When you pregnant, you straight know is twin birth??? =.=" (Or maybe i catch wrong meaning)
When you get pregnant, you need to inform your HR department immediately.
Other meaning i think of from your phrase:-
Twin children consider as 2 children only. So you still entitle to MT leave for the 3rd child.
Even you already give birth for 5 children already. When the 6th pregnant child, you can apply leave also. The different is MT leave is paid leave, for the 6th children is unpaid leave. Normally HR will approve. Unless your company want you give birth at office >.<