( ( By ( ( 21 weeks ) ( of pregnancy ) ) ) ( , ( ( development ( of ( the ( fetal ( digestive system ) ) ) ) ) ( ( enables ( the ( fetus ( to ( swallow ( amniotic fluid ) ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN By) (NP (NP (CD 21) (NNS weeks)) (PP (IN of) (NP (NN pregnancy))))) (, ,) (NP (NP (NN development)) (PP (IN of) (NP (DT the) (JJ fetal) (NN digestive) (NN system)))) (VP (VBZ enables) (NP (DT the) (NN fetus) (S (VP (TO to) (VP (VB swallow) (NP (JJ amniotic) (NN fluid))))))) (. .)))
By 21 weeks of pregnancy, development of the fetal digestive system enables the fetus to swallow amniotic fluid.
(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.