( ( ( ( ( ( ( The cause ) ( of ( uterine fibroids ) ) ) ( ( is not ) known ) ) , ) but ) ( it ( may ( be ( related ( to ( changes ( in ( estrogen levels ) ) ) ) ) ) ) ) ) . )
(ROOT (S (S (NP (NP (DT The) (NN cause)) (PP (IN of) (NP (JJ uterine) (NNS fibroids)))) (VP (VBZ is) (RB not) (VP (VBN known)))) (, ,) (CC but) (S (NP (PRP it)) (VP (MD may) (VP (VB be) (VP (VBN related) (PP (TO to) (NP (NP (NNS changes)) (PP (IN in) (NP (NN estrogen) (NNS levels))))))))) (. .)))
The cause of uterine fibroids is not known, but it may be related to changes in estrogen levels.
(ROOT (S (NP (NNP Estrogen)) (VP (VBZ causes) (NP (NP (DT the) (NNS changes)) (PP (IN of) (NP (NN puberty))))) (. .)))