is possible only,if unprotected sexual intercourse is done with ejaculation around ovulation time in regular period. Ovulation occurs approx 14 days before due date.
If your girl friend's due date was on 19th in next month,then ovulation occurs on 5th of the month.You did sex on ovulation time and your girlfriend took unwanted 72.
It is very much effective in preventing unwanted pregnancy with failure rate less than 1%. A withdrawal bleeding may occur and next period may be delayed/earlier.
You also did sex on 11th day and it was almost safe period with low risk.
If she does not get period,do a home pregnancy test with urine for B-hcg
after 1 wk of missed.
Practice safe sex
by using safe period/OCP/Condom etc to avoid unwanted pregnancy.