( ( ( ( ( Birth ( typically ( ( occurs ( at ( ( about 38 ) weeks ) ) ) ( after fertilization ) ) ) ) , ) so ) ( ( the ( fetal period ) ) ( generally ( lasts ( ( about 30 ) weeks ) ) ) ) ) . )
(ROOT (S (S (NP (NNP Birth)) (ADVP (RB typically)) (VP (VBZ occurs) (PP (IN at) (NP (QP (RB about) (CD 38)) (NNS weeks))) (PP (IN after) (NP (NN fertilization))))) (, ,) (IN so) (S (NP (DT the) (JJ fetal) (NN period)) (ADVP (RB generally)) (VP (VBZ lasts) (NP (QP (RB about) (CD 30)) (NNS weeks)))) (. .)))
Birth  typically occurs at about 38 weeks after fertilization, so the fetal period generally lasts about 30 weeks.
(ROOT (S (NP (DT The) (JJ fetal) (NN period)) (VP (VBZ lasts) (NP (NP (QP (RB approximately) (CD 30)) (NNS weeks)) (NP (NNS weeks)))) (. .)))