( ( The ( estrous cycle ) ) ( ( is ( ( ( the period ) ( from ( ( ( ( one ovulation ) ( to ( the next ) ) ) and ) ( averages ( ( ( 21 to ) 23 ) days ) ) ) ) ) ( in length ) ) ) . ) )
(ROOT (S (NP (DT The) (NNS estrous) (NN cycle)) (VP (VBZ is) (NP (NP (DT the) (NN period)) (PP (IN from) (NP (NP (NP (CD one) (NN ovulation)) (PP (TO to) (NP (DT the) (JJ next)))) (CC and) (NP (NP (NNS averages)) (NP (QP (CD 21) (TO to) (CD 23)) (NNS days))))) (PP (IN in) (NP (NN length))))) (. .)))
The estrous cycle is the period from one ovulation to the next and averages 21 to 23 days in length.
(ROOT (S (NP (NNP Ovulation)) (VP (VBZ occurs) (SBAR (IN before) (S (NP (DT the) (NN endometrium)) (VP (VBZ thickens) (PP (IN in) (NP (JJ estrous) (NNS cycles))))))) (. .)))