This was independent of whether a leiomyoma artery was identified [8].
Conversely, Mavrelos et al. found that intramural fibroids grew the fastest, followed by subserous then submucous fibroids [10••]. Intramural fibroids increased in volume by a median of 53.2% (IQR, 11.2–217.0) per year, subserous fibroid increased by 25.1% (IQR, 1.1–87.1) per year and submucous fibroids increased by 22.8% (IQR, − 11.7 to 48.3) per year (P = 0.012). However, due to a disproportionate number of small intramural fibroids, the authors concluded that fibroid growth rate appeared to be independently influenced by the fibroid size, rather than a true reflection of the effect of growth rate on fibroid position [10••].
Age
Fibroid growth tends to decline with age. Mavrelos et al. found that women less than or equal to 35 years old had a median fibroid of growth rate of 69.1% (IQR, 8.3–185.1) per year, whilst women more than 35 years of age had a median fibroid growth rate of 29.8% (IQR, 0.0–78.7) (P = 0.113) [10••].
A similar decline was also reported by Peddada et al.; however, this did not occur for all races [9]. In their study, only white women ≥ 45 years of age demonstrated a significant decline in growth rate, − 19.57% (95% CI, − 30.48 to 6.94) per 6 months when compared white women < 35 years old (P = 0.04). For black women ≥ 45 years of age, there was no significant difference in fibroid growth rate compared to black women who were < 35 years old, − 3.38% (95% CI, 17.31 12.90) (P = 0.67). Furthermore, fibroids from white women ≥ 45 years of age grew more slowly than those from older black of the same age (2% vs. 15% growth rate, respectively) [9].
Number of Fibroids
Peddada et al. found that women with solitary fibroids appeared to have a faster growth rate, 33.75% (95% CI, 7.58–61.17) per 6 months when compared to women with multiple fibroids (P = 0.06) [9]. However, only 6.9% (5/72) of women had solitary fibroids in this study. The influence of the number of fibroids on growth rates was not commented on other authors [10••, 11].
Radiological Life Cycle of Fibroids in the Gravid State
The incidence of fibroids in pregnancy is estimated at 0.1 to 10.7% [13, 14, 15••]. The conventional belief is that fibroids grow in a consistent manner during pregnancy due to a number of factors: the increasing steroid hormone levels, hypertrophy and oedema and stretching of the myometrium. Conversely, expansion of the amniotic cavity and ischemia-driven degenerative changes may lead to shrinkage or disappearance of fibroids.
A recent systematic review concluded that fibroids do appear to have a linear growth pattern in the first trimester of pregnancy. However, the growth patterns in later gestations remain contentious with inconsistent evidence [16]. There are also increasing reports that fibroids often remain unchanged or even decrease in size during pregnancy [17, 18].
Non-linear Growth of Fibroids in Pregnancy
Ciavenetti et al. performed prospective scans on 109 women at four intervals, before pregnancy (scan 1), 7–8 weeks (scan2), 10–13 weeks (scan 3) and 20–22 weeks (scan 4) [15••]. The authors found that of the fibroids measuring 10 to 50 mm at scan 1, 76.9% (110/143) increased in volume between scan 1 and 2, 72.0% (103/143) increased between scan 2 and 3 and 40.6% (58/143) increased between scan 3 and 4. The median growth rate was 122% between scan 1 and 2, 108% between scan 2 and 3 and 25% between scan 3 and 4. The authors concluded that most fibroids increase in size in the first 7 weeks of pregnancy; however, growth during pregnancy was non-linear as growth in the later trimesters was slower [15••].
Further suggestions of non-linear fibroid growth in pregnancy was found by De Vivo et al. who performed three ultrasound scans at 12 (scan 1), 21 (scan 2) and 33 weeks (scan 3) gestation [13]. The mean (± standard deviation) fibroid volume was 17.04 cm3 (± 29.1) at scan 1, 38.8cm3 (± 67.8) at scan 2 and 42.1 cm3 (± 69.0) at scan 3 (P < 0.05). The authors concluded that fibroids increased in size throughout pregnancy but growth rates were significantly greater in the first half of pregnancy as compared with the second half of pregnancy [13].
In support of rapid fibroid growth in early pregnancy, Benaglia et al. compared fibroid growth rates between a pregnant cohort (n = 46) and a non-pregnant control cohort (n = 41), following in vitro fertilisation (IVF) treatment [19]. Ultrasound assessments for fibroid volume and diameter were performed at the start of IVF stimulation and 5 weeks after embryo transfer in both cohorts. The median change of the diameter of all fibroids in pregnant women was + 34% and in non-pregnant women was + 2% (P < 0.001). The median change in volume of all fibroids in pregnant women was + 140% and in non-pregnant women 0% (P < 0.001). The authors concluded that fibroids more than double in size within 7 weeks of pregnancy [19].
Strobelt et al. performed regular ultrasound fibroid assessments on 134 pregnant women from 16 weeks gestation until term [20]. The authors found that overall, fibroid growth was only found in 14.9% (20/134) of pregnant women and the remaining 85.1% of women had no change in fibroid size after 16 weeks of pregnancy [20].