( ( At ( ( 8 weeks ) ( of pregnancy ) ) ) ( , ( ( the ( ( placenta and ) fetus ) ) ( ( have ( been ( developing ( for ( 6 weeks ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN At) (NP (NP (CD 8) (NNS weeks)) (PP (IN of) (NP (NN pregnancy))))) (, ,) (NP (DT the) (NN placenta) (CC and) (NN fetus)) (VP (VBP have) (VP (VBN been) (VP (VBG developing) (PP (IN for) (NP (CD 6) (NNS weeks)))))) (. .)))
At 8 weeks of pregnancy, the placenta and fetus have been developing for 6 weeks.
(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.