( ( Depending ( on ( ( the number ) ( of ( children born ) ) ) ) ) ( , ( it ( ( can ( be ( called ( ( ( ( ( ( twins ( -LRB- ( two -RRB- ) ) ) , ) ( ( ( triplets ( -LRB- ( three -RRB- ) ) ) , ) ( quadruplets ( -LRB- ( four -RRB- ) ) ) ) ) , ) and ) ( so on ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (VBG Depending) (PP (IN on) (NP (NP (DT the) (NN number)) (PP (IN of) (NP (NP (NNS children)) (VP (VBN born))))))) (, ,) (NP (PRP it)) (VP (MD can) (VP (VB be) (VP (VBN called) (NP (NP (NNS twins)) (PRN (-LRB- -LRB-) (NP (CD two)) (-RRB- -RRB-)) (, ,) (NP (NP (NNS triplets)) (PRN (-LRB- -LRB-) (NP (CD three)) (-RRB- -RRB-)) (, ,) (NP (NP (NNS quadruplets)) (PRN (-LRB- -LRB-) (NP (CD four)) (-RRB- -RRB-)))) (, ,) (CC and) (ADVP (RB so) (RB on)))))) (. .)))
Depending on the number of children born, it can be called twins (two), triplets (three), quadruplets (four), and so on.
(ROOT (S (NP (NP (CD Four) (NNS children)) (VP (VBN born) (PP (IN at) (NP (CD one) (NN birth))))) (VP (VBP are) (VP (VBN called) (NP (NNS quadruplets)))) (. .)))