Gravida = total number of pregnancies (current included). Para = number of deliveries at or after 24 weeks (each pregnancy counts as one parous event regardless of number of fetuses). The patient: previous twin pregnancy (1 delivery = P1) + currently pregnant again = G2. A twin birth is a single parous event. Therefore obstetric history = $G2P1$. \[\boxed{G2P1 \text{ (Option C)}}\]