I had a sex with my husband every 15 days and in a...

there are many tips to get pregnant which are :
1.you should do preconception checkup by your doctor to take a full medical history and to examine you and do CBC test
2. take folic acid supplememts
3. stop smoking or alcohol drinking
4. take multivitamins supplement
having intercourse three days before ovulation and through out the ovulation time.5.
6. lay for at least 15-20 minute after having intercourse
7. dont go to the bathroom after having intercourse
so . i advise you to visit your doctor to do preconception check up and to do the following tips
if there is still no pregnancy then you can do the following :
semen boosters and multivitamines for your husband
injection of sperm within the womb through a simple procedure under medical supervision
artificial fertilization of  ovum with an sperm and re-implantation of the fertilized egg inside the woman's womb. This is a last resort if the previous methods fail