Conception has to be timed just right. Your menstrual cycle plays an important role. In a perfect 28 day cycle (which not every woman has), during the first 1-5 days, an egg comes into maturity in an ovary. Between the 6th and 13th day, the lining in your uterus thickens in preparation for the egg.
Around the 14th day, you ovulate. Some women feel this sensation. Basically, the follicle bursts open and the egg gets released into the fallopian tubes.
From then on, if sperm has fertilized the egg, it will hopefully plant itself inside the uterus and begin to grow and be nourished. If you're not pregnant, the egg will be flushed out with your menstrual cycle and the cycle repeats itself.
When a man ejaculates inside a woman, he emits about 100 million sperm. Only approximately 200 of them survive to make it to the fallopian tubes and encounter the egg.
Sperm can live for up to 6 days in a woman's body if the conditions are right. The egg is viable for 12-24 hours, however many experts believe it is probably closer to 12 hours. Given this time element, it is extremely important to get the timing of intercourse right. In fact, incorrect timing of intercourse is considered the #1 mistake couples make when trying to conceive. If you don't know when you're most fertile, you may miss the window of opportunity that could allow you and your partner to have a baby.
When you and your partner have sex right before you've ovulated, the sperm will enter through the cervix and swim up to the fallopian tubes. Half will pick door #1 while the others will choose door #2. The half that picked the right fallopian tube will have a chance at penetrating the egg.
Within the next 24 hours, the sperm find your egg and surround it, each one trying to enter the protective layering. When one penetrates it, the others are locked out. The embryo then enters the uterus and implants itself in the wall. You miss your period - and a baby is on the way!