There are three main ingredients required for pregnancy to occur. These are 1) sperm, 2) eggs and 3) the Fallopian tubes with the uterus. When there is a problem with the production and or function of these, difficulties with conceiving can occur.
Conception or pregnancy occurs when the sperm fertilizes the egg i.e. The sperm enters the egg and fuses with it to form an embryo in the Fallopian tube. This embryo travels down the tube to implant in the womb, finally growing into a baby.
Whilst it takes only one sperm to fertilise the egg, several others are required to support the fertilizing sperm to enter the egg by helping to prepare the path by removing the cells that provide a large protective shell (corona radiata and granulosa cells) around the egg.
Generally a woman will release only one egg each menstrual cycle; however, on average a man releases about 250 million sperm per ejaculation. The vast majority of the sperm does not reach the egg.
The ejaculated semen which contains sperm is initially thick and within 20 minutes to an hour, it becomes liquefied. The sperm then travels through the cervix, then through the womb and into the fallopian tubes where they meet the ovulated egg.
The egg once fertilised will form an embryo and is just visible to the human eye measuring at about 10th of a millimetre. This embryo rapidly divides over the next few days from 2 to 4 to 8 to 16 cells and so on, till it becomes a blastocyst on day 5 at which stage it rapidly enlarges and implantation occurs in the womb two days later i.e. 7 days after fertilization.