Hello dear
I understand your concern
You become pregnant if you had sex during fertile phase of period.
If you have regular period of 28-30 days then 10th to 18th day is considered as fertile phase of period.
Pregnancy is possible when you have
Patent fallopian tube
Normal ovarian function: maturation of follicle and rupture of good quality of egg
Normal hormone level of FSH, LH,
estrogen and progesterone
Normal semen parameter of partner.
Sometime only one time sex dose not cause pregnancy.
You have to do daily intercourse during fertile phase of period.
If you have worry then you should go for reproductive hormone analysis, USG scan,
ovarian follicle study and semen anlysis of your boy friend to know problem.
But I think you should wait for your marriage.
If you will not become pregnant 1 year after regular intercourse then both partner require complete evaluation.
Avoid
stress, take healthy diet, drink plenty of water, do regular exercise
Hope this may help you
Contact further if follow up needed
Best regards
Dr. Sagar