In theory only one egg and one sperm is needed for pregnancy to occur. On average men produce 200-300 million sperm each time they ejaculate but most sperm die within the female genital tract before they reach to met the egg.

A normal healthy women ovulate only a single egg on each month of her menstrual cycle .For married couple having regular sexual relationship, during sexual intercourse, men ejaculated sperm in women cervix and within a short time sperm would pass through uterus and swim up into the fallopian tube where sperm met and fuse with the ovulated egg. The resulting fertilized egg further developed and move down along the tube and finally enter into the uterine cavity where it start to implant. If implanted successfully the fertilized egg would further develop to a viable fetus. During which the uterine blood vessel supply most of the nutrient to the fetus's growth.

For married couple, women need to have healthy fallopian tube and egg and men need to have healthy sperm in order to get pregnant. The chance of a normal couple with age of <30 year old who having regular sexual intercourse at the right time will have 25% chance of getting pregnant on each month of her menstrual cycle.