This was a spontaneous di-di twin gestation.
 The twins had separate chorions and amnions. 
(ROOT (S (NP (DT This)) (VP (VBD was) (NP (DT a) (JJ spontaneous) (JJ di-di) (NN twin) (NN gestation))) (. .)))
(ROOT (S (NP (DT The) (NNS twins)) (VP (VBD had) (NP (JJ separate) (NNS chorions) (CC and) (NNS amnions))) (. .)))
( This ( ( was ( a ( spontaneous ( di-di ( twin gestation ) ) ) ) ) . ) )
( ( The twins ) ( ( had ( separate ( ( chorions and ) amnions ) ) ) . ) )