Help with OB index

Hello all!,
I am an ADN student currently in my OB rotation and wondering if someone was willing to check out my answers to a part of a case study. I am having a little bit of trouble with the OB index and would like to know if you have any easy tricks for figuring these out.

Kathy, a 38-year-old married female is seeing the doctor to confirm a positive home pregnancy test. She relates that her first pregnancy ended in a miscarriage at 12 weeks; her second resulted in the live birth of twin daughters at 36 weeks’ gestation, and the third resulted in the birth of a 9 pound son at 37 weeks’ gestation. She expresses ambivalence about this pregnancy, stating, “I’m really scared. We just can’t afford another baby right now.” She complains of frequency and intermittent nausea. Her last menstrual period began on March 23, 2009 and its last day occurred on March 29, 2009. Her exam & pregnancy test indicate she is pregnant.

Here is what I came up with.

Two-digit OB index - G: 4 P: 2 Four-digit OB index – T:1 P:2 A:1 L:3

Five-digit OB index – T:4 T:1 P:2 A:1 L:3

Would appreciate any help anyone can give me.