Fibroids, sometimes referred to as myomas, are growths or benign tumours that form inside the uterus (womb). Around 4 in 10 women over the age of 40 years will have fibroids. While the exact aetiology of fibroid development is unknown, it is thought that the sex hormones, oestrogen and progesterone, play key roles. This is because fibroids rarely grow in girls before puberty and women after menopause. Pre-existing fibroids stop growing and can even shrink in women after menopause.