This was a spontaneous di-di twin gestation.
 The twins are originated from separate eggs. 
(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 (VBP are) (VP (VBN originated) (PP (IN from) (NP (JJ separate) (NNS eggs))))) (. .)))
( This ( ( was ( a ( spontaneous ( di-di ( twin gestation ) ) ) ) ) . ) )
( ( The twins ) ( ( are ( originated ( from ( separate eggs ) ) ) ) . ) )