( ( When ( ( the fetus ) ( is ( ( 18 weeks ) old ) ) ) ) ( , ( hearing ( ( begins ( to develop ) ) . ) ) ) )
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (DT the) (NN fetus)) (VP (VBZ is) (ADJP (NP (CD 18) (NNS weeks)) (JJ old))))) (, ,) (NP (NN hearing)) (VP (VBZ begins) (S (VP (TO to) (VP (VB develop))))) (. .)))
When the fetus is 18 weeks old, hearing begins to develop.
(ROOT (S (NP (NP (DT The) (NN development)) (PP (IN of) (NP (NP (DT the) (JJ respiratory) (NN system)) (PP (IN in) (NP (DT the) (NN fetus)))))) (VP (VBZ begins) (PP (IN at) (NP (QP (RB about) (CD 4)) (NNS weeks)))) (. .)))
The development of the respiratory system in the fetus begins at about 4 weeks.