( ( Between ( ( ( the 5th ) and ) ( ( the ( 12th weeks ) ) ( of pregnancy ) ) ) ) ( , ( ( while ( ( the fetus ) ( is ( ( growing and ) developing ) ) ) ) ( , ( ( nerve cells ) ( ( ( ( form ( in ( the ( digestive tract ) ) ) ) , ) ( ( ( beginning ( in ( the mouth ) ) ) and ) ( finishing ( in ( the anus ) ) ) ) ) . ) ) ) ) ) )
(ROOT (S (PP (IN Between) (NP (NP (DT the) (NN 5th)) (CC and) (NP (NP (DT the) (JJ 12th) (NNS weeks)) (PP (IN of) (NP (NN pregnancy)))))) (, ,) (SBAR (IN while) (S (NP (DT the) (NN fetus)) (VP (VBZ is) (VP (VBG growing) (CC and) (VBG developing))))) (, ,) (NP (NN nerve) (NNS cells)) (VP (VBP form) (PP (IN in) (NP (DT the) (JJ digestive) (NN tract))) (, ,) (S (VP (VP (VBG beginning) (PP (IN in) (NP (DT the) (NN mouth)))) (CC and) (VP (VBG finishing) (PP (IN in) (NP (DT the) (NNS anus))))))) (. .)))
Between the 5th and the 12th weeks of pregnancy, while the fetus is growing and developing, nerve cells form in the digestive tract, beginning in the mouth  and finishing in the anus.
(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.