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If you are reading this, it means you are alive, which means you were once just a single cell, and your zygote survived the unlikely odds of pregnancy and, eventually, you were born. Yes, you exist because you have DNA from one of your father’s sperm and one of your mother’s eggs, but do you know how unlikely it is for you to exist? If you can begin to understand conception, pregnancy, and birth, you may just take a different view of yourself and your life.For example, did you know: only 25% of fertilized eggs implant into the uterine wall. Of those that do implant, about 30–40% naturally miscarry (also known as spontaneous abortion) due to genetic abnormalities – and the woman may not even have known she was pregnant. So, of all fertilized eggs (zygotes), approximately 50% of those conceptions result in birth of a living baby. Those are pretty long odds, when you really think about it! This chapter will be focused on Prenatal Development. The word prenatal means before birth, so prenatal development refers to all of the changes that occur from conception until the day we are born. There is a lot that needs to happen during the prenatal period, so it is a time of incredibly rapid development. Over the course of about 40 weeks, we go from being a single cell to being a fully-formed newborn baby. Since the change is happening so quickly, that means that there’s a lot of potential for things to impact this development. Something that might have very little impact on an adult body can have huge impacts prenatally.