For pregnancy to happen, the female's egg must be fertilized by the male's sperm and attach itself to the lining of the uterus. At that point, hormones are released to stop the female's menstrual cycle and pregnancy begins.
- Every month, a female's ovaries are stimulated and release an egg into the fallopian tubes. This is called ovulation. Once the egg has been released, it has to be fertilized by the male's sperm.
- Almost always, an egg can only be fertilized if the female has vaginal intercourse with a man and he ejaculates semen inside of or near her vagina. The semen contains sperm, which swim up the woman's vagina, move in waves through the woman's uterus and into her fallopian tubes. Sperm are strong swimmers and are designed to find and fertilize the egg.
- Once the egg is fertilized by the sperm, it becomes a zygote. This develops into a multi-celled pre-embryo which travels to the uterus. There, it attaches itself to the lining of the uterus.
Check out this informative and entertaining video (http://www.plannedparenthood.org/teen-talk/watch/how-pregnancy-happens-26792.htm) from Planned Parenthood.